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Community and Q&A

Why must HVAC decisions be so complex? (Or… Is geothermal worth it?)

KSJeffery | Posted in Mechanicals on

In another thread, my choice for HVAC was appropriately questioned. The system seems inordinately complex, costly, and convoluted. At least, on the surface. But digging deeper, the reasoning behind the design becomes clear. But, does that make it right? Is this the best HVAC design, or is it redundant and wasteful?

Here are the pertinents:
• Climate Zone 6
• 2700 sq. ft. finished space in story-and-a-half (bungalow style) house
• 2000 sq. ft. unfinished basement (future completion for aging parents)
• 4 Bdrm, 3 ½ bath house
• Tight, highly insulated home
• Manual J heat load calc 56,000 BtuH, total cooling load 32,000 BtuH

Features important to my wife and I in designing the HVAC:
• Warm feet
• Low operating costs
• Ability to control zones as we have kids leaving, but may have aging parents coming
• Quiet, “I hardly know it’s there”, operation
• Right sized for our needs

After listening to our desires, the HVAC contractor designed the following system:
1. Geothermal ground source heat pump
a. 4 ton water-to-water unit – hydronic heat to basement and non-hardwood flooring areas of main floor
b. 3 ton split system – forced air for cooling of entire house, and forced air for heating areas not served by hydronic heating (loft area and hardwood floor great room/entry
c. Domestic hot water generation
2. Gas furnace with variable speed (ECM) blower
a. To serve as the air-handler (AH) for geothermal split
b. To provide for back-up heat to allow us to take advantage of reduced electric rates on geothermal system.

When presented with the estimate for installation, I about choked. I then began researching and exploring other options, and am STILL exploring other options. Such options include:
1. Trad’l gas furnace with central A/C
2. Ductless minisplits
3. Gas boiler for hydronic, with mini-splits for cooling and spot heat
4. Boiler for hydronic, with furnace for hardwood areas of home, and trad’l A/C
5. Geothermal forced air system alone
6. Ducted mini-splits

All involved a significant compromise, and systems I thought would be more cost effective proved not to be, when the cost of rebates, tax breaks, reduced electricity rates, operational costs, and functional efficiencies over the 25 years we intend to inhabit the house were considered.

I planned to start digging the geothermal pit for the loops in 2 weeks, but as yet, I’m not locked in. I invite all interested reader to criticize my HVAC, and discuss the alternatives. I very well may be missing something!

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  1. davidmeiland | | #1

    Since you want "warm feet" and apparently need A/C, you're going to have two complete systems installed, with no overlapping function. If you went entirely with forced air, you could have one split system for heating and cooling. If the contractor is proposing geothermal then I would expect a detailed breakdown that shows installation and operating cost vs. other types of systems (i.e. a gas boiler). I would also hope for clear evidence that the system will perform very efficiently as hoped, and frankly I don't know how a contractor could demonstrate that unless they can take you to homes where they have installed these systems and you can examine the very low energy bills for yourself.

    Anyway, your wish list is large, so your price quote is large too. The warm feet part is nice but you really only stand in certain small areas for any length of time, and installing geo to heat 100% of your floors in a major luxury.

    I would *always* look hard at the cost and options for entirely mini-split systems first.

  2. KSJeffery | | #2

    David - thank you for your input. Geothermal versus mini-split is a debate we've had. In fact, I installed two head mini-split system in my 1800 sq. ft. garage/shop to see how well the system worked. Here's what I learned: comfort was lacking. I fear that the house, with more doors and walls, would suffer even more from the limited ability to "mix and move" the heat, as others have mentioned on this site.

  3. GBA Editor
    Martin Holladay | | #3

    A couple of quick reactions:

    1. You calculated design heating load seems high to me, indicating that either (a) whoever performed the calculations made errors, or (b) your thermal envelope isn't as airtight or well insulated as it should be.

    2. You have two heat distribution systems -- hydronic tubing and ductwork. These components are expensive, so I advise you to choose just one system. If you want to have forced-air ductwork to deliver cooling, then use the same distribution system to distribute your space heat.

    3. I'm not a fan of ground-source heat pumps, which are much more expensive than air-source heat pumps without enough of an improvement in efficiency to justify the huge investment. Most experts advise that it makes more sense to choose one or more air-source heat pumps, and to spend the money you saved (by not buying a ground-source heat pump) for envelope improvements or PV. For more on this issue, see Are Affordable Ground-Source Heat Pumps On the Horizon?

    4. I'm not a fan of in-floor hydronic heating systems. To learn more about this issue, I recommend this article: Radiant-Floor Heating.

  4. KSJeffery | | #4

    Martin - you reference excellent articles, and as always, have salient points to make.

    1 - The heat load calc is accurate, but includes two areas that skew the numbers. First, we planned to heat the garage (radiant in-floor heat) to 60F, which adds 8,000 BtuH. Second, there is a nearly all glass sunroom that we considered heating also, to the tune of 11K BtuH. Heating the garage remains desirable, but the sunroom is on the chopping block (well, heating it is). So, a more realistic number is 45,000 BtuH for heat load.

    2 - I completely agree with you that we should choose one distribution methodology, and am inclined to go with forced air as we need ducting for cooling regardless, and the benefits of hydronic heat in a super-insulated home are subject to controversy.

    3 - This past winter, it was extremely cold in MN and the ASHP in my garage/shop did not seem to function well. Ignoring the COP inflation claims of GSHP manufacturers, accepting that variable speed compressors do serve to broaden the range of operational efficiencies for ASHP's, it remains easier to optimize GSHP efficiencies when the heat energy source arrives at fairly constant supply temps. I'd like to know how they truly compare, though.

    4 - Agreed. Except when changing oil on the car in the garage. Is this enough to justify redundant heat delivery modalities? Probably not. Although, here's my counter point. To get energy rates at 60% off for the geo, I need a back-up. Maybe I should use a boiler to provide hydronic heat to basement and garage slabs alone, and call it my back-up heat source? Of course, I'd still need an air-handler...

  5. Expert Member
    Dana Dorsett | | #5

    Even 45KBTU/hr, 37K if you don't include the garage in a zone 6 MN location for 2700' of fully conditioned space + 2000' of basement doesn't seem to fit the "Tight, highly insulated home" description, which makes me think either something in the description is way off, or there are some thumbs on the scale in that Manual-J. That's only 10% bigger than my sub-code 1920s 1.5 story bungalow + basement, and 37K is about the size of my (measured) heat load at -5F. Something just feels off about it.

    What's your ZIP code, and what were the indoor & outside design temperatures used?

    Also, if the garage isn't "Tight, highly insulated..." and/or the 2-head multi-split wasn't rated & sized for operation at your outside design it's not a great test of the concept. Got a model number for that unit? (They aren't all the same, not even close.) It doesn't take a lot of mini-split to deliver 8000BTU @ +10F, but most aren't even rated at -10F.

    The Daikin Altherma air-to-water heat pump is a fraction of the cost of GSHPs, and still has decent output at -4F if the water temp requirements are under 100F (concrete slab, gypcrete, or WarmBoard style, but not staple-up.) It's nowhere near 45, 000BTU/hr , but it's half that. A combination of cold-climate mini-split and Altherma could probably handle most of the load for an upfront cost less than half that of GSHP. Controlling the floor temp with the Altherma and the room temps with the mini-split(s) can be high on the comfort end and high efficiency. For doored off intermittent use rooms you can do a lot of temperature leveling with right-sized radiant cove heaters and occupancy sensors (or setback thermostats) rather than ductless heads at a low upfront cost, letting the heat pumps carry the bulk of the load.

    Within 5 years most analysts (including those at Citi Group and Sanford Bernstein investment banks, not just the usual renewbles cheerleaders) anticipate the pre-subsidy cost of residential scale photovoltaics to be $1.50 or less, at which point the lifecycle cost per MMBTU will be competitive with the current cost of natural gas, even at a COP of 1. Compare the projections in these two curves, bearing in mind the the cost axis on the Citi Group analyisis is logarithmic, to be able to show more detail, while the Sanford Bernstein is linear, showing just how precipitous the cost curve is on PV:

    Minnesota regulators just approved a statewide value of solar tariff (VOST) method that at current valuations would be a better deal than rough-justice net-metering, giving utilities the option to compensate solar operators either via net-metering, or a VOST value that is locked in for 20 years. It's not yet clear how many utilities will be opting in, but since the VOST is a fixed contract price and net metering is not, some will opt for the VOST. Either way, leveraging lower cost heat pump technology with perhaps some resistance heating for auxiliary or "backup" and spending the difference on PV is probably going to be a significantly better investment than GSHP, even in the shorter term. The actual price you'd have to pay for the PV depends on when you are building, and what subsidies might be available to you, but MN is poised to become a solar development hot-spot over the next three years, which should make PV pricing fairly competitive. In the parts of Texas where it has become VERY competitive it's already hitting the $2/watt range for all-in cost (pre-subsidy) for residential grid tied PV, whereas in my location (MA) it's still north of $3.50/watt.

  6. STEPHEN SHEEHY | | #6

    Kent- why heat the garage at all, let alone to 60 degrees?

  7. KSJeffery | | #7

    Dana - thank you for your insights. While I did attempt my own Manual J heat load calculations, I have neither the tools nor the training to know if my assumptions are reasonable. I've relied on the HVAC subcontractor (mistake?) At any rate, here are facts I used in my attempts: dry bulb temp 74F cooling, 70F heating. I'm building near Winona, MN zip 55987. Walls R40, ceilings R49 and R62 (have both traditional vented attic and cathedral ceilings)

    I think a big factor in the calculation that I'm uncertain about is infiltration. I know that when I set infiltration to 0.3 AC/hr, my calculations are close to the "summary" the HVAC contractor gave me. Is this value for infiltration reasonable for house with R-values above, achieved by spray-foaming the shell with BIBS?

    I've read with interest your many postings here on GBA, and know you are a strong advocate for mini-splits, or air source heat pumps. I'm struggling to get past my fears of cold bedrooms and obtrusive wall mounted appliances. Well, the cold bedrooms is my fear... the feelings about an appliance hanging on the wall is more my wife's issue! Seriously, though, how well will heat, and perhaps more importantly, cooling, get to the 3 bedrooms upstairs if I place a head centrally in the hall between them? My kids prefer to keep their bedroom doors closed... gender things, really.

    And as for photo-voltaics, I'm intrigued, but haven't spent time researching or pursuing this angle. It is a definite consideration for add-on down the road.

  8. KSJeffery | | #8

    Stephen - Good question. Simple answer? It's because I love my wife, and she wants to get into and out of a warm car during our 9 months of winter! I will trick her, though, and set back the heat in the garage to just above freezing when she isn't looking ;-)

  9. STEPHEN SHEEHY | | #9

    Kent- I love my wife too, but for what you'll spend to heat the garage, you could take your wife to Tortola for the winter.
    Seriously, insulating and heating a room for the car is, and I say this with the deepest respect, nuts. Go out 5 minutes before she needs to drive someplace and start it for her.
    I suspect MN is like ME. Lots of road salt, which will find it much easier to eat your car in a nice warm environment.

  10. KSJeffery | | #10

    Stephen - Agreed. I guess it is appropriate at this point to explain what we design for, and what we actually do, often differ. For instance, I will design for ASHRAE 62.2 ventilation standards. What I will do is turn back ventilation to what's comfortable. I have windows that open, but most often they're closed. Heat in the garage? Well, I'd like to make it available, but may use it less than expected.

    What I really want to figure out is: what heat source, and what manner of heat distribution, is best for my house?

  11. KSJeffery | | #11

    Dana - The mini-splits I have in the garage are Comfort-Aire, InverterFlex series. I don't recall the model number off hand, but could get it.

    When performing a Manual J heat load calc, one of the largest swing factors, in which I have the least confidence, is the infiltration. I use a program by TACO to perform my calcs, and note that whether I choose 0.3 AC/hr, 0.4 AC/hr or even 0.6 AC/hr makes an enormous difference. What should one expect for a home with a spray-foamed shell, followed by BIBS insulation to the R-values I listed above? I felt these values to be reasonable, but are they?? Diversity factors also can influence the results of course, but I'm certainly not knowledgable or experienced enough to start "tweaking". Of course, ventilation impacts loads too, but it again seems reasonable to use ASHRAE specified minimums.

    So, what is a reasonable value to use for infiltration in a tightly insulated house?

  12. davidmeiland | | #12

    The insulation itself has little to do with reducing infiltration. If you want low infiltration, you need to spec air-sealing details on the plans and make sure they are executed. Very likely some of it will best be done during the framing stage, and unless you have a savvy crew, they won't understand what to do or want to do it. There might be some value in hiring an insulation contractor who owns a blower door, does some air-sealing during the various stages of the project, and possibly even guarantees an outcome. Whatever you do, if the general contractor is not on board, you're likely to end up around 5ACH50, possibly worse due to the ductwork.

  13. KSJeffery | | #13

    House is part of Energy Star certification program, so will have blower door test performed. And yes, I understand that infiltration is a function of construction moreso than insulation. My point in bringing it up is that the spray-foaming the shell - rim joist, attic floors, window bucks, etc - will seal better than simply packing my double-stud walls with BIBS.

    Since blower door results won't be known until after contruction is well underway, and well after HVAC plans made, I need to know a reasonable estimation of infiltration for my house as described that I can use to perform meaningful Manual J heat load calcs. So I can appropriately size my HVAC. Can someone offer me advice or guidance here? I have the HVAC people proposing 0.4 AC/hr. The Energy Star team is only just getting started.

  14. LucyF | | #14

    Where are you in the construction stage? You can do a lot to reduce air infiltration on your own. I went through every stud bay and sealed every possible seam. I sealed the wall framing to the subfloor, sealed around every electrical outlet that penetrated the thermal envelope, I even sealed nails that held the rainscreen and siding to the house. We came back with an amazing blower door test BEFORE the insulation was installed.

    There are some articles in the Journal of Light Construction that is also posted on the Small Planet Workshop website that shows air sealing tips and tricks. Terry Nordbye is the author and he says that somebody has to be the ASS - Air Sealing Specialist at the building site to make sure the building is truly well sealed. Unless your contractor assigns someone that job, then you have to do it. Maybe you don't have to physically do the air sealing, but you have to say - we need to foam or tape this sheathing seam here, tape around this exterior electrical box, etc. And then it has to be inspected after it is done.

    Articles on air sealing:

    Maybe that will help with the infiltration.

  15. fitchplate | | #15

    I don't much like in-floor radiant (hydronic or otherwise), either. An now its not so relevant since we use super-insulation and super-air sealing technique, materials and designs.

    What makes people uncomfortable is drafts (convection and leaks) and contact heat loss (conduction to cold floors). Air sealing stops leaks, envelope and infrastructure details stops drafts, and insulating the floor stops contact heat loss (keeps the floors same temp as the rest of the room). These are passive solutions with low relative cost (i.e. no ongoing operating costs) and endless long term savings.

    I don’t much like forced air either (hot air furnaces or air heat pumps).And in cold climates, air heat pumps are weakest systems when they are most needed. But I must say that modern and knowledgeable duct placement, distribution patterns and the use of diffusers can make forced air, conditoned HRV and ventilation air streams surprisingly comfortable (i.e. draft free, zoned and occupant responsive).

    The point is to design, build and use materials to eliminate or prevent the unwanted conduction and convention cooling of the occupants. Instead of using energy to compensate for bad design and a suboptimal envelope, use building strategy to isolate occupants from the cold. Then if your radiant in-floor heating is the central heating system, it can be significantly thinned out, with costs cut by less materials and smaller scales.

    In a truly super insulated and super sealed house, if I really preferred radiant heat sources, I would consider electric radiant, instead of hydronic. This takes some relearning and sheding old predjudices learned from old and bad building technique.

    Properly sealed and insulated houses are nothing like the houses of the old days ... comfort comes easily.

  16. KSJeffery | | #16

    Lucy - thanks for your take on air sealing. I know it can be meticulous, time-consuming work to caulk, tape and/or seal every board and panel interface or overlap. Did you insulate with loose fill, or happen to apply ccSPF? I'm wondering how much caulking or taping OSB sheathing, for instance, adds if one plans to apply ccSPF?

    Flitch - tell me more about your dislike for forced air. Is it the equipment? The typical energy source used? The interior conditions that result? I too like radiant heat, but for reasons outlined by others in-floor radiant may not make sense in a house that I'm hoping can be heated for ~10 BtuH per sq. ft. or less.
    Electric heat does have some appeal, if I were to one day add PV as advocated by Dana for instance. Or if it were to power a heat pump for the energy multiplier. But in MN natural gas is much less costly per unit of heat than is electric.

  17. GBA Editor
    Martin Holladay | | #17

    My latest blog (posted this morning) is relevant to your questions. Here is the link: All About Radiant Floors.

  18. wjrobinson | | #18

    Lucy, looks like Owens Corning Energy Complete System which is done by an insulation subcontractor.

  19. wjrobinson | | #19

    Kent, as you know your CCfoam will seal up the areas that have it. Then you are left with sill plates, utility cuts, plumbing stack cuts windows cuts and more and more.

    As to your original post... put in a and be done with it. A separate system. I have met the rep for America great gent, he personally set up Alex Wilson's home, get him to do yours possibly. They don't actually sell that many, so personal service... maybe.

    Get a high tech tight home pro adviser on board if you can find someone in your area.

    Now go build that beautiful home, start a blog, post us some pictures.

  20. fitchplate | | #20

    Kent …I think you will find that if you analyze a super insulated and super air sealed house option, your heating energy bill will be astonishingly small – a couple of hundred dollars a year. That allows you to downsize the heating system infrastructure (components and distribution) to a point that electric heat will be less expensive than a natural gas fired boiler or hot air furnace. Blowers and circulators can use comparatively more electricity to run in a gas fired system than the savings of the lower cost of gas.

    In other words, the purchase/amortization and distribution capital and operating costs of heating in a super insulated/super air sealed house will be more competitive for electric resistance heating. This is why minisplits are being promoted here. They are very efficient electricity driven appliances that also use back up electric resistance heaters on the coldest days (in your zone)

    One really have to change ones paradigm to understand the new rule, variables and principles that do not at all resemble what we thought we learned by living in poorly insulated, leaky houses with old technology.

    By the way, you cannot use PV for electric resistance heating.

    If you want the cheapest radiant, a wood stove is it.

  21. GBA Editor
    Martin Holladay | | #21

    On another GBA page, Kent shared the bids he received for his HVAC systems. One bid was for $39,000, and the other bid was for $61,000.

    So here is the challenge for the GBA community: what kind of system can we suggest for Kent that will beat these bids? Sharpen your pencils.

  22. KSJeffery | | #22

    In the discussion following Martin Holladay's superb discussion about the perceived benefits of in-floor hydronic heating in the superinsulated house (, the conversation came around to discussing the costs of geo, and debating the merits of this "sacred cow" too. Martin posed the following: "...time to 'fess up: how high was your contractor's bid?
    Some people get a $20,000 bid for a ground-source heat pump system. Other bids are closer to $40,000. Where did your contractor's dart land?"

    Well, I solicited two bids. First bid is for 3 zone forced air geothermal with nat'l gas back-up, duct work, desuperheater for DHW, loop field and HRV. That bid is $39,000. Second is an incremental bid, adding hydronic heat throughout most of the house, but retaining forced air in the great room (hydronics w/ hardwood floor debate, anyone?) and second floor. That bid came in at $61,000.

    I no longer have any attachment to a hydronic, in-floor heating system for my house as it is impossible to justify $22,000 for a system that likely won't yield any significant benefit, real or perceived. Yet, I'm still quite interested in the possibility of going with a geothermal system.

    If I compare the forced air geo to a conventional forced air natural gas furnace and central A/C unit, the cost difference is $12,000. Quite a premium, I suppose. But, when one factors in the POCO's rebate, and the 30% tax credit for geothermal, the difference is only $2,000 for geo over natural gas. A quick and crude calculation of operating costs moving forward (I can get 50% reduced electric rate on geo via a dual fuel program) indicates a savings of $150 per year, meaning payoff of 13 years. To me, that makes it attractive.

    Now, I know someone will want me to consider mini-splits. OK, let's have that talk, but tell me first if ONLY considering conventional furnace & A/C versus GSHP, am I wrong to stay drawn to geo?

  23. GBA Editor
    Martin Holladay | | #23

    Here's my starting point: if your minimum budget is $39,000, what if we take 50% of your budget and invest it in PV?

    You'll have $19,500 to invest in PV. At $4 a watt, that will give you a system rated at 4.9 kW. In Minneapolis, this system will produce 6,267 kWh per year. If you have a ductless minisplit system with an average COP of 2.5, you'll end up with 53,471,000 BTU per year.

    In other words, a $19,500 PV system could pay your entire heating bill, for the life of the system (say, 35 years).

    Now, with the other $19,500, we can buy 5 ductless minisplits and a simple HRV system.

  24. KSJeffery | | #24

    See? There it is! the ductless mini-split always arrives to save the day! ;-)

    Well, here's the hurdle I must overcome. My wife wants an invisible HVAC system. She doesn't want to see it. Doesn't want to hear it. Doesn't want to smell it. Doesn't want to feel it. Doesn't want to know its there! I tell her you can't have it be all four. Pick two, I tell her. She picked the first two. So, you know what her opinion about a PV array in our yard would be (The roof? Are you kidding??)

    It's not that I disagree with your point. If I were widowed (God forbid, as that nearly was the case...), I'd be very inclined to take PV and mini-splits under consideration. Still am. I have 2 mini-splits in my garage/shop, along with in-floor hydronics, and things work reasonably well. But, the heads are obtrusive. There's also the little issue of getting the heat to the right spots. My daughter cannot sleep with her door open or the dog would eat the guinea pig, and her brother would complain about her noisy breathing! So, PV and ductless mini-splits are going to be a hard sell. If I'm shown that they unequivocally excel versus the GSHP, I can be swayed, though!

  25. Expert Member
    Dana Dorsett | | #25

    The lowest outdoor ambient operation temp for the Comfort-Aire InverterFlex series is +5F ( ), and the output at that temp is suspect at best. The smallest dual-head in the series is the 1.5 ton A-VFH18DA - it puts out a max of 22,600 BTU/hr @ 47F (and only if you had two 1-ton heads attached.) The +17F output specs don't look too encouraging. I couldn't find extended temperature tables downloadable on line, but it is clearly not designed as a cold climate unit. It MIGHT be able to keep up with your actual load at 0F, but I wouldn't count on it, and certainly not a -5F.

    By contrast the Mitsubishi M-series mini/multi splits deliver the full rated output at +5F, and 80% of full rating at -13F. The Fujitsu Halcyon XLTH mini-splits are similarly well-suited to low temp operation, with a fully specified outputs down to -15F. Even the slightly older Mitsubishi MSZ-FE18NA single head 1.5 ton can still muster ~22K @ +5F, and ~15K @ -13F, and the newer M-series has somewhat more favorable low temp derating at low temp than the -FExxNA.

    If you air seal to 2ACH/50 and use heat recovery ventilation you can set the infiltration numbers to ZERO on the heat load calculator, and while that introduces an error, that error smaller than the other factors that you can't really control. Most Manual-J calculations overshoot heat loads by a double-digit percentage against measured reality anyway, and the granularity in sizing of ductless, or cove heaters is such that if you picked the size that was just above your calculated output (even at the zero infiltration factor) you won't be left cold. Even when the infiltration cfm is correct (never is), the heat loss from those infiltration numbers are always exaggerated, since it presumes the air leakage is all parasitic and without heat exchange, which would require the leaks to be concentrated smooth bore pipes through the wall, which is (almost) never going to be the case for a high-R home that has been air-sealed during construction. With real air leaks some of the heat of the air on it's way out heats up the building materials on it's way in, lowering the local conducted heat load, and ending up outdoors only somewhat above the outdoor temp, not at the full interior temp. Similarly the incoming cool air is pre-warmed on it's inward journey and arrives indoors at a temp well above the outdoor temp. There are no really good models for this heat exchanger effect, so heat load calculators presume the worst-case, which is quite a bit worse than reality.

    Your 99% outside design temp in Winona is probably about -10F (since -12F the 99% design temp for Rochester MN to the west of you, the design temp for La Cross to the ESE is -8F), which means you would be able to specify any of the Mitsubishi M-series multi-splits & mini-splits or Fujitsu XLTH pretty easily against your Manual-J, since these all have well-specified outputs at temps a bit colder than that. (Unlike the Comfort-Aire InverterFlex series.)

    The analysts at Citi Group are projecting the average cost of grid tied residential rooftop PV to be in the $1.12/watt range by Y2020- the investment banker analysts at Sanford Bernstein think that even that may be too conservative. The D.O.E. Sunshot Prize pays the installer if they hit break the buck a watt barrier. These people aren't insane- it will become that cheap, and sooner than the average armchair analyst (like me) would have believed even as recently at last year. $1.50/watt by the time the 30% tax credit expires in 2017 seems more likely than not.

    Forget Martin's $4/watt napkin-sketch- that price point is SOOOOoooo Y2013- practically ancient history! :-) (That is unless you're planning to use a time machine to build it in 2013 rather than 2015 or whenever.) In the past month I've seen a quote (in MA) for 15kw of rooftop grid-tied (with 3-phase inverter no less) that came in at $3.72/watt, but I'm told the same system could be installed in some competitive local PV markets in TX for a bit more than half that, and that's right now.

    Given that the MN VOST was only very recently put in place, it will take awhile to sink in, but don't be surprised to see the PV market catch fire within 2 years, 3 at the most. Even if you waited, a couple of years for the price to drop, squeaking it in just prior to the 2017 tax subsidy deadline you'd likely be able to put up 6-8kw of grid tied for under $15K up front, and probably under $10K net after cashing in the 30% tax credit.

    Think carefully about your roof lines, shade factors, and house orientation- consider of every square foot of unobstructed un-shaded south facing roof area as ~10 watts of potentially productive photon-farming acreage to work with. It takes 800 square feet of south facing roof to hit 8kw with middle-of-the road PV but you can probably get there on a 2700 foot 1.5 story provided it isn't a hipped roof, and not broken up by south facing dormers. South facing shed dormers would still be rack-able for PV- gabled dormers not so much.

  26. KSJeffery | | #26

    Dana - thanks for your detailed response. I'm warming up to the mini-split idea, if I could just figure out how to hide them from my wife. PV on the roof is another matter. Our roof lines were designed for passive solar, with the garage on the north, the tall, gable end wall facing south to maximize exposed glass, and the long roof pitches typical of bungalow houses are oriented east and west. PV on the roof is not an option my wife (nor I) will entertain even if our roof were appropriately oriented. (We could set it up over the geothermal loop field, though!)

    I greatly appreciate the guidance on values "typical" when performing Manual J Heat Load calculations. Since that's where any HVAC design process should start, let me obtain numbers I can believe and trust, and see just how much heating and cooling I really do need. As has been pointed out be several knowledgeable persons in this and other threads, the economies of scale should play a role in HVAC equipment selection.

    My biggest hang-ups with mini-splits bear repeating. First, they are, in my opinion, obtrusive. Second, I am concerned about how conditioned air gets appropriately distributed to where it is needed. Third, even variable compressor units designed for cold environments as have a drop in operating efficiency at the type of low temperatures we experience here in SE Minnesota, corresponding to the very time that they are needed most. And fourth, are they really that much less expensive? Guess I'll solicit a quote and find out.

  27. wjrobinson | | #27

    Are any of you reading each others posts?

    Kent, build your home your way which suits you and your wife. Martin, Dana, flitch, you are not reading Kent's posts.

    Kent, build. Put in your system which exactly fits what you and your wife desire. End of story.

  28. LucyF | | #28

    You asked about the insulation after the air sealing. After that we installed a "smart membrane" Intello Plus from 475 Building Supply which was also meticulously air sealed.

    I've included a photo of the Intello Plus and the service chase. And we insulated with dense pack cellulose. There is a 2 inch layer of Roxul Comfortboard on the outside of the house.

    I don't think you have to go to these extreme details of air sealing each stud bay if you use closed cell spray foam.

  29. GBA Editor
    Martin Holladay | | #29

    I'm guessing that Kent posted his questions here because he wanted advice. (If I'm wrong, Kent, I'll stop posting.)

    If Kent was happy with his two bids -- one for $39,000, and one for $61,000 -- then I doubt if he would have asked for advice. He would have just proceeded to accept one of the bids and started construction.

    Of course, Kent, if you and your wife don't want ductless minisplits or PV, it's your house, and you should get exactly the system you want, as A.J. advises.

  30. wjrobinson | | #30

    Martin, I am a builder not a blogger. The number one thing I do is move from choices to... decisions. Not less is more, but even better, myself and the customer have to decide. There are a million decisions to building a home. Nothing wrong with exploring options but reread all of Kent's posts. If you read like I do, then you will see what I see which is giving kent and his wife the systems they are comfortable with and can afford and desire to afford. Some people want granite and some people want granite and much more like no solar on a roof and not hearing or seeing the heat system.

    I build what people want. We do go over choices but we listen and quickly zero in on what matters and what is nice but really does not matter.

    To me mini splits for this family would be totally absolutely not what they want.

    Time to build. Talk is cheap. Let's get to deciding. I am ready to put this house up just to be done with this endless merry go round.

    I'm calling for a concrete order. The excavator is headed to the site. My air guns are at pressure and loaded.


  31. KSJeffery | | #31

    AJ - you crack me up! And, I respect your opinions. You probably aren't even aware just how well you read me, and your imploring me to just "get on with it" made my wife laugh! She calls me the Two Year Man, because she knows that I research the crap out of every decision I make to the point that sometimes, I fail to MAKE a decision. It's at this point that my wife steps in and makes the call. In fact, the excavator was scheduled to start digging the geothermal pit in 10 days. (Though he just called and asked if he can push it back a week. I said, "sure", another week to make certain our call is the right one.

    Martin - you, too, are correct in that I came here looking for advice. Acknowledging that I don't know what I don't know, I think there are a number of knowledgeable people visiting this site who will steer me forward. While we previously owned a geothermal GSHP house in Northern MN (CZ7) that we were mostly happy with, I haven't felt good about the $61K bid from the start.

    I am of the opinion that a properly designed forced air geothermal system, with back-up natural gas furnace - to function as air-handler and qualify me for the PoCo's reduced geo electrical rate - will satisfy us. I'm also comfortable with my decision to forego the hydronic in-floor, a decision made easier by obtaining input from the great folks willing to share their knowledge on GBA.

    Again, if I can obtain the geothermal system for just $2,000 more net out-of-pocket than a conventional furnace and A/C (same furnace, ironically), is that not compelling? I am willing to follow through with my investigation of ASHP alternative, but lacking the 30% tax credit (though a rebate is available from the PoCo) I'm guessing net cost to be comparable.

  32. Expert Member
    MALCOLM TAYLOR | | #32

    Kent I am well beyond my comfort zone making suggestions on the efficiency of the various options that have been suggested, but it does seem some hybrid approach might satisfy all. Could you not find an unobtrusive spot for a small PV array, a similarly unobtrusive location in the common areas for a mini-split and provide back up heat for the closed off bedrooms by either baseboard or Cove heaters?
    I do agree with AJ. Much as you might want to save the planet, life on earth isn't worth living with an unhappy wife.

  33. wjrobinson | | #33

    I got concrete ordered and now exactly where am I going to pour it on the ground? The trucks are coming.

    The wife says she wants in by Christmas and better not have hotel room heating and cooling or something other than shingles on the roof cause some online green freak said get em....

    Kent you have a good plan. I await my New Years invitation.

  34. fitchplate | | #34

    AJ … Were we supposed to read other posts before opening our mouths?

    I am not agreeing with the heat pump and PV solution, just reacting to Kent’s occasional concerns about cost … i.e. his choice for gas as being a less expensive way to make heat. Clearly air sealing and insulation were not accurately factored into his assumptions.

    In northern climates I am not seeing any economics in favor of heat pumps or PV’s. The capital costs are not green, they are red … for 25 years until the mortgage is paid out. Government incentives are for contractors, not for home owners.

    If capital costs were equivalent – if electric and gas furnaces, boilers, radiant and force air were matched in price by PV’s, heat pumps and HRV’s, we would not be having these discussions.

  35. user-1072251 | | #35

    hang-ups with mini-splits: First, they are obtrusive.
    yes they are, but after you live with them for two weeks you forget they are there. They are quiet and unobtrusive until you look straight at them.

    Second, how conditioned air gets appropriately distributed:
    this the biggest difference between an airtight superinsulated home and a "standard" home, and the hardest to understand: when you essentially eliminate infiltration, and heavily insulate the envelope, the whole house basically stays the same temperature. You have eliminated the reasons there are cold rooms. One caveat: if you are determined to "outsource" the airsealing or rely on a single insulation method (ccsf) you are giving up the quality control necessary to assure that outcome.

    Third, even variable compressor units designed for cold environments as have a drop in operating efficiency at the type of low temperatures: So what? They are still at least as efficient as most other systems, and far better at higher temperatures.

    fourth, are they really that much less expensive? this one's easy: Yes.

    But in the end, spend whatever you need to build it the way you want.

  36. homedesign | | #36

    Pot Belly Stoves, Refrigerators, Ovens, Ceiling Fans, TV sets, Radiators, Toilets, .... there are many, many fixtures and appliances in our homes that we have grown accustomed to.... and learned to live with.
    Just look at all the can lights (ceiling acne) in many homes.

  37. homedesign | | #37

    Bob Irving wrote: " when you essentially eliminate infiltration, and heavily insulate the envelope, the whole house basically stays the same temperature. You have eliminated the reasons there are cold rooms"

    I believe that a High Performance Enclosure is certainly easier to "condition" than a typical home.
    The Enclosure alone does not ensure good temperature distribution.

  38. Aaron Birkland | | #38

    Hi Kent,

    I am curious about the reduced electrical rate. What is the the precise criteria for a reduced rate. You mention backup heat. Does it need to be from a different fuel source (other than electricity, i.e. nat gas)? If not, it may be cheaper and more straightforward to use an electric resistance heater before the plenum as backup/aux, which is a traditional approach for forced-air GSHPs, and consider that as the backup that satisfies the power company requirements.

    Typically, GSHP is sized for slightly less than the design load - a 50k load house may work well with a 3 ton GSHP (which would put out ~ 30K of heat at full capacity). This is a actually good thing for cost and comfort. The comfort comes from long running times (lots of air mixing, uniform temps). While the operating costs are a little higher (aux strips need to kick in every once in a while to give a boost to the heat output in order to meet the load of the coldest days), this is offset by the decrease in price between, say, a 5 ton and 3 ton unit. Also, the more continuously running 3T unit will produce more hot water via the desuperheater , compensating for some of the aux strip consumption.

    if the worst happens and the heat pump fails, the resistance strips would serve as emergency backup, covering all the load. This is a rare occurrence. If you can avoid having to put it in, I think you can save some money by avoiding having to install a gas furnace that does nothing. (A gas furnace can't really be used to provide aux heat, relegating it to strictly backup/emergency)

    Also, is this reduced electric rate given for ground-source heat pumps only - i.e. would mini-splits be eligible for it?

  39. KSJeffery | | #39

    Flitch - what's curios about the capital costs is that when rebates and 30% tax incentive are factored in, the geothermal system is only $2K or so more than the HVAC "standard" of HE gas furnace and central A/C. Payoff of 10 - 18 years, depending upon ones boundary assumptions. To me, a no-brainer as my past experience with GSHP in CZ7 was favorable.

    The conversation of PV should be clarified, however. I am not considering PV at this time.

    Air sealing IS important - paramount, even. The walls will be air-sealed at the exterior sheathing, including rim joists with caulk & ccSFP, then insulated to R-40. Ceilings air-sealed at attic floor, or built-on-site cathedral vent chutes, again with caulk & ccSFP, to R-50 & R-62.

    Bob - I have the final word on air-sealing, likely doing much of the work myself, and overseeing whatever work is done by others.

    John - I am in complete agreement. Easier to condition the air a tight house (e.g. less energy input for system), but that does NOT mean that the condition air arrives where it is needed/wanted without planning, work and effort.

  40. KSJeffery | | #40

    Bob - I appreciate your point-by-point comments on my mini-split "hang-ups". I believe you are correct in that the obtrusive nature is not likely to be as much of a concern as we (my wife, especially) make it out to be.

    I DO however worry about the cold-climate performance. It's one thing to operate inefficiently but still keep you warm. It's another issue altogether to not be able to keep you warm!

    I will be interested to see what sort of quote I get from the HVAC sub for mini split system option. I'll post the bid when received, so that we know the cost differential (for my system choices, anyhow).

  41. KSJeffery | | #41

    Aaron - Our local PoCo offers electric rates at about 60% reduced, IF you have a separate back-up heat source. It needs to be non-electric, and must meet total house demand for the point is that the PoCo may turn off the power to the geo system during exceptional periods of electricity demand.

    Thus, another reason the geothermal GSHP has some appeal... safety via redundancy, while heating with reduced electrical rates most of the time.

    I do not yet know if mini split would also qualify, but I have every reason to believe it would. I'm awaiting the PoCo's answer.

  42. rob kohaus | | #42

    I live in Zone 5, central Illinois and am currently in framing stage of building my house. Foundation is R-24 ICF, Walls are R-48 and Ceiling is R-60.

    To address a couple issues you've raised along the way.....
    1.) GBA gives great advice on a lot of topics but I'm going to somewhat agree with AJ.....Martin is Martin, Dana is Dana and you are you. Do what feels best for you and your family.
    2.) I had my HVAC guy AND an engineer run loads on my house. HVAC guy showed much much higher loads.....To the tune of engineer thinks we can get away with a 2 ton system, HVAC guy first suggested a "4, maybe even 5 ton to be safe." Another HVAC bid quoted 6 ton! The HVAC guys (for the most part) are programmed to oversize because the last thing they want is comfort call backs. They figure, oversize it and it keeps the masses happy (plus they're mostly used to sizing for tract homes with 2x4 walls and fiberglass batts and high ACH numbers). So I'd have someone else take a look at those numbers if you're not confident. (FYI....we settled on a 3 ton. Slightly oversized with ECM blower so we're hoping we found that sweet spot for performance.)
    3.) GSHP get bad mouthed all the time on here. And I think its based on local conditions up in New England. Here in Central Illinois, my HVAC guy quoted a GeoComfort GXT036 (COP 4.2 and EER 18.3 at full load conditions and some hot water heating cability) with 4 vertical wells (plenty of capacity in the ground...very important) for a total of $22k. After local and national rebates I'm just over $15k. That's pretty damn cheap compared to the numbers you see thrown around on this site for GSHP.....but that's what its like here where I live.
    I also asked for pricing on ductless minisplits. He quoted a price of about $5k per each installed. I wanted three for the load and layout of the house so I'm at about $15k to go with minis.

    I was absolutely sure going into this (based on research and the gospel here at GBA) that I would get minis, even planned the layout so I could hide the heads a bit. But you'd have to be crazy, IMO, to not go with the GSHP in my situation. My point.....local conditions, prices and experience varies widely so price your options and base your decisions on what YOU can get it installed for. Not what Dana says is happening in Austin or Massachusetts.

    Another example of this from my planning and getting bids from builders.....It was going to be more expensive for me to do a double stud cellulose packed wall than my 2x6 with 4.5" of rigid foam. But on GBA the double stud wall is (in some circles) the be all, end all of affordable, high R wall assembly.

  43. wjrobinson | | #43

    OK, thank you Robert for posting some sanity. I 100% agree you and Kent are making the correct choice to go Geo.

  44. KSJeffery | | #44

    I think so too. Thought I'd provide the updates that helped me move towards this decision, and throw out some additional questions to the mini split camp. Dana? Can you help?

    So, as promised, I redid the Manual J8 heat load calc with boundary parameters I've been told more accurately reflect my construction. For instance, rather than 0.4 ACH, I went with 0.1 ACH summer 0.19 ACH winter. I also added a diversity factor for human occupancy, lighting and equipment to more accurately reflect the fact that we wont have everyone home all the time, with all the lights on in every room. Plus I updated the windows for each room. Now, I'm looking at 26,200 BtuH cooling and 33,800 BtuH heating (just for house, not including garage).

    Thus, GSHP downsized to 3 ton unit, and back-up furnace/air-handler is a much smaller unit too. Only four horizontal loops needed now instead of 8, I upgraded to multi-zone electronic zoning, and will keep the desuperheater for DHW. Switched to ECM circulator pumps (Wilo Stratus or Grundfos Alpha). Tentative price $37,500, reduced to $26,000 after rebates and tax credits.

    At these heat load values, though, I definitely felt the mini split option was worth a closer look. Unfortunately, a first estimate for 4 head (7K, 12K, 9K & 9K), 3 ton mini-split was $20,000. Except, a suitable system for CZ6 could not be readily identified. Neither Fujitsu or Daikin evidently have a low-temperature multi-zoned unit, so four separate single head units were "suggested" as best suited to layout. Both companies' multi-zone units are rated to +5F --- well, we spent 6 days where the temp never rose above zero this past winter, with several consecutive night of less than 20 below! Mini split fans, are you telling me I ought not fret about this? Even the low temp units are not very reassuring. Dana, you posted earlier that the Fujitsu Halcyon XLTH are rated to -15F? According to Fujitsu, they operate down to temps of -15F, at greatly reduced efficiency, but "should not be counted on to operate at colder temps".

    Plus, in order to have the mini splits quality for the PoCo's Backup Relief Rates, I'd have to have another fossil fuel heat source capable of 100% of demand (which I think I'd want for safety as I would worry about sub-zero stretches next winter). If I went with a back-up gas furnace, like with the GSHP, I would have to add ducting which would drive costs to exceed the GSHP net cost. Gas fireplaces don't qualify, nor does a different electric heat source such as resistance heating.

    So, it's the ground source heat pump for me. Excavator is coming next week, AJ!

  45. wjrobinson | | #45

    Dig the damn hole already.


  46. Expert Member
    Dana Dorsett | | #46

    The "greatly reduced efficiency" at -15F means they only operate with a COP of about 2 at that temp instead of 4. The average winter outdoor temp will determines the seasonal average COP, which for a US zone 5 location will be a bit better than 3.0., maybe as high as 3.5 if you go with the Mitsubishi MSZ-HExxNA single-head units. The fleet-average of a cluster of 10-12 older Mitsubishi MSZ-FE12NA 1tons field-monitored in-situ in occupied homes in Idaho Falls for the Northwest Ductless Heat Pump study was a COP of 2.96. The newer ones are about 15-20% higher efficiency across the board.

    A guy in Quebec posting here as Jin Kazama heated his place with a few Fujitsu XLTH this past season, cruising through -27F weather without losing ground. You simply have to size them correctly for your loads at design temp, and it'll make it. Just because they don't spec the output below -15F, doesn't mean it just lays over and dies at -16F. Your outside design temps are nowhere near that cold anyway (even though once every 25 years it does get that cold.) For the coldest day of the century it's OK to turn on some auxilliary space heaters, or put on a sweater, eh? But the odds are if you sized it for your 99% outside design temp it already has margin at -15F or -20F.

    A 4 zone multi-split for $20K seems a bit on the high side, but depending on the installation issues it's sometimes quite a bit cheaper (and higher efficiency) to go with a few separate single-head units.

    Daikin doesn't have much with a short-form spec below -20C/-4F. Their air-to-water Altherma has a rating at -4F, and keeps going at lower temps, with pretty good output & efficiency if your radiation is a concrete slab floor.

  47. user-3256600 | | #47


  48. user-3256600 | | #48


  49. wjrobinson | | #49

    Kent, give us an update someday.

  50. KSJeffery | | #50

    AJ, and all -
    I remained unconvinced that mini-splits would meet my needs in this zone 6 climate, despite the reasoned opinions of others (with exceptional knowledge and experience on the subject, I imagine!) I also did away with in-floor hydronic heat for reasons I explained previously, and am very comfortable with this decision. Am going with a 3-ton ground-source heat pump, using 95+% AFUE gas furnace as air handler, and to serve as back-up heat source to allow me to take advantage of reduced electric rates on the geo system. Total installed price for heat pump, horizontal loop field, furnace, HRV, desuperheater for DHW, ductwork and commisioning will come in at $36,100. I expect to qualify for rebates and tax credits totalling ~$10,000. Break-even point for system I estimate to be about 13-18 years, depending upon initial assumptions. I know ground source heat pumps have their detractors, but I am satified this is the most appropriate system for my purposes.

    Concrete truck arrived yesterday to pour footings. Today, we're setting ICF blocks, AJ. Geo pit is 2/3 dug, and loops scheduled for install next Tuesday.

  51. wjrobinson | | #51

    Kent, I like the specs, great to hear you are pouring concrete and digging ditches. I have a project starting hear that was delayed due to a blown head gasket. As of today back in business as I found a big hole at the site.

  52. davidsmartin | | #52

    Kent, Did you follow through as planned? Are you happy with the results?

  53. KSJeffery | | #53

    Thanks for asking, David. I did go with the 3-ton geothermal setup, but building a house by oneself is a loooong.... and sloooow.... process. I got the shell up last fall, finished with insulation this spring, and had the HVAC installed this summer. It won't be commissioned until the drywall is finished inside, which is what I am currently working on.

    To recap, what I went with was a 3 ton geothermal system with four 800' horizontal ground loops at 8 to 10 depth. I have a desuperheater for domestic hot water. The system is forced air (both heating and cooling) and connected to a Daikin 96%AFUE 40K BTU gas furnace which acts as the air-handler, as well as the back-up heat source to qualify for dual fuel reduction rate for electricity.

    It is a $36,000 system, but should qualify for almost $9,000 in tax credits and other rebates from our Power Co. Funny thing is, I'm wondering if it's oversized for our house. I had the initial blower door test last winter, which came in at 0.62 ACH50. Since then, I've sealed up a number of leaks (well over 100 cans of Great Stuff in this house, now) that I've discovered along the way, and punched in a few new ones - like range exhaust hood and gas dryer vent. I heated the house last winter with nothing but a few electric space heaters, and so far this fall I haven't needed any supplemental heat to keep the inside at a nice toasty 58deg, despite night temps below freezing and daytime highs of 50 this past week.

    Anyhow, to your point, I'm not unhappy with my choice to go geo, nor have I second guessed it, but it isn't running yet so I can't say if I'm happy with it. My goal is to get it turned on in January. I'll get back to you then.

  54. iLikeDirt | | #54

    Y'know, when I read the original post, my first reaction was that electric resistance panels in each room would give you basically everything you want (right-sized, warm feet, quiet, zoned), plus a very low purchase price and essentially zero maintenance. Higher operating cost but greatly blunted by your excellent insulation. If you actually need any cooling in your climate with a shell that good, a few window AC units would probably be more than sufficient and it's doubtful you'd need to run them very much. You'd get zoning for free too, and replacing them would be cheap and simple.

  55. loracekim | | #55

    Kent if you are still out there can you give us an update on how it all turned out.
    I live in SE MN also and in the process of a caterpillar (slow) remodel of an old farmhouse. I currently have forced air (propane) but added a Mitsubishi Minisplit 2 years ago and so far I'm happy with how it is preforming. Cant wait till I finish all the insulation and air sealing modifications to see how well it does with out running the force air.
    One thing I never hear mentioned here is when it is cold out below 10 F the minisplit does cycle on and off and when it does it creates a load hum almost sounds like a plane flying outside. particularly noticeable at night. I mounted mine to the wall of the house to get it above snow level and plan to move it to a independent footing to see if that helps. But overall I'm happy with its subzero performance so far.
    Thanks for everyone's input here and Kent if you can let us know how it all worked out.

  56. loracekim | | #56

    Opps! Sorry Kent didnt see yo posted in November until I posted. Guess I didnt see the second page of comments. Thanks and keep us updated in the future. Good luck!

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