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HVAC options for a two-level house with basement in Climate Zone 4A

oldbungalow | Posted in Green Building Techniques on

Hi, I’ve tried to read through the Qs/As on this, I’d like to do a two-unit approach to a two level approx 2500 sqft home (as opposed to zoned single unit), heat & cool, with lows that sometimes hit the 10-15F range but not for extended periods (unless polar vortices become the norm). Prefer gas heat to electric. Assume not-passive standards, something like R50+ attic, 25 walls, 10 foundation walls, 10 slab.

new: just calculated about 4600 HDD and CDD = 1600 based on 2017-2019 data.

Common practice in our area seems to be a second furnace in the attic, not sure why, maybe  tight floorplans/space. To try and keep things inside the envelope, is it common/practical to put everything in the basement and run up to the second level?

Would this require an 18-20ft vertical run to get to the second level ceiling, and is that efficient? Or does everything go to second level floor instead?

Other combinations I’m curious about- Since heat rises, has anyone combined a first level gas heat and first level regular A/C with a second level air-source heat pump or mini-split setup ? My thought is this provides two unit A/C for the upper & lower levels, and with a gas furnace servicing the downstairs, the electric heating load upstairs is not as great. Or is that ridiculously complicated.

Ultimately, how much floor/ceiling space does one have to carve-out for getting things “inside the envelope” to heat/cool a second level?

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  1. Expert Member
    BILL WICHERS | | #1

    You want to avoid putting any ductwork in the attic, and you absolutely don’t want to put any mechanical equipment in the attic!

    The normal practice is to put your mechanical equipment (furnace, etc) in the basement and then run ductwork up the walls and across the floor of the second story to get where you need to go. If you want a return near the ceiling, locate it high on the wall but don’t go into the attic. It sometimes helps to have a large vertical chase somewhere central but where you can hide it (like the back of a closet), and then branch off from there.

    Even though in some regions it’s common to put ductwork in the attic, that’s not the only way to do it. There are many downsides to having any ductwork in the attic and it’s really best to avoid doing that even if adds a bit of complexity elsewhere to hide the ducts.


  2. Expert Member
    Dana Dorsett | | #2

    >"..a two-unit approach to a two level approx 2500 sqft home (as opposed to zoned single unit)


    >" Prefer gas heat to electric. Assume not-passive standards, something like R50+ attic, 25 walls, 10 foundation walls, 10 slab."

    Even the very smallest gas furnace out there are extreme overkill for ~1250' of conditioned space at that performance level. The 15K Detson mounted in a closet using their tiny ducts would be the closest thing to "right sized" in a gas furnace:

    See how they run the 2.5"round ducts through 2x4 partition walls (no soffits) in this video:

    The alternative would be to micro-zone it room by room with small thin-profile hyronic fan coils running of a condensing water heater, a concept HTP is promoting:

    Before committing to any solution run a room by room Manual-J load calculation- you may be surprised just how low your actual loads are.

  3. user-723121 | | #3

    I added a new 2nd story to a rambler in 1989 in Minneapolis. We used clear span 2x4 web trusses for the 2nd floor with an opening designed in for HVAC trunk lines. I used part of a first floor existing closet for a chaseway to the 2nd floor trusses to connect to the trunk lines and run the supplies.

    As I remember it we upgraded the existing main floor walls to R-26, new 2nd floor walls were R-26 as well. The ceiling had energy heel trusses with R-70 blown insulation. Windows were low E gas filled and the improved house was very airtight with an 8 mil Teno vapor barrier. At the time we used 8007 hdd for Minneapolis and the 3,600 sf total on 3 levels was heated nicely with a small high efficiency natural gas furnace.

  4. oldbungalow | | #4

    So I thought I'd do a back-of-envelope calc using Not sure if all the assumptions are right-on (hence back-of-envelope), basically seems like one would treat the whole house as one big box with 2 stories=18 ft walls.So a two story 30 foot wall would be 30x18 sqft. Is that the right way to use the tool? My first crack at it came out to something like 45000 btu heating and about the same cooling (assuming attic ducts) for 3,700 sqft (including basement) of conditioned space..

    1. Expert Member
      Dana Dorsett | | #5

      How much of the loadcalc load was ventilation & infiltration? Like many tools it tends to overestimate unless you use very aggressive assumptions on duct and building tightness, and even then will sometimes overshoot by 35-40%.

      45K @ +15F for that much space would be excessive even at current code minimum, and isn't very likely for the better than code house described, unless you have a crazy amount of window area, or a walk-out basement with sliders. For reference I have a 2x4 framed sub-code 1920s vintage ~2400' antique 1.5 story bungalow + ~1600' of insulated basement that comes in under that number even at 0F outdoors, but the loadcalc tool would probably put mine in the 55K+ range. I'm radiation limited to less than 45K at the water temps I'm running, and it stays reasonably comfortable in all zones but one (a radiant floor heated room with a lot of window area) even when it's ten below outside.

      Similarly a 3.5 -4 ton cooling load would be very unusual for code-min 2500' house unless all the windows faced west with no shading factors. Most would come in at around 2 tons.

      Since you're talking multiple units run the numbers for just the top floor using loadcalc, and pretend it doesn't leak any air and is unventilated. Since the top floor is the only zone that would potentially have ducts & air handler in the attic, that's the zone you really care about. The 15K Detson can fit in a cupboard and the mini- ducts that system uses can be run in partition walls or between/through floor joists.

      Are you planning to air condition the place with something other than window-shakers? Even though you specified gas, a 1.5 ton ducted Fujitsu heat pump could also fit in a tiny closet (less than 10 square feet of floor area if mounted vertically) though it would use bigger ducts than a Detson, and has over 20,000 BTU/hr of capacity at +15F. In my area (with higher than US averages for both electricity and gas) the operating cost of a mini-ducted Fujitsu is somewhat higher than condensing gas, but still much cheaper than propane or oil.

  5. oldbungalow | | #6

    Thanks! Got it down some by switching to ducts inside. Will definitely try with one level. Yes biggest load appears to be glass. I just assumed 3x5 windows (4) on each of the upper level and 3x6 windows (4) on each of the the lower level walls. Lots of sliders and french door space on one SW facing wall. I actually forgot to add windows/doors for the basement.

    central(?) A/C would be a must. if ducted, ideally would use the same system of ducts if possible, but I don't quite understand the Detson 2.5" system is it high velocity or just efficient and does an A/C plus heat set require 2 air handlers, 2 duct systems or can the equipment be piggy-backed. I assumed (perhaps wrongly) that the status quo approach with the attic furnace would be a furnace and A/C set (guessing A/C evaporator would be in the furnace/air handler package) for the upstairs, shared ductwork, shared air handler and an outside condenser.

    Is the Detson packageable that way and would the Fujitsu (or any other brand for that matter) stack with it? Is Detson's air handler equally tiny/compact?

    1. Expert Member
      Dana Dorsett | | #7

      I believe the Alize heat pump uses the same blower & ducts as the Dettson Chinook furnace, but have never dug into it that far. I'm not sure how much additional volume the cooling coil takes up, or what it's cooling and heating efficiencies are. (Couldn't find an AHRI submittal sheet on it.)

      The xxRLFCD Fujitsu units use the same type of compressor as their cold climate heat pumps, which gives it very decent capacity and efficiency at +15F, more capacity and higher efficiency (in both heating and cooling) than a simpler (cheaper) mini-split compressor that one might use in a "dual fuel" gas + heat pump system.

      It looks like my advice to turn the infiltration & ventilation to zero to get more realistic numbers out of that tool wasn't implemented. You have about 12.5K of heating load (out of 41.5K total) of fresh air + infiltration , about 3K of cooling load (out of 35K). That 12.5K number is fully 30% of the calculated load, and isn't realistic, especially if using balanced ventilation with an HRV or ERV (recommended, for new, tight houses.) The 41.5K heating number spit out by loadcalc is probably less than 30K, maybe even as low as 25K in reality, which would be a realistic range for the house described.

      With infiltration and ventilation subtracted out you're probably looking at 2.5 tons of cooling rather than 3, most of which is likely to be the top floor. A 1.5 tonner up top and a separate 1 ton for the first floor feels about right- the basement load is primarily latent, and with the door open to the first floor would be well managed by convection.

      1. oldbungalow | | #8

        Thanks for the insights! Sorry didn't do a screen grab w/o fresh air! was definitely lower load by a lot.
        Will have to futz more, as the 1st lvl sliders and glass will face a screened in porch so there's decent overhang there. I suppose if we sprung for lifter slides (sp?) they'd perform better than the model assumptions.

        I found the Alize on their site,
        looks like they have a 25 SEER 1 ton and 21 SEER 1.5 ton as well as 21 SEER 2 ton (overkill for 1300 sqft?). I thought I read it was Chinese made, not sure.

        So if the Chinook blower is the air handler (via same ducts) as in p.5 illustration, would that work with any model heatpump evaporator unit, i.e. the Fujitsu? Seems like some good options to consider.

        Wonder if both pairs of units could be put in the basement and run up 2 separate 6" mains, including up to the 2nd level.

        1. Expert Member
          Dana Dorsett | | #9

          The SEER isn't very closely correlated with HSPF heating efficiency. The 8.2 to 10 HSPF ratings of the Alize are on the low side. (That's 8200 - 10,000 BTU/kwh, under a presumed seasonal average load spelled out in the AHRI testing.) The xxRLFCDs are in the 11,000 BTU/kwh and up range, varying a bit by size. The most likely candidate, the 1.5 tonner is 11,500 BTU/kwh, compared to 9,800 BTU/kwh for the 1.5 ton Alize or 10,000 BTU/kwh for the 2 tonner.

          The minimum modulated output of the 2-ton Alize is pretty high (6800 BTU/hr cooling, 7500 BTU/hr heating), whereas the 1.5 tonner is almost as low as the Fujitsu. If you go the Dettson furnace + Alize route the 1.5 tonner is going to be more comfortable and possibly more efficient due to the modulation range and better match to your load.

          The control functions of modulating heat pumps usually don't allow mixing random compressors to random air handlers of comparable size, even within a manufacturer's different series. There is a fair amount of embedded firmware/software in the control algorithms- they ain't your gramma's Carrier.

  6. oldbungalow | | #10

    This has all been really helpful, thanks! Sounds like it's best to stick with a complete set from one company, i.e. Alize-Chinook combo. I like the heat pump idea as it opens up the possibility of electricity biased heat mix down the road if we ever go solar. Assume we could just "reprogram" the thermostat/controls somehow, switching from Alize A/C and Chinook heat to Alize A/C & Heat with Chinook backup below certain outdoor temp.

    I think I need to do a loadcalc workup for each floor individually, beginning with the basement (walkout, insulated slab, R10 continuous) and each of the upper levels. Not sure how the "floor" and ceiling assumptions work for the upper levels.
    Upper level 1: is a basement below equivalent to an "insulated crawl" of approx. the same R value?. Is the ceiling above, with conditioned floor above, equivalent to an attic?
    Upper level 2: is a floor below equivalent to an "insulated crawl" of approx the same R value as the floor below (say R25 walls?).

    Also rethinking the whole zone thing. We currently have 2 zone A/C, single zone hydronic heat in our old leaky home. For a tighter good but not passive new home, approx. 1300+ sqft per level including finished basement, does it make the most sense to do 2 zone A/c and 2 zone heat?

    Another thought, also raised earlier, is efficient Heat Pump only for A/C Heat on 2nd level, 2nd heat pump plus gas furnace for lower level only, relying on gas heat to rise (from basement & 1st level) if temps are too low to efficiently heat 2nd level. could be risky.

  7. Expert Member
    Dana Dorsett | | #11

    A basement and upper floor on the same zone (either heating or cooling) almost never works. Even in a tight home with multiple levels, zoning by floor is almost always necessary.

    A heat pump-only solution for the upper levels can work well if the compressor technology used is suitable for the local climate, the load calculations were done properly, and the heat pump is sized properly. There are high-R houses in locations that hit -20F being comfortably heated with cold-climate heat pumps. The Fujitsu xxRLFCD series uses that type of compressor, and even though it's not being sold as a cold climate solution, people are successfully using it in US climate zones 6 & 7. I haven't seen enough information on the Alize to know how much capacity it has at sub-zero F temps, and it wouldn't be my first choice as a heat-pump-only solution, even though that might work fine in zone 4A.

    What is your 99% outside design temperature? (Or share your location and we can look it up.)

    1. oldbungalow | | #12

      The two closest cities I could find in a table for "ASHRAE 99.6% HDB Temperature °F" list 15°F and 12°F. Honestly don't know what that means. Not sure if the same but the loadcalc tool was showing something like 15 and 90 for design heat/cool with indoor temps of 70 and 75 respectively. I no longer have the page. Sub Zero is not a thing unless counting windchill. I didn't know heat pumps were any good far below freezing.

      ALso finallly got around to trying 1 level model in loadcalc with no fresh air, ducts inside also did ducts in attic, not a huge diff. This is for the upper level, probably bigger than it'll actually be, with the floor assumption being a closed/vented crawl R21. In reality it'll be an insulated conditioned first level below the floor.

      1. Expert Member
        Dana Dorsett | | #13

        If you're in the US you'll find county-level design temps on this list:

        With a county the local temperatures may vary a few degrees either side of those numbers, but it'll be the right ball park.

        The 99% outside design temperature is the temperature of the 99th percentile temperature bin, which means than 99% of all hours in a 25 year period is that temperature or warmer. So about 87 hours out of typical year will be colder than that temperature. If one calculates the heat loss of the house at that temperature and specifies a heating system that can deliver that much heat with a bit of room to spare you're pretty much covered.

        The 99.6% outside design temp means only 0.4% of the hours (that's 35 hours) in a typical year are colder than that temp. The "HDB" in "ASHRAE 99.6% HDB Temperature °F" stand for "Heating Dry Bulb", which is the normal thermometer temperature. (There is also a "wet bulb temperature" that is a means of measuring the absolute humidity in the air, but that's more relevant for figuring out the latent cooling load.)

        ASHRAE recommends sizing the system to 1.4x the 99% load if one is using overnight setbacks, or want to be fully covered for the 25 year weather extremes, but even 1.2x oversizing is rarely a comfort problem.

        Since the capacity of air source heat pumps falls off with outdoor air temperature it's important to know the 99% design temperature and look the extended temperature capacity tables for the model to be sure it can deliver the heat. An AHRI "rated capacity" of a heat pump is only guaranteed down to +17F, though most mini-split manufacturers test and rate their equipment at modulation levels lower than the maximum capacity at +17F, so there is some built-in margin, but you have to look it up.

        There are mini-splits with specified capacity at outdoor temps as cold as -22F, but the two larger Japanese mini-split manufacturers Mitsubishi & Fujitsu only specify the capacity down to -13F and -15F respectively, and only on their cold climate specific models.

        The mini-ducted Fujitsu units are specified all the way down to -5F (though they keep going even at -25F), , so it would easy to specify one that could handle your load if the 99.6% design temp is above 0F. The heating capacity tables for that series start on page 16 (PDF pagination):

        The table column of interest is "TC" column of the 70F indoor temperature bracket and the °FDB column (= dry bulb temperature in Farenheit). The rows of interest would be 5 and 14 under the °FDB column. The units of the numbers in the TC column are 1000s of BTU/hr. eg:

        For the 9RLFC the 70F TC column and 5F row shows 15.4 , and the 15F ro shows 16.0. That means at +5F outdoors, 70F indoors it's good for 15,400 BTU/hr, and at +14F outdoors 70F indoors can deliver 16,000 BTU/hr.

        A code-min ~1300' upper floor might have a load as big as 15,400 BTU/hr @ +5F or a bit more, and if that's a realistic cold-snap temperature it might be worth going with the 12RLFCD, which according to the table for that model can deliver 16,800 BTU/hr @ +5F, 17,400BTU/hr @ +14F.

        It's unlikely that you would have to bump up to the 1.5 tonner, which is good for 19,700 BTU/hr @ +5F, 21,100 BTU/hr @ +14F, unless your cooling load numbers outstripped the capacity of the smaller units.

        Similarly, the cooling capacity tables start on p13. If your 1% design temp is 90F you'll be interested in the 95F row, and the TC numbers the 75F or 80F columns.

        Your loadcalc results of 17.8K cooling, 15.5K heating would indicate bumping up to the 1.5 tonner for the upstairs, even though the 1-ton could handle the heating load with some margin. The loads are usually somewhat padded using that tool, but not as much as most other freebie tools. The -18RLFCD covers the loadcalc calculated load, so it probably has even more margin than indicated, but dropping back to the 12RLFCD might not cut it- you'd need to have a pro with better tools run the numbers before making that call.

  8. Loya1ty | | #14

    Excellent information here. Thank you! And if advice from a professional isn't enough, take it from a 1 month new home owner that putting the ducts in the attic is a TERRIBLE idea. Aside from most likely poor insulation, the cost to properly insulate the ducting to replicate 'conditioned space' state is very high. The exception to the rule here would be if your attic was built to be conditioned from the start, because then you're essentially running your ducts in a 3rd floor (but even then your air is probably traveling quite a ways).

  9. d_barnes | | #15

    It sounds like you’re in the design stage of a new house? If so some things that will make a big difference regardless of the system, and will make it possible to use a small ducted Minisplit like the Fujitsu on each floor. First, Air seal your home. At the framing stage before insulation, after electrical and plumbing install, air seal All penetrations. Especially at the basement and the top floor attic plane/top plates. Heat doesn’t rise, Warm air rises. The stack effect causes warm air to rise, worse the more stories a building has. Air sealing chokes out the stack effect. Air sealing the attic/ceiling of my old 2 story house made a dramatic difference in warm air rising to the second floor and causing as big a temp difference between floors. Also air sealing a basement will cut the supply of that stack effect. This is all Very cheap at framing stage, harder after sheetrock. Use quality caulking, can foam on a foam gun, or a gallon of duct mastic with a cheap cut down paint brush. Stack affects heating more than cooling. Second, West facing windows affect the cooling load, so limiting West facing windows on the second floor might allow you to use a 1 Ton RLF12CD Fujitsu 1 per floor.
    Designing your duct system for high flow and low static pressure is a priority to get the full capacity and efficiency from your HVAC, but especially with ducted minis. Airflow is what delivers the heat and cool to the rooms.
    On my efficiency upgrade of my 2735 sq ft 1996 home, I used a 2 zoned 3 ton Rheem RP17 heat pump. It’s a 3 stage cooling, 3 stage plus overdrive heating, to put out full capacity at 17 degrees. 18 SEER, Good value for multiple stage unit, if you decide to go central HVAC.

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