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New HVAC in MA. Heat pump make sense?

ScottinMA | Posted in Mechanicals on

Hello green building advisors.

I thought I’d hit up this wise group for insight into my best options for replacing my home’s HVAC system.

Our home is in the Boston metro suburbs and is a roughly 1500 sq ft ranch built in the 40’s, full unfinished attic, and full basement (mechanicals, laundry, storage, one finished but mainly unused room).  We did have a home energy audit when we first bought the place a couple years ago, and we had the recommended air sealing  and weather stripping done and added a bit more blown in insulation in the attic.  They also recommended some blown in insulation in the walls, but this is whole different level of project so we didn’t do that.  I know there is some insulation in there, but not a ton based on the little experience I’ve had pulling some wire through the wall.  It is currently being heating with an oil burning forced hot air unit with A/C.  In the winter the house is reasonably comfortable, but I would say that both the family room and master bedroom are often on the chillier side compared to the rest of the house.  The house is shaped like a U, and these are the two rooms at the far ends and both have 3 exterior walls.  So I imagine the duct work and system as a whole isn’t balanced very well for this.

I followed the post on calculating an existing home’s heat load based on your existing fuel usage and came up with numbers between 31,000 BTU (Dec 2018) and 35,000 BTU (Jan 2019).  Not being sure if I was even in the correct ballpark, I did use loadcalc.net and came up with just over 32,000 BTU heating load.  So at the recommended 1.4 X sizing of that 35,000 BTU load, am I looking for a 49,000 BTU system?  Seems large for this size house, but I guess I do have a lot of exterior walls.

That 35,000 BTU month for example, was Dec 19-Jan 28 inclusive, 148.3 gal of heating oil, running at ~70% efficiency, 1005.2 HDD, 99% temp of 6 deg.

The oil, and perhaps everything else, is going to go, it’s just a matter of what to replace it with.

My initial plan was to have natural gas installed the Monday after we closed on the house a few years ago.  Boy was I surprised when they paved my street the afternoon of our closing – thus starting the 5 year moratorium on cutting in to the street to have NG installed.  But now that time has passed and a different NG supplier had an accident in 3 neighborhoods north of Boston,  I am wondering if NG is still the only play here.  About a year and a half ago I had a PV array installed, and we are currently generating 1500 or so kwh of extra juice over our current usage.  I now wonder if perhaps  there’s a heat pump alternative that would prevent me from putting in another fossil fuel burning system into the world.  I wonder, can this be done in a way to increase the overall comfort of the house and at the same time be more cost effective than oil, and at least within reason to NG (which I imagine can’t be touched from a pure $$ point of view).

One thought I had was perhaps three separate ducted mini splits (one unit outside) installed in the basement and ducted upstairs, one under the family room, another under the master bedroom, and then a third in the middle of the house.  That would allow me to control those two “chilly” rooms separately, and there already is duct work from the basement into these places.  But I’m new to the heat pump world – does this even make any sense?  Wall/floor mounted mini splits directly in the rooms are a no go for aesthetic/space reasons.

Thanks in advance.
-Scott

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Replies

  1. kjmass1 | | #1

    Did MassSave say there was insulation in the walls or not? They told me they won't touch a wall if there is anything in there because they don't know what it could be and don't want it blowing around.

    I would highly recommend getting your walls in shape prior to the mechanical upgrade. I just did blown-in and it wasn't too much of a hassle and the patches are easy assuming you have the original paint to match.

    Walls are a significant portion of your heatloss so tighten it up best you can.

  2. Expert Member
    Dana Dorsett | | #2

    A 32K load for a 1500' house is on the high side. Make some exploratory holes in some out of the way easy to patch area (a closet on an exterior wall is a good place) to see what's in the walls.

    What did you assume was in the walls in the LoadCalc run?

    If the foundation walls aren't insulated, it's worth doing even as a DIY using cheap reclaimed foam from Insulation Depot in Framingham or Green Insulation Group in Worcester, or one of the smaller reclaimers. (MassSave won't cover that, but they'll usually cover 2-3" of closed cell foam on the band joists and foundation sills, to be installed after your wall foam so that it seals to your wall insulation.)

    With improved air tightness, upgraded wall insulation and some foundation insulation you can probably bring that load down to the 25,000 BTU/hr range or lower. Just getting rid of the parasitic 25/365 load of the oil-burner's flue would be worth a few thousand BTU/hr at design temperature.

    DO NOT OVERSIZE A HEAT PUMP!!

    It's a recipe for lower comfort and lower efficiency. The ASHRAE 1.4x for low-mass fossil burners does NOT apply.

    With a 1-2 stage (or even modulating) ducted heat pump it's better to size the heat pump approximately for the 99% design load, not 1.4x load, or it's duty cycle will be too low for efficiency. The additional "---.4" to cover colder temperatures can be added as auxiliary resistance heat strip. So at the current load it looks like a 2.5 ton or 3 ton would be the right size, maybe a 3.5 tonner depending on heat pump model and your local 99% outside design temp (got a ZIP code?), assuming you're not planning further upgrades to the building.

    Chilly rooms are most likely due to the ridiculous oversizing factor of the fossil burner, which is probably more than 3x oversized, and certainly at least 2x oversized (the smallest oil burners out there would be ~2x oversized for a 32K load.) When duty cycles are low, room to room temperature imbalances get exaggerated. A right sized modulating system would run nearly constantly when it's actually cold out. See Nate Adams' take on that:

    http://www.natethehousewhisperer.com/hvac-102.html (watch the videos- they're short particularly this one: https://youtu.be/sehccCy2tyY )

    As a drop-in modulating replacement you can probably make it work with Fujitsu's mid-static 3 tonor 3.5 ton "mid static" mini-duct cassettes with the house as-is:

    https://ashp.neep.org/#!/product/25352

    https://ashp.neep.org/#!/product/25353

    You'll notice that the modulation range isn't huge on these (unlike their RLF series mini-duct cassettes), though the 5:1 turn down on the 3 tonner isn't terrible. Trane and Carrier have comparable performance modulating heat pumps as well. Nate Adams seems to prefer right-sized Carrier Infinity GreenSpeed heat pumps on his comfort-retrofits and it's not terrible choice, but it's not the only choice. Having local support for the product matters.

    Taking the time to air seal and recommission the duct systems is also going to be worthwhile. An unbalanced and leaky duct system can be worth 25% of the fuel use, and contributes to comfort problems as well. An Energy Star duct system would be balanced to where room to room pressure differences would be under 0.012" water column (3 pascals) under all operating conditions, doors open, doors closed, but at least getting it under 0.025" is usually possible as a retrofit. It's likely that at least some doored-off rooms with supply ducts and no dedicated returns will need some sort of jump duct to reduce room pressure differences and improve flow. Rooms that are too warm can be flow-adjusted with balancing vanes to reduce the flow.

    Getting rid of the cheap 1" filter and installing a large pleated filter ( Aprilaire 2400 or similar) cleans up the air far better without clobbering the system with high static pressures, well worth doing when installing a heat pump. At the lower duct velocity you're likely to have with a right-sized air handler the static pressure shouldn't be too bad at all.

  3. ScottinMA | | #3

    Kjmass1. Thanks for the reminder that keeping the heat in is the first place to start. I do remember there being some insulation in the walls of at least some of the rooms. I should probably take another look to see what state they are really in.

    Dana, thanks for all of those details. I’ll take some time to look into them. I did assume an R11 wall when I ran the load calc. Zip code: 02090. I like the idea of insulating the basement walls. I just wish there weren’t so many things in the way - wiring, plumbing, old built in work benches, electrical panels, ironically the current oil tank, blah, blah ...... I’ll need to give that some more thought. If I were to hazard a guess, my ducts should probably be completely rethought and redone. It clearly looks like a Frankenstein system added to over the decades.

    My DHW is also oil fired, and my plan would be to change that out for a heat pump unit. I guess that would be another vent open 24/7 that I could close off.

    Thanks again for the great info!! I’ll probably be back with more questions :-)

  4. Jon R | | #4

    More than you probably want to know about the benefits of moderate over-sizing of at least some variable capacity (aka modulating) heat pumps:

    https://aceee.org/files/proceedings/2014/data/papers/1-371.pdf

    1. Expert Member
      Dana Dorsett | | #5

      From that report on p. 1-52 & 1-53 (p.11 & 12 in PDF pagination):
      --------------------------------
      "Heating Season Energy Savings
      Analysis of data from two heating seasons found that right-sizing the FC system causes
      3.4% energy savings when evaluated at -22F delta-T (50F – 72F = -22F). Analysis found that
      oversizing the VC system causes 3.0% energy use reduction when evaluated at -22F delta-T.
      While these values are notable, they were not considered statistically significant given the error
      indicated in Table 5. However, since the 2-ton VC system is rated by the manufacturer to be
      4.2% more efficient than the 3-ton FC unit (HSPF of 10.0 and 9.6, respectively), the normalized
      energy savings from oversizing increases to 7.2% (calculation: 3.0% + 4.2% = 7.2%).

      Conclusion: rightsizing the FC heat pump and oversizing the VC heat pump both yield heating
      season energy savings, but more heating data is needed to gain more confidence.

      Peak Demand Heating Savings

      Right-sizing the FC heat pump reduced peak heating demand by 10.6% while oversizing
      the VC heat pump reduced heating peak demand by 10.1%. Conclusion: right-sizing the FC heat pump and oversizing the VC heat pump yielded significant heating peak demand reduction. "

      ---------------------------------- end quote

      It's known that given sufficient modulation range and good control algorithms oversizing by as much a 50% can be beneficial for mini-splits. The turn down ratio of the heat pumps in this study was only 3:1 (the Carrier Infinitys are only 2.5:1) with inclusive evidence on oversizing boosting average efficiency, but hinting that it might despite being less efficient at peak load compared to a right-sized system.

      Most single speed or 2-stage heat pumps don't fit that profile.

      In 99% outside design temp for ZIP code 02090 is about +7 - +8F, slightly warmer than Framingham's +6F. I assume the numbers from the LoadCalc runs were using Framingham's design temperature?

      Westwood is only about a half-hour drive to the Insulation Depot/Nationwide Foam on Waverly Street in Framingham, which is a good source for used foam board for a DIY basement insulation project. They sell in small quantities if you're picking up, but they're not open on weekends. At some minimum quantity they will drop-ship to your site at a reasonable upcharge. When you're ready to take on that project it's worth seeing what's in stock and getting quotes:

      http://www.insulationdepot.com/index.cfm

      http://www.nationwidefoam.com/index.cfm

      1. Josh Durston | | #12

        IMHO, most residential multi splits should be treated as FC capacity and 1:1 as VC. There are many stories here of people being disappointed by the modulation of the multi head setups.

  5. ScottinMA | | #6

    Forgive me if this is a colossally dumb follow up question, but how does one control these newer more sophisticated systems?

    To my knowledge, even the smart thermostats with fancy UI, voice control, self-learning, etc. in the end just send an on or off to the heat pump. A two stage one would, at some trigger point, send an on to the second stage. Do I have that right? Is that enough to control these variable inverter heat pumps, with multi speed air handlers (for ex ample a Mitsubishi PVA/PUZ pairing)? They don’t need to know how far off the set point the internal temp is, or whether to run max out or scaled back, etc.?

    Does one need to use the controllers from the HVAC manufacturers to make these systems work well? The industrial design is horrible on those units.

    Thanks in advance.

  6. Jeff Wasilko | | #7

    We're in the Boston suburbs and recently replaced our AC with a Mitsubishi heat pump and 2 SVZ air handlers. We kept our boiler and installed new hydronic coils for backup heat. We're on a town electric utility with National Grid for gas. Our economic balance point to switch from heat pump to gas is 15F.

    You're right that most of the thermostats are just switches.

    We're using the PAC-US444-CN-1 thermostat interfaces along with Honeywell Prestige IAQ thermostats. One of the drivers for that choice is that we have a steam humidifier on our 1st floor system, and the Honeywell thermostat does a great job of managing humidifer set points as the outdoor temps drop (to avoid over-humidifiying). The Honeywell thermostats use a wireless outdoor temp sensor for the humidity adjustments, and also uses it for the economic balance point switchover (aka compressor lockout).

    http://meus1.mylinkdrive.com/item/PAC-US444CN-1.html has tech notes on how this interface is connected to a thermostat, and how it operates. The flow chart in App note 3044 is really good at showing how it modulates the heat pump. In my experience, it starts low and ramps up, and then shuts off when the thermostat meets the set point. It never ramps down.

    The Mitsubishi system is seen by the thermostat as a 3H/2C heat pump system. The thermostat interfaces offers a modulating 1st stage, and a full capacity 2nd stage. We also have the 3rd heat stage (emergency/aux) provided by the boiler and hydronic fan coil.

    1. ScottinMA | | #8

      Jeff: Thanks so much for sharing your setup, and providing the link to the awesome info. It does a great job explaining exactly how all of this works together

    2. MAheatpumpguy81 | | #13

      Your heat pump behavior is similar to mine and I believe this behavior is resulting in some loss in efficiency. I have two outdoor units, each with 3 indoor heads connected. Following being off for a period, they will ramp up and sometimes hold at around minimum capacity (see SouthHeatPump in the attached fig) for approx 5 min and then ramp up to higher power only to then shut off after a time, sometimes as short as 15min and sometimes much longer. Then the cycle repeats. They typically shut off after overshooting the set point by 1F.

      If they would instead ramp down upon hitting or overshooting the setpoint, I feel a much more efficient equilibrium of holding temp and electricity consumption could be achieved. I'm currently working with my installer to troubleshoot... but maybe this is how they're designed to operate.

      P.S. Regarding my figure, I flipped the breakers off at around 12:16pm then flipped them back on again at 12:30pm to see what would happen.

  7. Jay Thomas | | #9

    For MA, cold climate heat pumps can easily provide you the heat you need without a backup. (You don't need the complexity of an automatic backup system.)

    Modulation can be an issue - so you are probably better with several outdoor units than just one. (On mild days you can turn them off, which gives you much lower minimum modulation.)

    Your setup sounds a lot like BFW577. You probably could get away with the standard non-ducted common areas and mini ducted bedroom setup that you see recommended.

    You also should definitely think about adding a small heat pump for the the basement. If you insulate it - the loads will be quite low - but you'll get warm floors - and - maybe more importantly - you'll get better heat distribution.

  8. MAheatpumpguy81 | | #10

    Recently MassSave was covering in-wall blown-in cellulose to 75% of cost. You should definitely insulate first, then tackle the HVAC system.

    1. ScottinMA | | #11

      Definitely a good idea for anyone in MA to pursue first. We actually did that already. They came out again about a month ago and poked around int he walls a bit, and found some batt insulation. Is it great?, probably not. But being there pretty much means that's what I get - they won't/can't blow anything into the cavities with that there, and I'm not in the position to open up all of the walls.

      But thanks for the good idea.

  9. Expert Member
    Dana Dorsett | | #14

    Anybody looking for heat pump solutions in MA should also check out the huge amount of contractor & pricing data available on the MA CEC site:

    https://www.masscec.com/cost-residential-air-source-heat-pumps

    It's possible to filter by manufacturer & town, and they show the number of installations from individual contractors that went through the CEC program.

    In general it's better for YOU or a competent third party to spec the equipment, then put it out for bid among the contractors working in your area. Simply opening it up for proposals almost universally ends up with more crazy-oversized and inappropriate equipment being proposed than proposals that make sense, wasting everybody's time.

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