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Community and Q&A

Heat pump recommendations (central system)

Sean Herbert | Posted in General Questions on

Hello Gents,
I have been lurking here, pretty much since I bought my first new home, (11 years ago) and realized how little I knew about this huge purchase.
It’s a 1650 sq. ft. two-story house, in Montreal, Quebec, (zone 5a) built in 2007. It has central heating, electric furnace with the original 2.5 ton heat pump, which has just let go.

So I’m in the market for a new heatpump, and possibly a more efficient furnace to go with it. Of the 5 quotes I have had to date, nobody has suggested a manual J, and salesmen look at me like I’m an alien, if I show them the heat load calculation data that I have. Knowing that most systems are oversized, would it make sense to go with a variable speed heatpump and furnace?
I have seen talk of how it is hard to goof up on over-sizing the newer mini splits because of their ability to ramp down, and have seen a few central heat pump systems that can do the same.(obviously with less efficiency) On company has even suggested going up to a 3ton, because it can ramp down 50%. Does this make sense?

The home has been tested through the local energy audit programs, and has estimated that the home’s design heat loss would be 29900 Btu/hour, cooling load would be 18200 Btu/hour. Air tightness is 3.39 ACH/50
Any input would be appreciated.
Thanks
Sean

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Sean,
    Are you talking about installing two heating appliances -- a heat pump and a furnace?

    If so, will the furnace burn natural gas? Or are you thinking about an electric resistance furnace?

    If I were you, I would choose just one type of heating appliance. Instead of a central furnace or a ducted heat pump, it may make sense to install two or three ductless minisplits.

  2. Sean Herbert | | #2

    Sorry for not being clear.
    Heat pump and an electric furnace. I am mainly interested in the new furnace/air handler as a way to get an ECM motor and move the air around a little more efficiently. (And it gets me an extra energy rebate from the government)
    Ductless is a no go, the wife doesn’t like the look, and the house already has it’s existing ductwork.

  3. User avatar
    Dana Dorsett | | #3

    Averaging over 4000 heating degree days per year (base 18C) Montreal is definitely zone 6A, not zone 5A.

    https://montreal.weatherstats.ca/metrics/hdd.html

    https://www.greenbuildingadvisor.com/blogs/dept/building-science/all-about-climate-zones

    A 3-ton Mitsubishi MXZ-4C36NAHZ and a pair of MVZ series air handlers (one for each floor) sized properly for the zone loads would cover you without using much heat strip. The MXZ-3C30NAHZ + MVZ air handlers would too, but would need to engage resistance heating under design conditions, but may be the better option:

    http://meus1.mylinkdrive.com/files/MXZ-3C30NAHZ_Submittal.pdf

    http://meus1.mylinkdrive.com/files/MXZ-4C36NAHZ_Submittal.pdf

    MVZ air handlers do not modulate, but by sizing them correctly for the zones they would have a high duty cycle, and most of the season the calls for heat would overlap somewhat, allowing the compressor to step down to the higher efficiency lower modulation levels during the shoulder seasons with out a lot of excess cycling on/off.

    Whether it's easy to break the duct system into separate zones with separate air handlers would make or break this potential solution.

    If the ducts remain as-is and it's all operated as a single zone, the Mitsubishi PVA-A30AA/PUZ-HA30NHA4 combination or PVA-A36AA4/PUZ-HA36NHA4 would give you some modulation, but would need the supplemental heat strip options to cover the shortfall at colder temperatures:

    http://meus1.mylinkdrive.com/files/PVA-A30AA4_PUZ-HA30NHA4_Product_Data_Sheet.pdf

    http://meus1.mylinkdrive.com/files/PVA-A36AA4_PUZ-HA36NHA4_Product_Data_Sheet.pdf

    http://meus1.mylinkdrive.com/files/OptionalElectricHeatKit_ProductDataSheet.pdf

    The MXZ hyper heating units are probably higher efficiency than the fully modulating PUZ systems, but may be more expensive up front since there are two air handlers, not one. The PVA modulating air handlers would run nearly continuously a the low end of their range during winter, which may be more comfortable.

    None of these solutions are cheap though.

    If the ducts are easily separated for zoning by floor, it is conceivable that a pair of fully modulating 1.5 ton Fujitsu 18RLFCD mini-ducted mini-splits could work, which would be cheaper than the large air handler Mitsubishi solutions:

    http://portal.fujitsugeneral.com/files/catalog/files/18RLFCD1.pdf

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Sean,
    You don't need an electric furnace, as long as you buy a heat pump that meets your design heat load at expected exterior temperatures. Lennox and Carrier now make heat pumps with published specifications down to -15°F (-26°C).

  5. Walter Ahlgrim | | #5

    Martin are the cold weather heat pumps Lennox and Carrier offer conventional products they make or are they the rebranded mini splits they offer as bidding options.

    Walta

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Walter,
    No, they aren't minisplits. I'm talking about conventional heat pumps designed to be hooked up to an American-style forced-air duct system -- units like the Lennox XP25, Carrier GreenSpeed, and Trane XV20i.

    I suppose these could be re-branded Chinese units -- these days, it's always hard to tell where anything is manufactured.

  7. Walter Ahlgrim | | #7

    Sean
    It is a common HVAC sales practice to offer to install the next size larger equipment as an up sell option. Do not fall for this as you duct work will be under sized and make the equipment short cycle and leave you uncomfortable with higher electric bills.

    How well did your old unit work with your home? Was it able to cool the house to the set point on the hottest days? Did it cycle off often on the hottest days?

    If you have decided you like your conventional heat pump with electric back up and understand the heat pump does not do much work when it is below 5° F and still want something similar look at the units with a SEER rating of 17 or above the key features you want are
    1 variable speed compressor
    2 electronic blower motor in the indoor unit
    3 electronic expansion valves

    Walta

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Walter,
    It's no longer true that heat pumps don't work when outdoor temperatures drop to 5 degrees F.

    If you choose to install that kind of equipment, I suppose you could. But you don't have to.

    In Montreal, Quebec, there's no reason that Sean shouldn't choose equipment that has performance specs at outdoor temperatures of -15 degrees F or -17 degrees F.

  9. Sean Herbert | | #9

    Walter,
    I’ve never had issues with the old 2.5 ton heatpump. I think it was generally oversized for the cooling load. This was my main reason for looking at a variable speed system, to adapt to the different load conditions. So I’m focusing more on the HSPF than the SEER rating.

    Martin,
    I guess I don’t need an need an electric furnace, but I’d like a more efficient way to move the air around than my older PSC blower motor. Government rebates will pay about $1200 of the $2800 cost.(furnace +air handler) But you bring up a good point, that maybe I should invest a little more in the heat pump to stay off the heat strips as long as possible.

    Dana
    Thank you for all the links, I did not even realize that Mitsubishi made more conventionally ducted systems. (As soon as I saw the horizontal heatpump fans I figured they were mini-split units.) I also never knew that 2 air handlers was an option. I love the idea of have zones, and splitting the home, but could this also be accomplished with so sort of directional damper in the ductwork? After separating the duct work into first and second floors?

    Is slightly over sizing with a variable speed 3ton helping or hurting me from and energy perspective?
    Thanks
    Sean

  10. User avatar
    Dana Dorsett | | #10

    Zoning with dampers and a single air handler can be done with any of these systems, but that approach reduces system efficiency and comfort.

    The MXZ multi-split compressors compatible with the MVZ air handlers need at least two active heads/cassettes/air handlers to be able to function, but the second zone could be a half-ton mini-split or a mini-duct cassette etc- it doesn't have to be anything as expensive as another MVZ air handler.

    The single three ton modulating PVA air handler solution isn't really oversized for your calculated load by very much (if at all) and it's minimum modulation level at +47F/+8C is the same 18,000 BTU/hr as the 2.5 ton version, so they would operate at about the same efficiency, the only difference is the number of hours the 2.5 tonner would have to engage the auxiliary electric heat. But the number of hours per year that the 2.5 tonner wouldn't keep up without the heat strips is pretty limited. I'm not sure how low the extended temperature capacity tables go for the PUZ/PVA modulating systems, but I'd be surprised if it didn't have a specified output at -20C/-4F. The fact that the 2.5 ton PVA-A30AA4 & PUZ-HA30NHA4 can deliver as much as 32,000 BTU/hr @ +5F/-15C makes me believe it will cover all or almost all of the ~30,000 BTU/hr load at your 99% outside design temp of -22C/-7F without the auxiliary electric heat option, but when it's colder than -22C it won't keep up on heat-pump alone. The 38,000 BTU/hr @ -15C of the 3 tonner makes me believe it would cover the 30K load @ -22C with a bit of margin ( but not huge margin) without using auxiliary heat. But the efficiency is low enough at -22C (a COP of about 1.5) that it doesn't make much difference in the electric bill whether shortfall of less than 5000 BTU/hr is being covered with resistance heaters (at a COP of 1.0) for those few hours. Getting a quote for both a 2.5 tonner + auxiliary heat option vs. a 3 tonner without auxiliary heat would make sense. Paying an extra $2000 for the bigger unit would probably never "pay off", but an extra $500 might.

    The MXZ-xxxxxNAHZ multi-split compressors have a specified output capacity down to -25C/-13F, so you would be able to choose the compressor that can keep up at -22C with whatever air handlers/cassettes are properly sized for the zone loads, but the MVZ air handlers won't modulate.

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