GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Sizing an air source heat pump

Chuck Baynton | Posted in Energy Efficiency and Durability on

I have a design for an 1800 sq ft slab on grade house in the Minneapolis MN climate, targeting 1.0 ACH50, R80 above the ceiling, R40 walls (but also >100 sq ft of triple pane glass), R35 under the slab.  My builder’s chosen HVAC subcontractor, who promotes himself as a ground source heat pump specialist, has been asked to propose an air source heat pump.  He recommends a 3 ton model (Mitsubishi Hyper Heat).  That seemed significantly oversized to me.  Here is an excerpt from our email exchange:

 I have some specifications of the HVAC system you propose for this house.  Could you share with me how you arrived at a recommendation for  a 3 ton (versus smaller) air source heat pump? (He answered:)


With a modulating heat pump, you can over size the cooling load to cover more of the heating load before using AUX heat/electric baseboard. (see below) The unit will still run at a lower speed for cooling for dehumidification and then a higher speed for heating. I can decrease the size if you’d like?

3-ton balance point is 22 degrees. (Heat pump will 100% of the house to this temp without any help from the electric resistance baseboard) Annual KWH heating 13,683
2.5 -ton balance point is 27 degrees. (Heat pump will 100% of the house to this temp without any help from the electric resistance baseboard) Annual KWH heating 14,328
2-ton balance point is 32 degrees. (Heat pump will 100% of the house to this temp without any help from the electric resistance baseboard) Annual KWH heating 15,186 

I don’t understand his answer, except that it seems not to involve a calculation or an estimate of the peak heating load.  I’ll be taking this up with him, but can anybody make sense of what he is saying?

Might he have meant to say that the air source heat pump, in light of its declining effectiveness as ambient temp gets lower, if oversized to 3 tons can keep expensive, inefficient electric resistance backup out of the picture even when ambient temp is as cold as 22 Fahrenheit? 


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Expert Member
    Dana Dorsett | | #1

    A 3 ton MXZ series Hyper Heating Mitsubishi is good for 45,000 BTU/hr @ 22F. There is no way in hell even a code-minimum 1800' slab on grade is going to have a heat load that high at 22F unless you leave some windows open. The balance point numbers seem really gonzo even for a code min house.

    https://ashp.neep.org/#!/product/25915

    The 2.5 ton is good for 28, 600 BTU/hr @ 22F:

    https://ashp.neep.org/#!/product/26173

    Your heat load at 22F might be that high in a code min house if you have a lot of window area or build a really crummy leaky house. Odds are pretty good that your heat load is in that range at -11F (the 99% outside design temp for the Twin Cities), but it doesn't put out that much at -11F, but it'll be over 20K.

    The 2- tonner is good for 24,600 BTU/hr in an all-ducted situation:

    https://ashp.neep.org/#!/product/29018

    http://meus1.mylinkdrive.com/files/MXZ-3C24NAHZ2_ProductDataSheet.pdf

    That too is probably more than this house needs.

    Without the actual series name/numbers of the systems being proposed it's hard to drill down too deep, but on the surface his discussion seems outside the bounds of realistic loads. Was it MXZ compressors and MVZ air handlers? PUZ-HAxx air handlers and PVA air handlers? Some other series?

    This sort of thing is why it's important to run a Manual-J or similar load calculation on the house design before even talking to HVAC contractors. Your house is considerably better than code min, and odds are pretty good both the heating and cooling loads could be covered with a 1.5 ton or smaller cold climate heat pump. Paying a professional engineer to run the load numbers would likely "pay off" in smaller equipment size, higher efficiency , and higher comfort levels. This is one you don't want to screw up.

  2. Matt F | | #2

    Totally wacky stuff. They both oversized relative to reality and undersized relative to their design numbers. It would be insane to put in a heat pump in Minneapolis with an actual balance point of 22F.

    Is there a missing negative sign? That would seem plausible if we didn’t have the numbers for all three units.

    Fortunately a heat load call for your house is very straight forward and and an engineer should be able to run it for a very reasonable cost.

    Only thing to watch out for in a house like yours is humidity management due to the low loads in summer and low infiltration in winter.

    1. Expert Member
      Dana Dorsett | | #4

      >"They both oversized relative to reality and undersized relative to their design numbers. "

      We don't know that they're undersized relative to the design numbers. This is a tight house with triple pane windows, R80 attic and R40 walls. Even at a 15% window/floor area ratio the window heat loss is less than 4000 BTU/hr...

      ...the 1800' of R80 roof is less than 2000 BTU/hr...

      ...and with something like 1500 square feet of R40 wall maybe another 4000 BTU/hr...

      ... for about 10,000 BTU/hr total at the code-min 68F indoors, and a 99% design temp of -11F outdoors.

      Even the 2-ton MXZ-3C24NAHZ may be sub-optimally oversized, and the -4C36NAHZ is almost certainly ridiculously oversized. Without the capacity tables in front of me I'm guesstimating the -3C24 is good for about 18-20,000 BTU/hr @ -13F (70-80% of it's max capacity @ +5F), or getting on to a 2x oversize factor for the 99% load, whereas 1.3x - 1.5x would usually be the max optimal range beyond which efficiency starts to take a dive.

      Which is all why a competent person needs to calculate the load numbers on this non-standard way-better-than-code house before shopping for the HVAC solutions. The self-promoted "...ground source heat pump specialist..." who came up with the proposal is clearly out in unfamiliar waters that are way over his head.

  3. Jon R | | #3

    +1 on getting properly done Manuals J and S. That will still leave you with some decisions about how much electric heat you want to be used. The best way to decide that is to review the expected operating costs in $ (but few can calculate this accurately). A guesstimate - design for the heat pump to be capable of supplying the load at a -5F balance point.

    That heat pump will shut down on rare, very cold days - so design the electric backup for about 125% of the Manual J design day load.

  4. Chuck Baynton | | #5

    Thanks to all who replied, especially Dana. Took me a while to get model numbers of hardware in the HVAC contractor's proposal. Compressor is allegedly MXZ3C36NAHZ; however, it seems no such model exists, and this should probably be MXZ4C36NAHZ. Air handlers are one each of MSZGL24NA and MSZGL12NA, and there is also a branch box.

    When I went looking for specs on the compressor, somehow I got routed from Mitsubishi to a Richair Solutions web page, which tells me that the MXZ4C36NAHZ has heating capacity at 47º F of 45,000 BTU/h, and the same capacity at 17º F; no data provided for any other outdoor temp.

    It seems (from the specs page at richairductless.com) that the smallest compressor in this series is MXZ2C20NAHZ, which has ductless cooling capacity 18,000 BTU/h so it is identified as a 1 1/2 ton model. It accepts no more than 2 air handlers--works for me. But the specs say that its heating capacity at ambient 47º F is 22,000 BTU/h, and 13,700 BTU/h at 17º F. Quite a remarkable decline, and suggests that in my Minnesota climate, the 1 1/2 ton unit would be quite challenged whenever ambient is below +10º F. A substantial portion of our annual heating degree days--I don't know the actual number--comes at times when ambient is below plus 10. As with the larger compressor, no data are provided about heating capacity at any other temperatures.

    I am far from sophisticated enough to understand on my own why the smaller compressor exhibits such a performance decline over the range 47 to 17 degrees, which the larger 3 ton unit has no decline. Can this be explained short of a trip deep into the design of these compressors?

    Also, it might be useful to know the heating capacity of these units at temperatures lower than 17º F. Mitsubishi promotes these units as working down to -13º F.

  5. Expert Member
    Dana Dorsett | | #6

    The MXZ-2C20NAHZ, but it should still be worth 22,000 BTU/hr @ +5F, and something like 16-17,000 BTU/hr @ -13F. The specs you were looking at with the big capacity hit are not the cold climate versions.

    The 2C20NAHZ comes in a few subtly different flavors- you'd have to dive into the engineering manuals to sort out what's different about them. Capacity wise they're identical, per their AHRI submittal sheets.

    http://meus1.mylinkdrive.com/files/MXZ-2C20NAHZ_Submittal.pdf

    http://meus1.mylinkdrive.com/files/MXZ-2C20NAHZ2_ProductDataSheet.pdf

    All the MXZ______NAHZ series are all good for at least 70% of their "Rated" capacity at an outdoor temp of -13F. Some will be good for about 80% of their rated capacity @ -13F. That too would need to be looked up in the engineering manuals.

    >"But the specs say that its heating capacity at ambient 47º F is 22,000 BTU/h, and 13,700 BTU/h at 17º F."

    Only the non "-HZ" versions , the MXZ_____NA (no HZ) series take the big hit, with only about half as much max capacity at +17F as the max capacity at 47F eg:

    http://meus1.mylinkdrive.com/files/MXZ-2C20NA2_Product_Data_Sheet-en.pdf

    http://meus1.mylinkdrive.com/files/MXZ-3C30NA2_Submittal.pdf

    ^^These you absolutely don't want in your climate.^^

    The MSZ-GL24NA is a whopping HUGE high-wall coil that's good for 27,600 BTU/hr (with is more than half the heat load of my 2x4 framed 2400' sub-code antique +1600' of basement at -13F). It has that much capacity at any outdoor temperature, whenever there is sufficient MXZ compressor capacity to support it. You really REALLY don't want that anywhere in an 1800' high-R houseL

    http://meus1.mylinkdrive.com/files/MSZ-GL24NA-U1_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf

    The -GL12NA is also pretty hefty, good for 14,400 BTU/hr when married to an MXZ compressor.

    http://meus1.mylinkdrive.com/files/MSZ-GL12NA-U1_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf

    I'd hate to just take a WAG at your heat load in a high-performance- it really should be calculated, but I doubt there is any single zone in your house that needs 27,600 BTU/hr of heat even at -20F (when the thing shuts down). With an open floor plan there might be a zone that could use a GL12, but even that isn't a sure thing.

    A couple of 3/4 ton or 1 ton mini-duct cassettes, or perhaps a 1-ton ducted cassette + 1 ton wall coil would probably be more than enough. You're allowed to hook up more (cooling rated) cassette capacity than compressor rated capacity, up to a max of 1.3x the compressor capacity, so if it's really called for it's not a problem to go with a pair of 1-tonners rather than a 3/4 ton + 1 ton.

    http://meus1.mylinkdrive.com/M-Series/R410A+Systems/Horizontal+Ducted+/index.html

    Also, in heating dominated climates such as yours the floor mounted coils are a better choice than the typical high wall types:

    http://meus1.mylinkdrive.com/M-Series/R410A+Systems/Floor+Mounted+/index.html

    Even the smallest in series KJ09 pr KA 09 is good for 11,000 BTU/hr in heating mode:

    http://meus1.mylinkdrive.com/files/MFZ-KJ09NA-U1_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf

    http://meus1.mylinkdrive.com/files/MFZ-KA09NA_For_MXZ_MULTI-ZONE_SYSTEMS_Submittal.pdf

  6. Expert Member
    Dana Dorsett | | #7

    The MXZ-2C20NAHZ, but it should still be worth 22,000 BTU/hr @ +5F, and something like 16-17,000 BTU/hr @ -13F. The specs you were looking at with the big capacity hit are not the cold climate versions.

    The 2C20NAHZ comes in a few subtly different flavors- you'd have to dive into the engineering manuals to sort out what's different about them. Capacity wise they're identical, per their AHRI submittal sheets.

    http://meus1.mylinkdrive.com/files/MXZ-2C20NAHZ_Submittal.pdf

    http://meus1.mylinkdrive.com/files/MXZ-2C20NAHZ2_ProductDataSheet.pdf

    All the MXZ______NAHZ series are all good for at least 70% of their "Rated" capacity at an outdoor temp of -13F. Some will be good for about 80% of their rated capacity @ -13F. That too would need to be looked up in the engineering manuals.

    >"But the specs say that its heating capacity at ambient 47º F is 22,000 BTU/h, and 13,700 BTU/h at 17º F."

    Only the non "-HZ" versions , the MXZ_____NA (no HZ) series take the big hit, with only about half as much max capacity at +17F as the max capacity at 47F eg:

    http://meus1.mylinkdrive.com/files/MXZ-2C20NA2_Product_Data_Sheet-en.pdf

    http://meus1.mylinkdrive.com/files/MXZ-3C30NA2_Submittal.pdf

    ^^These you absolutely don't want in your climate.^^

  7. Expert Member
    Dana Dorsett | | #8

    The MSZ-GL24NA is a whopping HUGE high-wall coil that's good for 27,600 BTU/hr (with is more than half the heat load of my 2x4 framed 2400' sub-code antique +1600' of basement at -13F). It has that much capacity at any outdoor temperature, whenever there is sufficient MXZ compressor capacity to support it. You really REALLY don't want that anywhere in an 1800' high-R houseL

    http://meus1.mylinkdrive.com/files/MSZ-GL24NA-U1_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf

    The -GL12NA is also pretty hefty, good for 14,400 BTU/hr when married to an MXZ compressor.

    http://meus1.mylinkdrive.com/files/MSZ-GL12NA-U1_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf

    I'd hate to just take a WAG at your heat load in a high-performance- it really should be calculated, but I doubt there is any single zone in your house that needs 27,600 BTU/hr of heat even at -20F (when the thing shuts down). With an open floor plan there might be a zone that could use a GL12, but even that isn't a sure thing.

    A couple of 3/4 ton or 1 ton mini-duct cassettes, or perhaps a 1-ton ducted cassette + 1 ton wall coil would probably be more than enough. You're allowed to hook up more (cooling rated) cassette capacity than compressor rated capacity, up to a max of 1.3x the compressor capacity, so if it's really called for it's not a problem to go with a pair of 1-tonners rather than a 3/4 ton + 1 ton.

    http://meus1.mylinkdrive.com/M-Series/R410A+Systems/Horizontal+Ducted+/index.html

    Also, in heating dominated climates such as yours the floor mounted coils are a better choice than the typical high wall types:

    http://meus1.mylinkdrive.com/M-Series/R410A+Systems/Floor+Mounted+/index.html

    Even the smallest in series KJ09 pr KA 09 is good for 11,000 BTU/hr in heating mode:

    http://meus1.mylinkdrive.com/files/MFZ-KJ09NA-U1_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf

    http://meus1.mylinkdrive.com/files/MFZ-KA09NA_For_MXZ_MULTI-ZONE_SYSTEMS_Submittal.pdf

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |