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

Conventional heat pumps with mini-split low temp efficiency?

Mai Tai | Posted in Mechanicals on

There seems to be quite a few options for extra high efficiency heat pumps when you go with mini-splits, but I am struggling to find conventional heat pumps that stand out. It will be connected to a standard ECM motor air handler, like a normal a/c unit would. The ducting is basically in the plans, so there is no going back now.

I understand there is a decent efficiency penalty to be paid when driving a large fan motor vs a mini-split head and low powered fan, but are there still some products that stand out, efficiency wise? I am considering a heat pump as my only source of heat for the first winter, so it would have to have decent efficiency (cop>1.0) down to -20decC or better (stretch goal -25degC).

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

    Modulating Carrier Infinity Greenspeed or Bryant Evolution etc can hit HSPFs in the 11-13 range, depending on which air handler option is used and the quality of the duct design/implementation. The turn down ratios are pretty limited compared to mini-splits- best case of about 2.5:1, compared to at LEAST 3:1 for even the crummiest modulating mini-splits, and more than 10:1 for some. (eg. The Mitsubishi FH09 modulates from 1600 to 18,000 BTU/hr @ +47F in heating mode.

    With low turn down ratio getting the efficiency out of it requires much closer attention to sizing it for the load than with mini-splits. It's often better to undersize by a half-ton and use heat strips to cover than to size it to where it will cover the 99% load entirely with heat pump, only to suffer a steep drop in shoulder-season inefficiency related to increased cycling rather than modulating.

    Play around with this tool a bit- click on the Heating Capacity tab, and pick a location with an outside design temp of -4F/-20C or colder, or just adjust the outside design temp to -4F (or -13F/-25C) to see the capacity with different air handler options:

    http://www.tools.carrier.com/greenspeed/

    While they only show the 2 ton and 3 ton options, the Greenspeed comes in much bigger versions too. A 3-ton Greenspeed only delivers ~13,000 BTU/hr @ -13F/-25C, but is good for about 18,000 BTU/hr @ -4F/-20C. That's half it's "rated" cooling capacity BTUs, so it's not exactly comparable to cold-climate mini-splits. (Cold climate mini-splits use different innovative refrigrant valves to deliver higher capacity & efficiency at low temps.

    Mitsubishi's PVA full-size conventional ECM drive air handlers have comparable turn down ratios to the Greenspeed et al, and can hit HSPFs it the ~ 10F range, but aren't compatible with their "hyper heating" low ambient temp compressors. Non-modulating MVZ air handlers can be married to MXZ mulit-split compressors and do a bit better in both capacity and efficiency at low temp when sized correctly to the loads.

    [edited to add]

    Note: Hyper-heating MXZ compressors need at least two zones to be hooked up in order to function properly, even if the second "zone" is a half-ton mini-split head. I don't have the engineering manual in front of me to verify then -25C numbers (but you can look it up), the 3-ton MXZ-4C36NAHZ is good for 45,000 BTU/hr @ +5F/-15C, and can still deliver something like 30-35,000 BTU/hr @ -13F/-25C, and is compatible with the 3-ton PVA-A36AA7 full size air handler:

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

    A 3-ton PVA-A36AA7 for the primary zone and MSZ-FH06NA ductless head (or even a cheaper GE06) for the second zone would be enough to run a -4C36NAHZ, delivering a bit better than HSPF 10 for seasonal efficiency.

  2. John Semmelhack | | #2

    Small correction to Dana's comment: the Mitsubishi PVA conventional air handler IS offered as part of a "Hyper-heat" package (operation down to at least -13F)...but only in the following nominal capacities: 2.5-ton, 3.0, and 3.5. Look for the PUZ-HA outdoor models here: http://meus1.mylinkdrive.com/P-Series/R410A+Systems/P-Series+OD+Units+/index.html

    Expanding on Dana's comment regarding duct design/install - this is a critical element to any ducted system for comfort, IAQ and energy efficiency. Regarding just the energy, you could see a factor of 10 spread between low and high power input to the air handler with world class duct design/install versus bottom of barrel.

    If you want to do more research on cold climate heat pumps, the NEEP database is still the best available...though the data is not always accurate. Make sure you sort by "ducted" systems, since the database contains both ductless and ducted systems. Main page here: http://www.neep.org/initiatives/high-efficiency-products/emerging-technologies/ashp/cold-climate-air-source-heat-pump#Listing%20Products

    Excel spreadsheet of products here: http://www.neep.org/sites/default/files/ColdClimateAir-SourceHeatPumpSpecificationProductListing-Updated6.22.18.xlsx

  3. Expert Member
    Dana Dorsett | | #3

    John- thanks for the correction regarding hyper-heating PUZ-HAxx compressors!

    As an example a 3 ton PVA air handler on a 3 ton "HA" compressor tests at an HSPF of 11, and is good for a max of 38,000 BTU/hr @ +5F/-15C, and about 80% of the "rated" capacity @ -13F/-25C, which would be ~23,000 BTU/hr. (Would have to dig into the engineering manuals to be more specific on low temp capacity & efficeincy.)

    http://meus1.mylinkdrive.com/files/PEAD-A36AA7___PUZ-HA36NHA5_Product_Data_Sheet-en.pdf

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

  4. John Semmelhack | | #4

    A couple more options from Mitsubishi...it looks like they recently released 1-ton and 1.5-ton single-split models using a traditional air handler.

    http://meus1.mylinkdrive.com/item/SUZ-KA12NAR1.html#Submittals
    http://meus1.mylinkdrive.com/item/SUZ-KA18NAR1.html

    Unfortunately, these are not "Hyper-heat" systems and will cut out at -7F according to the specs.

  5. Kevin Spellman | | #5

    <>

    Dana and John--can you expand on your comments about good vs. bad ductwork installs or point me to somewhere I can learn more about it? I have the Bryant Evolution planned for my new build. We did an aggressive Manual J and sized it right. If the duct job can make this much difference, I would like to know how to plan for the best install.

  6. GBA Editor
    Martin Holladay | | #6

    Kevin,
    For an overview of duct design issues, with lots of useful tips and links to online resources, see "All About Furnaces and Duct Systems."

  7. Mai Tai | | #7

    Thanks Dana and John, very good information.

    I have a calculated heat loss of 36kbtu, and need about 24kbtu of cooling, so a 2.5 ton may work for me. Keep in mind I also have natural gas, and will be installing 2 fireplaces and a high efficiency water gas heater with radiant, , so I can supplement when the efficiency equation does not make sense.

    The only real question is this: what is the premium involved in getting a hyper heat heap pump vs a decent/mid grade ac unit? Although very reliable, Mitsubishi units are not cheap around here. Although I'm attracted to the efficiency and dual purpose of a good heat pump, and also has to make financial sense. The house will be solar ready, which would make the heat pump a real winner, but I don't see the solar install happening for 5-10years now, so may be it would be better to run a standard ac unit for the time being, and switch when I get the panels installed.

  8. Irene3 | | #8

    Daikin's SkyAir ducted systems are another possibility, but I'm not sure about pricing. They may be more than Mitsubishi.

  9. Andy Kosick | | #9

    When sizing for heating watch the capacity loss at low end due to frequency of defrost cycles, I've found this to be far more of a factor than the pump output with the Mitsubishi I have. They used to have a chart for it but I can't find it now.

    This may be worth a whole different thread, but does anyone know why Mitsubishi won't give us a single zone hyper-heat with a ducted head (or Fujitsu for that matter)?

  10. John Semmelhack | | #10

    Andy- for MITSUBISHI see comment #2...hyperheat units are available, but only in the pricier P-series, and only starting at 2.5 tons capacity.

    As for FUJITSU, they already have what you’re looking for, they just don’t advertise it. The AOU/ARU series has no known lower limit (yet). I have colleagues who had them operating as low as -20F over the past 2 winters. Engineering data only goes down to -5F. Heating capacity for each of the models (9k, 12k and 18k) is greater than 100% of nominal/rated capacity at -5F. For the 12k unit, for example, capacity is 15kBtu/hr at -5F.

  11. Mai Tai | | #11

    John,

    Fujitsu AoU/ARU sound like a good option, I'll just have to find a decent dealer in Southern Ontario. Fujitsu dealers are already few and far between, let alone dealers who carry more than ductless mini-splits.

    I remember Fujitsu also makes airtowater pumps, and since I have a radiant floor option, that might be a path forward. How is the efficiency and price vs air to air? It might be a coup if I could avoid buying a water heater also. i was going with an hydronic air handler, so it would not be much of a stretch (except I probably would have to run the ecm fan at a higher speed).

  12. John Semmelhack | | #12

    As far as I know, Fujitsu doesn’t offer their air-to-water heat pumps in N. America.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |