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Central air heat pump: sizing via kBTU/hr vs CFM?

finePNW | Posted in General Questions on

My Manual J output for a ducted heat pump in the Seattle, WA area (climate zone 4 coastal) indicates 27 kBTU/hr heating load with CFM needs of 1001 CFM, and I’m confused by how to meet what seems to be an atypically large CFM-to-kBTU ratio. Is it a bad idea to get an air handler below this 1001 CFM value?

With fantastic advice from folks on another thread, it seems the best Mitsubishi option for me is ~3ton (32 kBTU rated) unit — the next lowest option (2.4 ton I think) would make me nervous as it’s rated at 26 kBTU/hr, which is 1kBTU below my Manual J based needs. Even with this larger ~3 ton unit, the air handler model (PVA A30AA7) that’s supposed to go with it maxes out at 845 CFM according to its spec sheet. What should I do here? Go another step up on the ODU? Mix an oversized IDU with the 3 ton ODU? Ignore it because I’m misinterpreting stats?

Air handler for 2.5-ton cc ODU, “air flow rate, dry” = 613-744-875 CFM:

Air handler for 3-ton cc ODU, “air flow rate, dry” = 613-744-875 CFM:


Air handler for ~ 3.5 ton cc ODU, “air flow rate, dry” = 788–956–1,125 CFM: 


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  1. paul_wiedefeld | | #1

    CFM is irrelevant here.

    1. finePNW | | #4

      Thanks! Why is that? My concern is that even if the ODU and IDU are producing enough heat, it's potentially not supplying heat quickly enough to rooms to overcome loss, and that it might create an uneven and less comfortable indoor environment. Is that not quite right?

      As I've convinced my wife to let me pick a heat pump over standard gas furnace, my mandate is (reasonably so!) to meet her indoor design temps -- even if I wanted to wear a sweater instead! :) Which, honestly, I don't... I'm maybe less devoted to the cause than I should be in that record.

      1. paul_wiedefeld | | #5

        Nope that's not related. You want to supply enough Btus to keep the temperature constant, but there is no magic CFM value.

        CFM is not a fixed value. 500 CFM would work. So would 2500 CFM. The formula is: BTUH = CFM x ΔT x 1.08. So 850 would work with a Delta T of 29, which is easily within the range of a Mitsubishi. It seems the program defaulted to a delta t of 25. That’s a reasonable, but more or less random, software decision.

        Don't sweat the furnace vs. heat pump! I'm in a colder climate and my Mitsubishi is substantially more comfortable than the furnace it replaced. For a heat loss of your size, you'd had a massively oversized furnace.

        1. finePNW | | #6

          That's very helpful. Thanks!!

          Apparently depending on how the ducting ends up being installed (within the scenarios that actually apply to how we might do it, at least), the Manual J heat load may be as high as 31 kBTU, so we might get a tightly sized unit out of the 32kBTU ODU after all. Whew.

  2. joshdurston | | #2

    My recommendation is to size tight (even slightly under your case) and have an electric heater in the ductwork, entrance, or warm bathroom floors to make up any gap.
    The tight sizing typically gives you a lower min capacity which may actually translate to less seasonal energy even if it uses a bit resistance under high loads (less cycling and overshooting). If you do go with any electric heat you really need to make sure the control strategy is done in a way that isn't wastefully running it. There are options to run the duct heater during defrost which helps comfort but drastically ups the usage. You don't want it running unless the heat pump can't keep up. .
    The BTU's are what matters and the BTU/cfm ratio is going to vary a bit depending on the equipment. In cooling you want a reasonable SHR to get decent dehumidification, more CFM at the same capacity may hurt your comfort.

    Also, note if you're slightly short on BTU's on a cold day, it's not like your system collapses and completely fails to heat. Your space just reaches equilibrium slightly below setpoint.

    1. finePNW | | #3

      Thanks! Interesting suggestion re: resistive heating element. Do you know any good articles I could read up a bit more on how considerable the trade-offs are between under-size + resistive heat vs. oversize?

      Not meeting equilibrium at set temperature would probably (and justifiably) cause my wife to reduce her faith in my green-building ventures. Electricity is cheap here ($0.10/kwh) and we're likely getting solar soon, so whatever I need to do to attain the set point is what I'll end up doing. I just need to figure out the best way in terms of longterm cost and proper/even heating & maintaining a reasonable RH.

      I did forget to mention that I'll be integrating an HRV into the system ... unfortunately, costs will probably make us go with shared ducting vs. HRV having it's own ducting. But maybe that fan will help the air delivery system some?

  3. walta100 | | #7

    The manual J is the first step.

    You use the data from manual J in manual S to select the correct equipment. Then you do the manual T &D calculations for each room with manual J data for each room you design duct work large enough to move the required number of CMF to each room.

    Note the Asin blowers tend to be less powerful and are designed for lower static pressure levels in short, your ducts will be bigger with fewer bends and need to be careful designed compared to US products.


    1. finePNW | | #8

      Ah, gotcha. I assumed (stupidly) that the CoolCalc provided Manual J and more... but I'm not sure why they'd do that if they only boast being ACCA certified for Manual J.

      My HVAC contractor is a "Diamond" installer, so hopefully will be able to see the ducts properly, but I'll try to find tools to complete Manuals S, T, D. I downloaded spreadsheets from ACCA, but they seem to be locked.

  4. Expert Member
    Akos | | #9

    Look at the M series SVZ/SUZ combination. The P series is the semi-commercial version which tends to be more expensive. The 2.5 ton M series hyper heat will do 32k BTU in your climate.

    The CFM is an output for duct sizing nothing to do with heat pump selection. Once you have the heat loss, you select the equipment. Each heat pump will provide a certain BTU/CFM in heating and a different BTU/CFM for cooling. You look at the BTU load for each room and calculate the CFM that room needs by dividing the two.

    1. finePNW | | #10

      Thanks! This is great. I can’t quite tell if the SVZ/SUZ will operate with only a single indoor unit (a multi position air handler for central air), or if it needs at least two units connected to it. The M-Series product guide tables are a little confusing in that regard (to me, anyways).

      1. paul_wiedefeld | | #11

        The SVZ/SUZ combo is one to one. It’s what I have, it’s great!

        1. finePNW | | #14

          Thanks! Any chance you have an HRV, too? If so, how is it connected and are you satisfied with it?

  5. walta100 | | #12

    Consider getting two separate systems.

    Yes, it will cost a little more but almost every post from an unhappy minysplit owner turns out to have multiple heads mostly because extra heads limit the low-speed operation.

    An added benefit is redundancy in that sooner or later there will be a fault and the multisplit will leave you without any heat while one of the twin systems will still be working making it an unconvince instead of an emergency.


    1. finePNW | | #13

      Interesting point! Thanks.

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