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Whole-House Heat Pump System

SouthshoreJohn | Posted in Mechanicals on

I’m looking to have a whole house heat pump system or systems installed and after 5 quotes and a ton of spec comparisons I’ve decided on a Fujitsu system. The question is will a multi system with 24k slim duct and 2 12k heads work or do I go with a mid static…

The plan is a 2 ton slim duct with all new ductwork for upper level of a raised ranch/split level and 2 12k ductless heads downstairs. The upper level is 1100sq ft and I’m concerned the 24 k slim with static pressure of 0-.20 is too low.

The installer says he’s put a ton of these in and they work good but not to use s 5” filter only a 2” filter. Filtration is important to us and wondering if better to go with 2 24k condensers and use mid static instead or is this going to work well?

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  1. Expert Member
    Akos | | #1

    How were the units sized, which climate zone are you in and is this for heat or just cooling?

    2 tons of mini splits for 1100sqft of space sounds awful close to 500 or 600 sqft per ton rule of thumb which is guaranteed to be wrong. Add on there two more zones of 12k for the living space and I can pretty much guarantee the units are oversized by at least a factor of 3. This generally means they will not run well, create comfort issues, have trouble with dehumidification and use more power than a right sized unit.

    More likely, you only need a 12000BTU unit for the upper level which still produces almost 1.5 tons of heat at 5F which is also good for 0.36" WG. That should be plenty of pressure for a simple ducted setup.

    As for the rest of the house, generally best to avoid multi splits. These tend to have much lower modulation range and even right sized tend to be less efficient than having each indoor unit on its own outdoor unit.

    1. SouthshoreJohn | | #2

      It was designed for heating and cooling at 5 degrees(Boston area). House was built in 1971 but all updated windows/ doors so average insulative values I would say..

      One of the larger contractors said he did manual j and commented I could put an 18k Mitsubishi that ran 21k heating btu’s but would be cutting it close..For comparison I currently use a 10k window shakers ond two 5k units other end of house which do well unless it hits high 90s and humid for days.

      The midstatic modulates from 5400 to 29000/32400 which seems much better than the 12000 to 48/54 on the multi…I’ll look at the submittals for a 1 to 1 18 k unit as that would probably be most efficient

      1. aunsafe2015 | | #3

        A contractor telling you that he ran a manual J isn't very convincing. Contractors typically use rules of thumb (e.g., 1 ton per 500 sq ft), and/or they use estimations that tend to greatly inflate the amount of heating/cooling needed. Not saying this is true of all contractors, but if a contractor tells you he ran a manual J and that you need 1.5 tons, it's quite possible--likely even--that if an independent designer ran the manual J, the result would be 1-ton or even less.

        Again, not saying there's a problem with your contractor's manual J -- just saying to be skeptical until you've seen the inputs used and verified that they are actually correct.

        1. SouthshoreJohn | | #4

          I appreciate the input so far and agree 2 tons of cooling is oversized but it’s the heating side I believe they are sizing it for. I could be getting all off the cuff 500’ per ton quotes but one contractor said he ran it thru his Mitsubishi builder where gave several options from a few 1:1 and a central condenser/d box all sized 2 ton upstairs for a little over 1000 sq ft and 2 9s or 12 s downstairs…

          I would think Smaller is certainly cheaper and less to run in the long term as long it can handle the heat load

          How can I go about getting an independent manual j & d?

  2. Expert Member
    Akos | | #5

    The simplest is to run through the calculations bellow using the fuel usage of the existing system:

    This won't tell you what each area will need but it will give you an overall number for how much heat you need.

    As a rough number, we can ballpark it assuming:
    -R10 walls
    -U0.3 windows, 20% wall area as glazing
    -R20 ceiling insulation
    -3/4 of 1100 sqft as exterior walls
    -say about 8ACH@50PA air leakage

    At 5F you loose
    -walls (70F-5F)*750sqft*80%/R10=3900BTU
    -ceiling (70F-5F)*1100sqft/R20=~3000BTU
    -glass (70F-5F)*750sqft*20%*U0.3=3000BTU
    -air leaks ~75CFM * (70F-5F) *1.08=~5200BTU

    Add on some solar gain and heat gain from people/lights/appliances and looking at ~12000BTU. If over uninsulated basement, it will add a bit of extra floor losses but still well bellow 20000BTU.

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