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Ducted heat pump retrofit

doug_m_ | Posted in Mechanicals on

We have a 1200sqft, single story (with basement) in Zone 6 (northern New England). Built in the 1950s, it’s been upgraded with insulation (R60 attic, blown fiberglass in the 2×4 walls, foamed rim and basement) and air-sealing. Blower door is 3.87ach50, Manual J heat load ~35,000btuh (attached).

We have a single 18000 btuh minisplit (Fujitsu ASU18RLF/18RLXFWH) in the combined kitchen/living/dining area. 

We would like to remove our old oil system entirely, since it’s failing anyway. 

We received a quote for a ducted unit to cover the remaining rooms (two bedrooms and bathroom). The indoor unit would be mounted in the basement, with vents coming up through the floors. The contractor is proposing a Mitsubishi SEZ-KD18NA4 (specs attached).

I’m concerned by the listed minimum temp as -5F (we see -20F a few times every winter) and the listed rating at 17F dropping to 13,000btuh. Think this unit plus the 1:1 unit we have could cover the whole house? Should we consider a different unit? or add electric baseboards to supplement as needed? Any thoughts on ducted units would be appreciated.

Thanks in advance,
Doug

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Replies

  1. paul_wiedefeld | | #1

    A bit tricky to size using fuel usage since you have one minisplit already but here goes: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler.

    Installing without electric baseboard and including that later should be no big deal.

    1. doug_m_ | | #3

      Thanks Paul. We use a woodstove as our primary heat source and don't track exact quantities burned, so we cannot accurately use a consumption-based estimate.

  2. Expert Member
    Akos | | #2

    The ManJ seems a bit high. Is your mudroom on an uninsulated slab? Doing a rough calc on wall losses assuming roughly a 30x40 building with 8' ceiling, I'm only getting 5000BTU not 12000BTU.

    I would run through the link suggested above to see what ballpark you are in. For the mini split, take your winter time electrical usage and subtract out electrical use from a non cooling month. This would be roughly the energy used by the mini split. That unit should be about a COP of 3 so.

    Mini Split kWh * COP 3 * 3414= BTU supplied by mini split.

    You can add this onto the fuel use in the link.

    In either case, in cold climate you want a hyper heat unit not the -NA4. That would be SEZ-KD xx NAHZ. Once you get the heat load dialed in you can look at what you need. I would guess probably a SEZ-KD09NAHZ.

    1. doug_m_ | | #4

      The mudroom is on an uninsulated slab (at the moment - planning to insulate foundation exterior next year).

      Thanks for the hyper-heat recommendation and model numbers.

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