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Community and Q&A

Small ducted heat pump?

roygoodwin | Posted in Mechanicals on

My wife and I are 69. We’re in the process of designing a house for our retirement with our architect. It’s going to be a “pretty good house” with a very small heating/cooling load. Neither my wife nor I think the ductless minisplits are all that attractive, AND the air filters would be hard (relatively, because of the height) likewise for the “ducted” minisplits that I’ve found via Mr Google. ( they’re all ceiling mounted and we’d like to avoid having to get on a step ladder to change air filters as we get older & older). We’re looking for something like a conventional heatpump (ducts & all), that the air filter(s) would be easier to access. Any suggestions.

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  1. GBA Editor
    Martin Holladay | | #1

    One possibility is to mount your ductless minisplit units low on your wall, near the floor. For more information on this approach, see Minisplit blowers mounted low or high?

    [Click the photo to enlarge it. Photo credit: Peter Talmage.]


  2. Expert Member
    Dana Dorsett | | #2

    There are also floor units offered by several ductless manufacturers that look la bit like very thin/small wall-furnaces:

    Like hydronic fin-tube convectors, most of the ductless floor units take & deliver air out of the front face of the unit, which means they are easily set into walls or cabinets with the face pretty much flush with the wall. (See page 7 of that last document for instructions on how to embed it in a wall behind a grating without impeding function.)

  3. user-626934 | | #3

    There are a handful of ducted mini-splits that might be appropriate, depending on your loads and design temperatures. Mitsubishi has versions in 9, 12, 15 and 18kBtu/hr nominal sizes. Fujitsu in 9, 12, and 18. I'm attaching spec sheets just for reference for the 9k versions from both manufacturers. The Fujitsu units are unique (for now) in that they can be mounted in horizontal or vertical (upflow) position. The vertical orientation allows for a "conventional" installation, similar to a standard upflow furnace or heat pump air handler.

    Note that all of these units are designed to operate only at low static pressures (up to 0.36 in.w.c. for Fujitsu and up to 0.20 in.w.c.), so careful duct design and installation is essential.

  4. roygoodwin | | #4

    How well would they work with a filter with a higher MERV rating ? We'd like to use something pretty high performance for dust as well as allergies. What about something like the Trane xv20i which is variable capacity. It would probably never get anywhere near it's rated capacity (2 ton) but would more than likely handle higher static pressure(s). Would running it at the low end on it's rated capacity, using the variable speed compressor, be less efficient ? would it have a negative effect on it's life expectancy ?

  5. Expert Member
    Dana Dorsett | | #5

    In general modulating systems run at highest efficiency when modulating at or near their lowest speeds (effectively making the coils oversized for the amount of heat they actually need to exchange). So if your average heating load is around the minimum-modulated output it can work out well, but if your PEAK load is below it's minimum modulation it becomes very sub-optimal.

    When the average load is below the minimum modulated output of the system you start to lose a lot of efficiency to standby power use and start-up surges, short-cycles, etc. (The cycling losses with scroll compressors and ECM drive air handlers it's as severe as with old-school heat pumps, but it's still a loss.) For the same reasons, with mini-splits anything more than 1.5x oversized for the peak load starts slipping down that efficiency curve, but below that runs pretty close to optimal efficiency. (At very slight oversizing there is a small efficiency boost, but beyond 1.5x it gets more than eaten up in standby and cycling losses.)

    The Lennox XP25 and Carrier GreenSpeed 2-ton units are comparable to the Trane XV20i- if you go that route you may want to compare specs and get competing quotes. Any of them are quite a bit more expensive than right-sized ductless (or mine-duct cassette) solutions though, with distribution losses that ductless solutions don't have.

  6. user-626934 | | #6

    One can use high MERV filters in any of the low-static, ducted mini-split heat pump systems...again, it's a matter of system design/installation, and carefully accounting for the various parts of the system, including filters. I recently designed and tested a Fujitsu 9k ducted system that has a 2" MERV 11 filter. The in-field measured total external static pressure was 0.23 in.w.c. at the design airflow.

  7. Expert Member
    Dana Dorsett | | #7

    It would be good to know both the climate zone and at least the rough estimates of the design loads to be able to make reasonable recommendations.

    Terms like "pretty good house" has a pretty squishy definition, and "very small heating/cooling load" is even more vague. A 5000' pretty-good house in Moose Jaw, SK may have very low loads relative to a code min house that size, but it would be many times the loads of an 800' pretty good house in Isla Vista, CA.

  8. roygoodwin | | #8

    We're in the mountains of Western NC ( climate zone 4 ) . The house will be about 2,000 Sq ft. We don't have an estimated heating/cooling load yet. The walls are currently being planned to be 2x6 with blown in cellulose with 2 3/4" of polyiso on the exterior + siding, about 20" of cellulose in the attic. r13 basement walls & 2" of EPS under the basement floor. We're still working out details & perhaps I've gotten too far ahead of myself, but I figure it's better than playing catchup.

  9. Expert Member
    Dana Dorsett | | #9

    As long as you have windows with glass (and a reasonable window/floor area ratio) the heat load of that house is going to be under 20,000 BTU/hr, and with decent peformance windows would probably come in under 15,000 BTU/hr, making a 2-ton high-efficiency ducted heat pump a bit silly. Your 99% design temp is probably going to be between about +15F or maybe a bit higher (See: ).

    A 1.5 ton single floor unit would probably cover your loads with margin:


    Or maybe a pair of 3/4 ton floor units (or a 1-ton and a 3/4 ton) tied to a single compressor unit, one floor unit upstairs, the other downstairs eg:

    MXZ-2A20NA + (2x) MFZ-KA09NA ( + MFZ-KA09NA + MFZ-KA12NA),_43-51%29.pdf

    The efficiency of a dual-head might be a bit behind a best-in-class perfectly implemented 2-ton ducted heat pump, but the installed cost would be less than half.

    Spending the difference on rooftop solar would likely pay most or all of the heating & cooling bill, provided your house design is keeping the rooftop solar option open and optimized.

    About the R13 basement walls- I'm assuming that's 3" of Type-II EPS? R13 fiberglass between studs would have moisture issues up against a concrete foundation in your climate, unless you put an inch of foam between the studwall & concrete (or CMU.)

    Is it two stories + full basement, or one-story with a full or walk-out basement or ???

    Got a zip code? (For weather & climate data purposes.)

  10. roygoodwin | | #10

    House will be single story, with possibly a full basement partially fully exposed due to the site slope. Zip code is 28715. The architect is suggesting a half basement for the mechanical area & the other half piers. I'm concerned about air sealing the underside and any plumbing leaks in the covered/insulated area, so we're discussing that some more. We're looking at casement windows with the integral blinds ( triple glazed - IG with a storm panel)(Pella or Andersen Eagle). Yes 3" of EPS. Having said that, the basement walls are another subject for discussion with the architect. I've considered durisol ( could be up to R21 ) or thermomass .

    We do like the ease of zoning with a mini-split along with the cost, but still aren't sure that is sufficient to make us chose that for the long haul.

    Perhaps I'm being obsessive about this, but the ability to change the filters as we age is a big factor. My wife has had psoriatic arthritis for over 40 years and while it's not too bad now, she's already had one knee joint replaced and the she could have the other one replaced any time she's ready to endure the physical therapy that follows. I've got stage IV lung cancer which is under control right now, but I don't know what tomorrow will bring. We want to be able to stay in the house for as long as we can. We'd rather not have to call a service person to change/clean the filter(s). Also, while vents aren't the most attractive, they're still less obtrusive than the head unit of a mini-split.

  11. user-626934 | | #11

    Return air filter grille/s low on the wall/s seem like they'd be in order. This can be part of the design of any ducted system (mini-split, or otherwise).

  12. Expert Member
    Dana Dorsett | | #12

    The design temp for Candler will be about +16F, which is a challenge for old-school ducted heat pumps, but a walk-in-the park for ductless.

    Changing the filters on ductless floor-unit cassettes such as those I pointed to in my previous post (post #9) is a piece of cake- probably easier than many ducted heat pump systems. They are not over head- they are at floor level, in front of a wall (or embedded in the wall if you design & build the detail to accomodate it). You don't need a ladder, and you don't need to kneel- you can even park a chair or stool next to it to sit on while changing the filter if you like. They look a bit like a very small wall-furnace or hydronic convector. Here are a couple example images of what they look like:

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