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Ductless minisplit vs. air-to-water heat pump

youngb556 | Posted in Mechanicals on

Hello everybody, I am in the very beginning stages of my house design and I keep bouncing between using mini splits vs Air-water heat pumps and fan coils and possibly a radiant slab.

I intend to insulate my house very well, basically ICF construction with rigid foam above the roof sheathing and cellulose loose fill making up the rest of my roof insulation. Trying for somewhere around R-60.

The house will be set up with an open concept living dining and kitchen with vaulted ceilings taking up approximately half of the 1400 sqft the other half will be a master suite and a second bed and bath. Above the bedroom area we were planning on doing room in attic trusses to create a bonus room, possibly to be a third bedroom and storage area.

We were planning on doing a slab on grade so naturally my first thought was to include radiant heat but I would like to be all electric, a pv solar array is likely in the future. The mini splits are an attractive option because of their low price and proven track record but I have concerns with humidity control and even temperature throughout the house. I live in Climate zone 4a and we tend to have hot humid summers.

The air-water heat pump is the more expensive option but there is no limit of fan coil units that can be installed as long as maximum output of the unit is not exceeded and it would work with the radiant heating. I have looked into the chiltrix system and it can be purchased with a separate humidity control system. It seems like a good product just have no experience with these types of units.

So basically I’m wondering if the added comfort is worth the extra expense? How much of that warm foot feeling that is so popular with radiant am I missing out on? Also being a smaller house am I correct to assume I would need two “centrally” located fan coil units using the mini split heat pump? Sorry I know that was a lot of questions. Thanks in advance for an responses.

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  1. GBA Editor
    Martin Holladay | | #1

    Since you are asking for advice, I'll provide it: go with the minisplits. Here is my reasoning:

    1. Air-to-water heat pumps are exotic, and few HVAC contractors understand the equipment well enough to provide design assistance or service. For more information on this topic, see Air-to-Water Heat Pumps.

    2. With a well-insulated slab, hydronic tubing will almost never provide you with the "warm foot feeling" you seek, because your heating needs will be low, and the slab temperature will never be very high. For more information on this topic, see All About Radiant Floors.

  2. Jon_R | | #2

    In a well air sealed house, I expect that the Chiltrix humidity controller's primary effect would be to not over dehumidify, providing an efficiency increase. How much/often depends on how dry the outside air is. Not clear to me if they have any options for dehumidification without cooling.

    I'd look at Manual J numbers, desired temperature and the need to close bedroom doors to estimate bedroom comfort. For example, if you like the bedroom as cool or cooler than the rest of the house in summer, you can't do it with a central mini-split.

  3. Expert Member
    Dana Dorsett | | #3

    Without knowing the climate particulars (such as the 1% and 99% outside design temperature) and the calculated load numbers there's no way to estimate what the required peak water temperatures are for an air to water system or how exotic/expensive it would need to be. But the heating & cooling load of even an IRC 2015 code-min 1400' + 700' one-and-a-half story home in zone 4A can come in comfortably within the capacity range of a 2-ton 3-head multi-split, or even a 1.5 ton mini-ducted Fujitsu mini-split, which are pretty cheap compared to any hydronic solution.

    Since you're at the beginning design stages...

    A minimalist 2.25" + 2.25" EPS foam, ICF meets code-min in all US climate zones, but isn't necessarily insulated ''...very well..." for zone's 6 & higher. (It just BARELY makes code minimum in US climate zone 7.) For zone 4A it's a significant upgrade beyond code-min though.

    An R60 roof isn't a huge improvement over a an IRC code-min R49, and depending on climate zone could require a LOT of foam above the roof deck if insulated at the roof deck rather than the attic floor or a vented cathedralized ceiling. In zone 4A for an R60 roof you'll need a MINIMUM of R18 exterior foam (say, 3.25" of roofing polyiso at about $3.50 per square foot) for dew point control , but if went that route the rafters could then be standard milled 2x12s rather than I-joists or trusses. A vented R49 cellulose solution can be done with 15" I-joists rafters, probably for less money. To hit a vented R60 would require 18" I-joists. All of these solutions would require at least 2.8lb density cellulose (minimal dense-pack) to work.

    Price it out multiple ways, but it'll still won't be cheap. The cost adder of an R60 cathedralized ceiling might be better spent on higher performance windows, and going with an 18" energy-heel truss (for a settled depth of 17" open blown low density cellulose) vented attic approach. Making it a full second floor (rather than half & half) might even be cheaper than a cathedralized ceiling.

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