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Community and Q&A

Low ambient minisplit heating

William Ayers | Posted in General Questions on

Hello, I’m new to GBA forums. I’m an inveterate diy-er,  and I’m doing my best to research through the strategies of using mini splits to heat and cool my home. We currently use a combination of wood heat and plug in heaters in the winter and windows units in summer. I did a heat load calculation using HVAC-Calc and the heat loss in winter is greater than the gain in summer. Given that heat pumps generally taper off on heating btus with low outside ambient temps, if I supplement one zone of a multi zone mini split with our wood stove on our coldest days will the other zones benefit and perform better?  Or will they still be saddled with the decreased performance inherent to the condenser outside.  If I size the system to handle our most extreme winter days I’m afraid I’ll  oversize for good summer dehumidification. Even if I hire a contractor I want to understand these things. Thanks

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Replies

  1. Trevor Lambert | | #1

    People are going to need some numbers. What zone are you in, what are your heating and cooling load numbers? It's not unusual for the cooling load to be less than the heating.

    Based on the fact you did a heat loss/gain calculation, you already know more than 99% of HVAC contractors.

    1. William Ayers | | #3

      Thanks for the quick reply, Trevor. I don't know a zone number that was used in the calculations, but we are in steamy east Texas. I think the outdoor temps of 24F-97F reflect the program using a 97% factor of excluding the most extreme days. We definitely have higher and lower days than that, but most of the units I have looked at have more capacity than their nominal btu rating. So shouldn't I be ok using the attached figures?

  2. Jon R | | #2

    Over-sizing causing significant lack of dehumidification is mostly false. Some background:

    Figure 7 here:
    https://pdfs.semanticscholar.org/62f8/9eb6e689fe45e3fd723b225cab0a875fe3f3.pdf

    https://publications.energyresearch.ucf.edu/wp-content/uploads/2018/09/FSEC-PF-473-18.pdf

    A multi-head system will allow some load shifting between the indoor units - up to their maximum output.

    1. William Ayers | | #4

      Jon, thanks for the info. I haven't yet waded through it all yet; it looks to be useful though. I think it falls under the category of "be careful what you ask for, you just might get it". Seriously, though, once I digest it I may post another question. Thanks again

    2. Walter Ahlgrim | | #7

      Jon Please help me understand your point. When I look at the first few lines in both papers you linked to seem to be at odds with your statement.

      “Cooling coils provide both sensible cooling and moisture removal. Data from field test studies have demonstrated that the moisture removal capacity of a cooling coil degrades at part-load conditions, especially when the supply air fan operates continuously.”

      “This is due to low sensible loads, variability in moisture removal effectiveness of air conditioning, and variability of moisture sources. As a result, some builders and highperformance home programs resort to using dehumidifiers.”

      If I missed where the papers suggest an oversized unit is preferable please point me in the right direction.

      Walta

      1. Jon R | | #9

        I'll ignore the straw man attempt about preferable.

        Jump down to the specified Fig 7 - which will make it clear that with a normal intermittent fan, even 5x load may only shift SHR from .73 to ~.77

        The second quote isn't at odds - it also applies to even perfectly sized AC.

        Here is an example (page 30) of an AC unit that it going to spend a lot of time with no dehumidification (SHR ~= 1 at < 1/2 load), even when perfectly sized.

        https://www.nrel.gov/docs/fy11osti/52175.pdf

        The right answer is "it depends" - check the figures vs making assumptions about about one moderately over-sized unit under performing (humidity) some other perfectly sized unit.

  3. Steve Knapp CZ 3A Georgia | | #5

    William,

    This link (https://www.energy.gov/sites/prod/files/2015/10/f27/ba_climate_region_guide_7.3.pdf) opens a document that identifies climate zone by county.

  4. Walter Ahlgrim | | #6

    Modern heat pumps are not what they were in the 1980s my conventional heat pump heats my house down to 6°F without any back up heat.

    By modern I mean you need 4 feathers built in to your heat pump. 1 Variable speed compressor. 2 Two electronic expansion valve. 3 Variable speed indoor blower. 4 Two way communication from the thermostat to both the indoor and outdoor units. No this will not be the cheapest heat pump in any company line up.

    Being in Texas you are likely to need a dehumidifier because your dew points are so high. Also in Texas your equipment is almost certainly in your attic and attached to leaky flex ducts that look like a dead spider. Find and fix the leaks and straighten the kinks.

    I say size the unit close to your cooling loads. I am guessing that is going to cover 90% of your heating with a backup heating element to cover the few hours when it is very cold.

    You may find Matt Risinger videos on YouTube interesting as he is in Texas and has similar issues.

    Walta

    1. William Ayers | | #8

      Steve, according to your linked source were are in 3A. Hot-humid. Which I didn't need to be told, but now it's official. Thanks

  5. Expert Member
    Akos | | #10

    You are VERY far from needing low ambient heating. That is for places that regularly hit sub 0F temperatures.

    In your mild climate with a 24F design temp, almost any heat pump will work. You still want one with decent low temperature performance just for efficinecy. Your best bet is to search through here:

    https://ashp.neep.org/#!/product_list/

    Also your heat loss calcs seems to be a bit too high. 29k BTU is about the heat loss for a somewhat insulated 2000sqft older home up here in zone 5. You can try some of the other online ones such as coolcalc or loadcalc and see what they come back with.

    Zoning tends to add unnecessary complexity to most setups. For a single story structure, it doesn't make much sense.

    Focus instead on getting good air distribution in there and keeping your ducting out of the attic. That will make much bigger difference for comfort than zoning the place and cost less.

    1. William Ayers | | #11

      Thanks Akos, I understand that I would not need one of the hyper-heat type units for here. We rarely get down to the low to middle teens in any given year. I just want it to be reasonably adequate when it does. The mini split option appeals to me because it doesn't make sense to me to put ducting in the hot attic or in the crawl space which is not enclosed(don't want to close it in for particular ground moisture problems). We have poor air circulation from the living areas to the bedrooms so that is why I was considering zoning. One unit in the living/ kitchen area, one small unit for our master br/ bath, and a concealed ducted unit for three small bedrooms that are clustered at the end of the hall. I would also like to be able shut down the three seldom used bedrooms to a minimal level most of the time. The concealed unit would require only minimal ducting. I'll take a look at the load calculators that you mentioned. If our experience with our windows units is of any value it does take an 18k plus three 5k units to keep the whole house comfortable in midsummer. I'd appreciate any thoughts, and thanks for your time.

      1. Expert Member
        Akos | | #12

        Generally for single story, the best solution is a single wall/floor mount in the living space and a slim ducted unit for the bedrooms.

        If your living space is chopped up, this might not be enough, than a second ducted unit would work best. Each zone adds a lot of extra cost and also forces you to a multi split, which are less efficient and have much worse modulation range. If you can stay with two zones, with each mounted on its own outdoor unit, you can have a much better setup (efficiency and comfort) for significantly less cost than a three zone multi split.

        Generally one central wall/floor unit in the living space does a decent job of cooling the space but doesn't work well for heating. One option here is to keep some of the electric baseboards around the perimeter rooms but only use them to provide just a bit of extra heat there to even out room temperature. The central unit would still do most of the heating but you'll get much better comfort.

        A decent setup in the ballpark of your load would be
        living space:
        https://ashp.neep.org/#!/product/28983
        bedrooms:
        https://ashp.neep.org/#!/product/29059

        I would be tempted to go with a Fujitsu 12k hyper heat for living space and 9k hyper heat for the bedrooms, as they get much better low temperature performance at 17F. There are also some other brands that make excellent mini splits that are much cheaper than the Mitsu/Fujitsu/Daikin.

        For the unused bedrooms, you can partially close the registers when not in use, just make sure you are still getting reasonable flow through the unit. Most minisplits have no issue with a fair bit of reduction in flow, which is actually a good thing for humidity removal.

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