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Community and Q&A

Heat pump for frost protection in an unoccupied house?

Irene Heim | Posted in Energy Efficiency and Durability on

We have a vacation home that gets only very occasional use in the winter. Currently, the only source of heat there is a Rinnai 560 WTA propane heater, which we set to 42F when we are not in the house. This has worked to keep the water pipes from freezing, but propane bills can run quite high.

We are now thinking of adding a Mitsubishi Ductless System that both heats and cools (MUZ FH12NA + MSZ FH12HA). (There was so far no cooling system in the house.) Bu we are wondering whether the heating function on this system will meet our needs. Apparently, the thermostat that comes with it cannot be set lower than 50F. So we would either have to keep heating the house to a much higher temperature than previously when we are not there — i.e. to 50 rather than 42, thus much beyond what’s required for frost protection. Or else we would be using the heating function only during those times when we are there (typically only every other weekend or so iin the winter). The first option probably means that any savings due to the heat pump’s higher efficiency vs the propane heater would be more than offset. The second option may mean that it’s not worth it for a house used so little.

My question is: Would it be feasible, and would it make sense, to combine the system with a different or an additional thermostat? E.g., could we just add an external thermostat that simply cuts power to the entire system unless indoor temperature is below 42F?

Thank you for any advice.
–Irene

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Irene,
    As far as I know, it is not recommended to try to jury-rig a ductless minisplit with the aim of lowering the minimum thermostat setting. For one thing, that would invalidate the warranty.

    Depending on how much you are paying for propane, and how much you are paying for electricity, it may still be cheaper to heat your house to 50°F with a ductless minisplit than to heat your house to 42°F with a propane space heater.

  2. Charlie Sullivan | | #2

    What climate is the house in? Or, better, what's the zip code so we can look up specific climate data. From that we can figure out how much of a difference it makes to heat to 50 vs. to 42.

    Just as heat pumps are more efficient at warm outdoor temperatures, they are more efficient at cooler indoor temperatures. Unfortunately, that characteristic is not well documented because heating to 50 isn't the standard application. But I think that a heat pump could have amazingly good performance in this application.

    You might be able to rig a kludge that would make the thermostat think it's 50 in the house when it's really 42: get a box about the size of a shoe box, and put a very low power electric heater in it, like maybe 2 W. Adjust the heater or the box so that the box temperature is about 8 degrees warmer than the room temperature. Put the thermostat in the box.

    At first I though the box should be metal, for extra fire safety, but then I realized that the wireless connection between the thermostat and the heat pump probably wouldn't make it through the box. Then I thought is should be a ceramic urn, but that has more thermal mass than you'd want. So maybe a large plastic electrical junction box or "NEMA enclosure" which is made of plastic with nasty toxic fire retardant chemicals in it. If it doesn't get warm enough, you could put some insulation on some of the faces of the box; if it gets too warm you can drill holes.

    If you have some small always-on electronics in the building (an alarm system?), it probably provides at least 2 W of heat, so you could use that as the heater. Even a doorbell transformer dissipates about 2 W all the time.

  3. Randall Thomas | | #3

    You could add a thermostat, to control a contactor, that would disconnect the feed to the heat pump.

    But: I'm not sure that the savings would cover the cost; I'm not sure it could be configured to meet code; Plus, I think the unit might loose any programming that you have in the heat pump thermostat.

    Personally, I'd look at trying to install heating wire and insulation on the pipes. Or, possibly insulate the piping and install return flow pumps to protect the plumbing. What exactly are you trying to keep from freezing.

  4. Expert Member
    Dana Dorsett | | #4

    Methinks it's the wrong question and probably the wrong solution for he freeze-protection part, but maybe we can narrow in on that a bit.

    How cold is the climate? (Can you share it's location?)

    Is draining the plumbing for freeze protection an option?

    How air tight and insulated is the house?

    What type of windows, and total window area?

    Have you run a heat load calculation on the house, at the coldest outdoor temperature that you'd actually be using it?

    Is a 1-ton air conditioner big enough for the cooling load?

    What are your electric rates?

    What is the price of propane in your area?

    The average outdoor temperature, and the output capacity of the mini-split at the outdoor & indoor temperatures makes a large difference in it's operating efficiency. A mini-split running full speed at an outdoor temp of 25F with an indoor temp of 70F might have a COP of about 2, but if you drop the indoor temp to 50F and it's running near it's minimum modulation it could have a COP of 4 or even 5. If you are sizing it for the heat load at a 99% outside desgn temp of +10F with an indoor temp of 70F it'll almost never run at anything other than minimum speed at an indoor temp of +50F, and will beat it's HSPF tested efficiency by a large margin.

    But with the information we have to work with so far it's impossible to even take a WAG at the as-used efficiency, &/or whether it would be cheaper than heating with propane in an 80-82% efficiency burner.

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