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Community and Q&A

Heat pumps and traditional heating

user-982243 | Posted in Mechanicals on

We are about to install a Fujitsu minisplit heat pump for our cooling and heating needs in our 100 year old 2000 sq foot row-home in Montreal Canada, (zone 8 million). We currently have electric hot water radiator heating.

I am curious how I manage to keep the two units from “fighting” each other over who gets to heat the house. I know that when the depths of winter arrive, the heat pump alone will not be able to heat our home.

Is it simply a matter of setting the thermostat for our main heating system a degree lower than the minisplit?

Also, the old heating system has a really old manual thermostat. I was thinking of replacing it with something like the Nest, but again, I’m not sure that this would be a good idea as the minisplit will be doing most of the “thinking.” Maybe a slightly stupider programmable thermostat would be enough?

Basically, I don’t want to be micro-managing which system is doing what.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is it simply a matter of setting the thermostat for our main heating system a degree lower than the minisplit?"

    A. Yes, although I might choose a difference of 5 F° rather than 1 F° for the difference in the two thermostat settings. Note that there isn't always a good correlation between a minisplit thermostat setting and your indoor air temperature. If you like (for example) your indoor air temperature to be at 72°F (22°C), then you should fiddle with the minisplit controls until you end up with the result you want (indoor air temperatures of about 72°F). Then set the thermostat for you hydronic system at 67°F (19.5°C).

    During a protracted cold spell, you can always turn up the thermostat on your hydronic system temporarily to improve your comfort.

  2. Expert Member
    Dana Dorsett | | #2

    Since the mini-split infers the room temperature from the temperature of the air entering the head a the top, there is an offset between the setpoint temperature of mini-split and the average room temperature. That usually increases during colder weather, less in milder weather. It's not uncommon to need to set the mini-split to 24-25C to maintain a room at 20C in mid winter, then have to back it down to 22 C during the shoulder seasons. When in doubt, aim for the high side for the mini-split.

    For the electric boiler system, leave the mini-split at a fixed temperature, then use a programmable setback thermostat, rather than a learning thermostat (such as the Nest). Behavior during the set back periods would allow you to assess whether the mini-split is already running at maximum and just can't keep up, or whether it simply needs to be bumped up to deal with the offset between room temp and the entering air temp at the head. Decent programmable setback thermostats can be had for under $30. A learning thermostat might mask the evolving change in that the offset, running the electric boiler at a higher duty cycle or modulation level than it needs to.

    For a few hundred you can add a wired wall thermostat for the Fujitsu to eliminate the shifting offset issue, in which case you install the thermostats to the two systems near one another, and set the mini-split 2C higher.

  3. user-982243 | | #3

    These answers are hugely helpful, thanks to both Dana and Martin!

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