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My new house has a natural gas boiler with three zones of radiant baseboard heat (one per floor). It also has a mini-split system (again one head per floor). The mini-split was advertised as air conditioning, but I realize that it's actually a proper heat pump. My situation is somewhat similar to Mini-split vs gas boiler for heating, although I think my numbers are a bit different — my last gas and electric bills were $1.20/therm and $0.22/kWh (both excluding fixed charges).

It's my understanding that there's probably a cost-savings cutoff in favor of the mini-split when we're just keeping the chill away and the exterior temp is above 60°F. And there's a better-for-the-world emissions cutoff somewhere below that — maybe as low as 45°F (and which will only get better as my state switches to more renewable sources for electricity.)

But of course there's a complication. As the lone answer to the other question suggests, the radiant baseboard heat is hard to beat for comfort. But in this house

  • the first floor mini-split head is in the kitchen which, because of the layout, only has a small section of baseboard radiator and so gets kind of cold;
  • the second floor mini-split is a in a guest room which we only use rarely, and the other rooms are bedrooms which don't have their doors open all the them; and
  • the third floor mini-split is in my home office.

So, all of this together comes to the question. On the second floor, I'll probably just use the radiant heat, because it heats the bedrooms which are in use. But on the top floor, when it's above 50° or so outside, I'm thinking of leaving the radiant heat set to 60° and running the mini split in my office (when I'm there) to keep the heat up to 68°. The baseboard thermostat isn't next to the mini-split, so it might turn on if the other rooms on that floor dip below 60°. On the first floor, I'm thinking to primarily use the radiant heat, but if we're in the kitchen and it's chilly, also running the mini-split to warm up that room.

Are there any perils in running the system this way? It seems optimal for comfort and might also increase energy efficiency.

  • What make and model is your multi-split? How do you want this setup to be controlled, for that matter: via the multi-split's remotes, or via separate thermostats in each zone? – ThreePhaseEel Dec 28 '18 at 22:16
  • Mitsubishi Mr. Slim, GE-15NA. Right now, the mini-splits are controlled by their remotes, but I have a grand design of getting the flair.co pucks and automating the whole deal. – mattdm Dec 28 '18 at 22:41
  • Are the minisplit heads ducted or ductless? The Flair system is basically an IoT VAV system for a traditional central ducted system, and probably not one whit useful for your application, by the way... – ThreePhaseEel Dec 29 '18 at 1:30
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    @ThreePhaseEel Ductless. Flair has two different products — the duct valves, but also an IR controller for mini splits. – mattdm Dec 29 '18 at 2:13
  • Do you want the baseboards to work even if the mini-split is non-functional? Also, how attached are you to the Flair's smart features? – ThreePhaseEel Dec 29 '18 at 3:09
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Running them at the same time won't cause breakage

As Ed Beal indicates in his comments, running both systems simultaneously with the mini-split set for heating won't cause trouble -- in fact, your mini-splits have support in their controls for running an auxiliary heating system while the heat pump runs if it can no longer keep up with heating demand.

However, you'll need to get clever with controls here

Normally, what I would suggest for your type of situation (no significant concern for running heat during power outages) would be to have the mini-splits control the baseboards using their auxiliary heat control facilities. However, due to the lack of a second floor head outside the little-used guest room, this approach won't work on the second floor, for sure, and also seems not quite compatible with your control goals in general, given that your economic crossover point is rather high.

As a result, you'll need to do some dancing with your control setup to get the right mix of baseboard and mini-split heat for a given situation -- most low-voltage thermostats should be able to handle controlling the hydronic zones on your behalf, although you may need to route C wires to them, while the Flair device you linked in the comments can handle bending the mini-splits to your will.

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I have installed both systems and lean to gas even with baseboard because it has been cheaper in my area if it is available. But since my current home is only electric I changed it from partial baseboard and central electric to mini split, my power bills are lower, but if you live in a very cold area they don't work as well, many of todays systems work well below freezing and into the teens F. Would it hurt to have both systems? NO it would be fine but you would need to understand if it is cheaper to run the gas fired or electric mini split and this depends on the outside temps and your utility rates. 50-30 years ago in my area electric heat was the way to go, I changed my last house over to gas and no gas at my current location so I now have mini split system. So it depends on the local temps and cost of electric or gas.

  • There's also situations where the minisplit might be running full blast but not able to do the job by itself, and in that case, you want the boiler to fire and make up the rest using the baseboards. – ThreePhaseEel Dec 29 '18 at 2:54
  • I agree and in my answer I said it would not be a problem, run the system that cost the least is common sense in my opinion if it won't cause problems. – Ed Beal Dec 29 '18 at 3:16
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    I guess the crux of the question really is if there's a problem with the "make up the rest" idea. Can they run in tandem without issue, and does that make sense? – mattdm Dec 29 '18 at 3:20
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    Yes they can run in tandem, the mini split control will be more accurate in my experiance but if you need more heat run both not a problem because when the temp is reached one or the other will stop for that room. – Ed Beal Dec 29 '18 at 3:27

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