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.
Best Answer
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.