How to balance minisplits and oil furnace for winter heat?

Question

We had a full set of Carrier mini-splits installed (1600 sq. ft, 2 stories, 5 heads), added to a very senior oil furnace.

We're still getting used to them. We opted out of the "smart" thermostat.

We used them for a couple weeks of heat. Then, it got very cold and we had company, so we switched to the oil heat -- oh! it was so beautifully warm. The ancient thermostats for the oil furnace are all wonky, so set at 58, the house was like a greenhouse. Decadent!

And now we are back to the splits. It's chilly. Our daytime comfortable temperature is maybe 64, even 65, and our nighttime temperature is about 61.

So, I am writing to ask you how to balance these two heating systems.

(1) put them both at (what we'd expect to be) 63 and let them fight it out? (2) set the splits at 63 and the furnace rather lower, to kick on only when the splits can't get it done (will they waste a lot of power trying and failing)? (3) turn the furnace way down generally and only "ask" for heat when cold?

We're sort of doing (2) right now, somewhat grumpily, but there is one super-cold bathroom, where the radiators reach but the warm air from the mini-splits doesn't.

All energy is expensive here, so we sort of want to prioritize the electric for the environment (nuclear/gas mix). And we just got attic insulation, but the windows are old (historic).

Answer 1

As someone who heats only with Mini-Splits: if we make the generous assumption (lacking details) that your dealer didn't foist something unsuitable for your climate upon you, the most likely cause is not being used to how mini-splits typically work when you set a temperature.

If you're cold, turn up the temperature setting. The split measures the indoor air at its head, and almost always no place else. The temperature where you are is almost never going to be the temperature the head is set to, and that doesn't matter. You just need to figure out what temperature at the head equals comfortable where you are.

You might also need a fan to help move air around if the locations of the split heads are not such that they do a good job stirring the air themselves. So if you don't want to run the oil for the super-cold bathroom, you need to bring it warm air from a mini-split head nearby (or from the top of a room the mini-split head nearby reaches.)

Second-hand advice from a factory rep (who spoke to a friend, thus second-hand) is that it's generally best to leave air-air inverter-drive heat pumps set at the same temperature rather than dropping them at night - because the additional heat load when you crank them up in the morning comes when the outside has (typically) cooled down overnight, and forces the pump to operate in a more inefficient part of its operating envelope than it would be at from simply maintaining temperature without a sudden load increase when it's colder outside.

Answer 2

There are two factors to consider:

Outside Temperature

Some smart thermostats are actually able to either measure directly (with a remote thermometer) or possibly estimate (based on online information for your approximate location - I don't have this myself, but it seems logical and I expect some systems would do this if they are internet-connected anyway) the local outside temperature. That is key to determine efficiency of a heat pump and also whether it is even practical to run the heat pump. If a heat pump is 2.5x as efficient as your oil heat when the outside temperature is 50 F and 2x at 40 F and 1.5x at 30F and 1x at 20F (totally made up numbers and probably not a linear relationship) then once the temperature gets below 20F it makes sense to switch over to oil heat. But your ordinary indoor thermostats have no idea what the outside temperature is and so they can't make the decision for you.

Automation

While the specifics vary (e.g., if your heat pump uses 24V controls but your oil heat uses 120V controls then you will need some relays/contactors/etc. to connect everything together, which may be rather non-trivial), there are smart thermostats that will automatically run the heat pump at higher outside temperatures, run the oil heat (auxiliary heat/emergency heat, terms vary) at lower outside temperatures based on user-configured setpoints and possibly even run both in certain situations (outside temperature high enough to safely and effectively run the heat pump but inside temperature low enough that getting more heat quickly would be beneficial).

Answer 3

If economy, as well as comfort, is a goal, then you'd have to check, for your home, the heating oil usage and cost vs. cost of electricity to maintain the same temperature. Admittedly, measuring usage of oil is tough, unless you check frequently (with a dipsticK?).

Another issue is that the changeover point depends on outside temperature. A heat pump is most efficient when there is only a small temperature differential between inside and outside.

If it's 50°F (10°C) outside, and 64°F (18°C) inside, a heat pump is fairly efficient, likely costing less to run than heating with oil. If it's -10°F (-23°C) outside, and 64°F (18°C) inside, a heat pump might be unable to derive sufficient heat from ambient air, and it's resistance heater would have to turn on, possibly costing more to run than heating with oil. Of course, if you have a ground loop, i.e., a geothermal heat pump, it's far more efficient in winter than one using air alone.

So the crossover point depends on cost of oil vs. cost of electricity, as well as temperature.

However, your concern is the environment, including production of CO2, a greenhouse gas. That depends on the source of electricity -- there are still locales using coal-fired generators, and burning coal produces more CO2 than burning oil or, better, gas, for the same amount of energy. If the source is nuclear (or solar, wind or hydroelectric power), using the heat pump 100% would be best for the environment. Since you state your locale uses a mix of nuclear and gas-fired generation, the answer is murky. Gas produces less CO2 than oil, but inefficiencies in generation and distribution might give home heating with oil the edge in reducing CO2 emission in coldest winter weather... or not.

The simplest solution to minimize carbon footprint:

• Get accurate thermostats for both systems.
• Mount them in the same locations.
• Set the heat-pump thermostats to a comfortable temperature in each room.
• Set each oil-fired heating thermostat to about three degrees colder that the nearest heat-pump thermostat.

That way, the heat-pump will run most of the time, and, in coldest weather, the oil-fired boiler will kick in only if the heat-pump can't maintain the temperature.