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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).

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  • What's "very cold"? Are you really at the lower bounds of the mini-splits' efficacy? I wouldn't expect that you'd need to bust out the ol' heap until below zero F, or darn close.
    – isherwood
    Commented 18 hours ago
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    2 seems like the obvious solution to me, but the nuances of airflow in your home will dictate what you need to do. You should also install a proper modern thermostat, even if it is "dumb". You need accuracy, at least. I'm not sure asking the internet is the right move here. Experiment and find what works best.
    – isherwood
    Commented 18 hours ago
  • This needs considerably more detail to answer well. Carrier probably makes many models of Mini-Split with many different operating characteristics. While I doubt they have bothered to equal Mitsubishi's performance, they may be adeqaute for you needs or not, but without details we can't tell you. How cold is cold, what specific model you have, how cold that specific model operates, and your cost per kWh and gallon of oil would be a minimum starting point.
    – Ecnerwal
    Commented 15 hours ago

3 Answers 3

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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.

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  • Good info. Regarding that last point, lowering temps at night is a great way to save energy (use more bedding), and it also makes sleeping easier for many of us to have cooler air. Maybe a longer, more gradual ramp-up in the morning is an option.
    – isherwood
    Commented 40 mins ago
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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).

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    Just a note to go along with the automation: mini splits commonly use proprietary controls. Some are wireless: the remote tells the indoor head what to do and the temperature sensing is done in the indoor head. Others have a wired control but it isn't the traditional 24v type. However, many mini split systems have an accessory available which bridges from the proprietary controls to something else: 24 v, modbus, or other. A bridge like that could be the missing link in having a single control system for the minis and the furnace.
    – Greg Hill
    Commented 17 hours ago
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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.

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    Please stop acting as if all heat pumps are "That '70s Heat Pump." Lacking an exact model, it's hard to know what themperatures the specific units installed operate to efficiently. Mine work to -15°F, but then, they are not Carrier, either. At 14°F they are quite efficient.
    – Ecnerwal
    Commented 15 hours ago
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    Modern air source heat pumps have a COP of about 2.2 even at 0⁰F, nowhere near as good as at higher temperatures but still well above resistance heating. Their ability to move enough heat is more the problem. Commented 10 hours ago
  • The answer has been amended in light of the comments. Commented 46 mins ago

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