We bought a house, it was built in the 1920s and the main floor uses hot water radiators as the primary heat source (boiler is a Weil-McLain CG-4). I also have a three zone Mitsubishi mini-split system, with two zones upstairs and one on the main floor. The upstairs also has electric baseboard heat in all rooms.
Understanding which sources of heat to use this winter is a challenge. The mini-split can be an effective heat source compared to electric baseboard heat, but the efficiency depends on the outside temp and how much heat each indoor unit is requesting. My understanding is that mini-splits are most efficient when running on low. This means setting large setbacks temps at night could actually end up using more electricity than keeping the temp constant.
However, there are other factors that can affect this. Depending on how insulated and air sealed my house is, constantly heating with the minis-splits could be cost-ineffective if my house is a leaky sieve.
Electric baseboard heat should be my last option, and when it’s below ~5 F outside, it’s my only option for the upstairs, as my mini-split can’t heat below that temp (newer models can operate another 15-20 degrees below that).
One of the better resources I’ve found on heating using minis-splits is a report prepared for the U.S Department of Energy, Long-Term Monitoring of Mini-Split Ductless Heat Pumps in the Northeast.
Temperature setbacks are not an effective strategy with MSHPs: when temperatures are set up, the unit runs at maximum capacity (and lowest efficiency) to return to set point temperature.
The report is over 120 pages long, many interesting observations. However I am cautious about immediately applying all of their recommendations as my house is not as well insulated as the studied houses, and I also have the main floor boiler heat system.
I’m still new to this heating thing, I only have one partial month of utility usage billed. I hope to understand the efficiencies of my radiators/mini-split/electric heat and when to use them and not use them.