I just had a 4 head Fujitsu mini split system (not the XLTH) installed. They work really well currently. Our home is pretty well insulated so they do a good job keeping the majority of the living areas at set point.
We have a 200K steam boiler that is probably 20 years old but runs perfect and gives us nice even heat in the winter.
What the most cost efficient way to run these in the winter? I believe they give out 100% heat down to 5 degrees. Heat load per manual J is about 55K BTU/hr and specs for these should put out 34K BTU/Hr (not including one in attic space not used a lot) at design temp. Do I just leave them on all winter or at a certain point should I just switch them off?
I also have an ecobee thermostat with 4 wireless sensors. I could run sort of a hybrid system where I shut off the radiators that overlap with the mini splits in the main living areas, and use the wireless temp sensors/steam heat in the other rooms that are a bit removed.
Any mathematical forumla to figure this out? I just don't want to run these for a month in the dead of winter and then get a huge electrical bill.
Some interesting cost comparisons on this site:https://newenglandcleanenergy.com/energymiser/2017/02/20/how-ductless-mini-splits-beat-oil-heat-even-in-a-new-england-winter/