I have a newly installed (this summer) Mitsubishi mini-split system. It consists of two zones supported by one outdoor unit. It serves about 1800 square feet of living space, a large open cathedral ceiling area (zone 1) and smaller area (zone 2). The house is located in south western New Hampshire. The other heat source for the house is a hydronic system served by a propane fired boiler. The outdoor unit is a MXZ-3C30NAHZ2-U1 3 ZONE H2i HYPER HEAT PUMP, 2 1/2 ton, 30K BTU. The larger living area has a MSZGL24NA wall unit at 24K BTU and the smaller unit is a floor unit, MFZ-KJ12NA-U1, at 12K BTU. The system worked great this summer with the cooling and it so far its been great for the heat in both zoned areas. My question is how to operate this system to achieve the lowest electricity consumption. I see a lot of “set it and forget it” recommendations but still question this. Case in point, in the morning on a cold day the system is running and providing heat with a temperature setting on both zones at 67F. If its a bright sunny day, with lots of south facing glass, the sun eventually does its job and will warm the area up to over 70F. Is it less costly to just shut the units down during that time and turn them back on as the sun goes away? Also the smaller zone is for a bedroom area, during the day no one is spending any time there, should it be shut down? So my questions are; does it cost more to cold start this system vs leaving it running all the time and am I loosing efficiency by running only one zone? Thank you for any insights you can provide.

I would set and forget.....your house is continually loosing heat. The mass inside the house slowly changes temp if you turn your heat off. Then when you turn it on that mass has to be heated backup and your unit usually has to run at a higher power less efficient setting to get the house back to temp thus no savings.

I think it has been proven that set back therms really don't work in majority of cases. The only time i see them working is if the house looses so much heat quickly and the occupants leave for extended time that so the heat just doesn't run and saves money.

Yes, unless you have a house with very little interior thermal mass, set and forget is best.

If your heating system and heating zones were designed and installed properly, you should have thermostats that will stop calling for hydronic radiant floor heating and mini split heating in areas that receive passive solar heating.

Unfortunately, it is not uncommon for passive solar to be designed and implemented very poorly. There's lots of info about this you can read in the instructions for our passive solar design calculators on our website. Without overly boring you about the issues and details, suffice it to say that the clear sky maximum passive solar heat gain should never be more than the house heat loss that occurs at the average daily outdoor temp.

I question the difference between the mini-splits operation vs the propane fired boiler. When the sun warms up the house the room thermostat no longer calls for heat and the boiler is completely off, no propane consumed, no circulator pumps running, etc. The mini-split on the other hand continuously runs, albeit it may not be pumping out much heat, if any, into the living space but the outside unit is running and the wall units are moving air very slowly, therefore some current is being drawn. Without throwing a power monitor on it, I don’t know how much power is being consumed during those times. So my question is what consumes less power, letting it run all day or shutting it off for the times during the day when there is no call for heat. Does starting up the mini-split consume more power to bring it up to heat operation that it negates any benefit of shutting it off for 6 hours or so when there is no call for heat?