Blowing Hot and Cold

[Juvenal]

Any Forum-ites with experience with a heat pump?  I have a house (not young) with oil heat, but had no A/C for many years, aside from one wall unit in a small "refuge."

Finally, I thought, "Well, they say a heat pump is a good idea."  They did not say they are cheap.  They are clearly not, if somehow added on top of the system that came with the house.  No way to make a heat pump connect with my ductwork (what I have is baseboard heat).

Yet I still went and got one (hate, hate summer heat!).  The hot/cold diffuser on the wall in the LR is in the only sensible place for it in the house, but that leads to the foyer being the warmest/coolest place--and I spend minimal time there.  Still, it seems to deal with hot/cold problems well enough, if one can take the back rooms as "just" warm/cool enough as needed and thermal balance drifts around.  I seem to manage.  Although this winter will be a signpost of expense, since I'm using the heat pump for some days on end ("mellow" as it's been here in the NY Metro area).

My question is: At what temperature does the balance between oil/gas heat and heat-pumping balance out?  For no real reason I make the decision at about forty degrees and that's the switch-over.  Is lower a good idea?  Yes, yes, every heat pump touts its benefits, but is a there a m.o.l best point?  If it's at zero or lower (Fahrenheit extreme), how well does one or the other do best?  On the other hand I could put in a wood stove and shovel dollar bills in, laughing at expense.

[Puget]

I have two air source heat pumps (aka ductless mini-splits) put in last fall. The newer models designed for cold climates are very efficient even well below freezing. The only day I turned them off this winter was when it was in the single digits to below zero, and it seems to be running its defrost cycle constantly.

The rest of the time, I leave the heat pumps on since they are most efficient when you leave them set to constant temperature (they aren't so good at quickly changing the temperature).

I have the boiler set to a few degrees lower, with the thermostat for the boiler (Nest sensor) in the living room (heat pumps are in the dining room and main bedroom). That way, the boiler kicks on if the heat pump in the dining room isn't keeping the living room warm enough. I also have a gas fireplace in the living room that puts out a lot of heat and that I like to have on in the evenings and when working from home when it's cold. This system has the boiler use down to just a few hours cumulative per day.

Of course, the relative cost of operating each depends on your electric rate and your oil/gas rate. In my case, I also got solar panels, so I'm generating part of the electricity to run the heat pumps. I'm also more motivated by reducing my environmental impact than I am by saving money.