Just thinking back to all the info when we redid our HVAC. Has your furnace pro done a load calc? If your furnace is like most, it is probably "oversized" already!

I would think, keeping your lower level at 70 degrees might INCREASE the efficiency of the whole house, unless, of course, there is little to no insulation in the lower level.

Heat loss is low for a basement below ground if you do a few simple things. Insulate the rim joist area and insulate the walls, but do some research to make sure you stud and insulate the right way for a below grade basement, critical or you'll have problems down there.

Heat will transfer to a source lower in temperature. so some will move into the floor but since heat rises the ceiling above will be hotter than the main level floor below and will be transferred to the main level, so there won't be much waste and this is why you insulate the walls.

Under Ideal conditions your heat pump can produce 36,000 Btu's so does the backup need to kick in often? I'm guessing it does during the colder months and even though I'm not a big fan of mini splits but in your application a 12000 btu unit would work fine to supplement the basement.

Thank you Mary. I don’t know if he ever did a load calculation. I will definitely ask. I am surprised when you say oversized. I was in the impression that they would undersize just to keep cost down. :-)

Thank you Kevin9408. I was not planning to insulate. There was insulation blanket on the concrete wall but I placed the stud against it. My propane backup does kick in below 30F. I was actually planning to even increase it as I have 1000g buried tank and so far prices are pretty low. I prefer gas over heat pump warmth. Would baseboard heating worth the cost?

IMO hot air rises so keeping the basement warm all the time will pobably actually be more efficient and IMO you needed a lot more insulation if you are worried about money saving.

To add 10,000 btu's to your basement you'd need 2 1500 watt electric baseboard heaters. It's a lot cheaper than adding a mini split but operating costs will be 3 or more times higher than the mini split in heating mode. I'd advise running a dedicated circuit for each heater using 12 gauge wire with a 20 amp breaker for each 1500 watt heater if this is the route you decide take.

You could run the wiring (less than $100) now and drop the heating vents per the HVAC companies recommendation, and if it just isn't enough heat buy and install the baseboard heaters later.