I am remodeling the basement. Area is ~ 1000sft which includes 2 bedrooms and a bathroom. This is below grade and we are in zone 3 (PA). The basement walls are insulated by code with the insulation blanket. There are 2 HVAC units (house is built 2015). The first floor is 1500sqft with HP 60k btu with propane backup (buried a 1000g tank). Second floor is HP with electric backup.

I requested some options from local HVAC companies. One suggested to open 4 supplies and 1 return. I am concerned that this will be not enough to keep the basement at 68-70F. Is 60K (95.5% efficiency) big enough to provide heat for both 1st floor and basement?

I am open for either putting a second furnace / mini split but I would like to avoid using baseboard heating for the main area (OK for bedrooms).

I keep remembering about colleague who once told me.. cry once when you buy it, or cry every time when you use it.

I would definitely appreciate any guidance.