I've got a 900 sqft in-law apt with baseboard heat + central AC. The current AC unit is 10 seer, oversized, and runs with flex duct through a cathedral ceiling. The system is about 13 years old and now there is a very slow leak somewhere, which dropped the pressure and caused the coil to freeze up. I figure one charge now to get my parents through the hot months and then upgrade the system in the Fall. In the Winter I get major ice dams over this part of the roof. I'm sure its a combination of recessed lights, the penetrations of the duct work, and shoddy sealing of the envelope. I was planning to attack this problem this year by replacing the cans with track lighting and covering the AC vents during the Fall/Winter months, but now I'm thinking of abandoning/sealing off the duct work and installing a dual-zone mini split system. This way I won't have the penetrations in the cathedral ceiling (maybe even blow insulation into the ducts?) and hopefully make it a little more comfortable for the folks. My other hope if I go this route is to install a heat pump system instead of straight AC. For a majority of the shoulder season the main house stays pretty warm with the help of the wood insert, but the in-law is being heated from the boiler (90% eff system with outdoor setback). My thought is it may be more efficient/cheaper to use the heat pump during mild weather than the boiler. HSPF of heat pump is 8.4 KW/h cost is $.18 Boiler is 89%+ eff heating oil is $3.50+ (and rising) I'm sure the system eff. of the oil boiler is closer to low 80%. Are there other factors to consider? At what outdoor temp will it make better sense to turn off the heat pump and use the boiler? From my understanding heat pumps are only efficient when its warm.