I live in Seattle and want to install a heat pump in my house for heating (don’t need much AC around here). I’m trying to decide between a 2-stage compressor and a single-stage compressor unit. I’ve read that the HSPF ratings for region iV are reasonably accurate for a properly sized heat pump,so I can get those from the dealers and compare units. For the two units I priced, the single stage is 4% less efficient than the 2-stage IF PROPERLY SIZED. But I will never get a heat pump that is sized exactly right..

What I can’t figure out is how the seasonally averaged efficiency (seasonally-averaged COP, or adjusted HSPF) is affected by over-sizing. Is a 2-stage compressor significantly less sensitive to losses due to oversizing? Or, in our mild climate, is a 2-stage compressor complete overkill?

Robert