Heat Pump efficiency loss with oversizing
![](https://secure.gravatar.com/avatar/55bd81ad86ab52633dfd9fad30da1a0f?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Fr.png&r=g)
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
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies