On Tuesday, September 28, I was in the audience at the Pacific Energy Center in San Francisco as Alex Wilson, Founder and Executive Editor of Environmental Building News, gave a presentation about misguided pursuits in green building. He covered all-glass buildings, building-integrated wind turbines, and residential ground source heat pumps.
An ellipsis (…) indicates that I was not able to capture the words or thoughts skipped. The presentation is transcribed as accurately as possible – punctuation choices are mine.
– – –
Let me move on the last issue – this is more at the residential scale. I’ve been frustrated for a while. This issue first struck me when I was a juror for a design competition maybe 10 years ago … almost without exception, these homes had ground source heat pumps. They went to extraordinary lengths to reduce heat loads…yet they didn’t benefit from that on the cost side by reducing the cost of the heating systems.
Ground source heat pumps are great … They can move heat from outside a house to inside, even if it’s colder outside than inside. That’s how a refrigerator works … does it by changing phases, alternately compressing and expanding the refrigerant …
Okay, so on the surface they’re very attractive, but they’re expensive. Let me explain the difference between an air source heat pump and ground source heat pump. Air source uses the air as a heat exchanger. Ground source uses the ground, which is more of a constant temperature during the year … if you’re heating, you want as warm a source as possible. The heat pumps can be operating in reverse in the summer to provide cooling. That’s the theory of why a ground source should be better than an air source heat pump. But there’s a lot of cost … either digging wells or trenching pipe … the costs are pretty high.
In our area in New England, we’re seeing ground source heat pumps for residences for about $30,000 … … As with wind turbines, there’s remarkably little data on the performance of ground source heat pumps. I don’t mean lab data, but actual in situ performance. There was a study just published by The Energy Saving Trust in the UK that compares ground source heat pumps with air source heat pumps. The results are pretty interesting … … There’s a pretty dramatic range in the performance of these systems … the range of performance is pretty identical, ground source to air source. That’s pretty interesting because air source is a lot cheaper to install. If you’re planning to spend a bunch of money on a heating system, you should instead take most of that money and invest it in the envelope and get the heating load down so low that you can meet the heating load with whatever system you want … A very efficient home can be heated with a few strips of electric baseboard heat or a small pellet stove … instead of a very expensive system. It’s the same argument I have about radiant floor heating systems– it’s a great way to distribute heat for a lousy building … I digress.
The basic argument with ground source heat pumps is take the money and put it into the building envelope. If you want a heat pump, get an air source heat pump …. these can be put in for a lots less money than ground source heat pumps. The performance is now neck and neck, and the performance of air source heat pumps keeps going up … I actually predict that the ground source heat pump industry disappears in the next ten years, it wouldn’t surprise me at all …
- – -
This exchange was followed by a question and answer session with the audience.
- – -
- – -