Let's be realistic about ground-source heat pumps
"Heat pumps really are earth savers. They can provide a lot of savings, and they also lessen the carbon footprint of your home. Though installation can be quite expensive, it will last long and the savings you get from it will basically pay off your expenses."
This comment was posted recently here at Old House Web in response to a column I wrote about ground-source heat pumps nearly two years ago. It shows that both interest and expectations in these systems are still high.
Sometimes, the expectations aren't very realistic. Ground-source heat pumps can be attractive, but they're not magic.
They need electricity to operate
Heat pumps don't generate heat by burning fuel directly; they move heat from one place to another. A heat pump extracts heat from air or water and moves it to another place, just like a refrigerator removes heat from its insulated box.
Heat pumps run on electricity. And unless the house has photovoltaic panels or a wind generator, the electricity comes from the utility's grid. Typically, the energy source is coal, oil or gas.
Here's the catch: Grid electricity is only about 30% efficient, meaning that only about 30% of the energy potential of the fuel burned by the generating station actually makes it to your house.
So, when someone says a ground-source heat pump lessens your carbon footprint, it may or may not be true. You'd really have to compare the true efficiency of using utility electricity for the heat pump to what you would get by burning natural gas or oil in a high-efficiency appliance in your own house.
Henry Gifford, a well-known heating system designer from New York City, discussed some of these issues in an article in Fine Homebuilding magazine last year.
Efficiency is hard to measure
Ground-source heat pumps are often credited with being very efficient. In the business, efficiency is described as the "coefficient of performance," which is the ratio of energy going into the system vs. energy coming out of the system. When a heat pump has a COP of 3, it means that for every watt of electricity it consumes, it produces 3 watts of heat.
Manufacturers may claim COPs of 3 or 4. But as Gifford and others have pointed out, there have been no wide-scale studies of installed systems that could confirm this. In fact, based on his own measurements and the work of others, Gifford thinks a more realistic COP is 2. In that scenario, the heat pump could actually use more fossil fuel than a high-efficiency gas or oil burner.
Be realistic and work to reduce heating loads
Ground-source heat pumps can produce both heating and cooling. That's a plus. And they can lower energy bills for some people. But they are not a one-size-fits-all solution. The cost of installation is usually very steep, and it takes a real pro to sort through the many variables in system design.
In the end, I'm more convinced than ever the real solution to the high cost and environmental penalty or heating and cooling is to build tighter, better insulated houses.