I read Alex Wilson’s “Heat Pump Primer” in HP149, and was surprised about the claim that an air-source heat pump (ASHP) with variable refrigerant-flow attains the same coefficient of performance (COP) as a ground-source heat pump (GSHP). While I can imagine that an ASHP is very efficient down to 20°F, I wonder how ASHP can work well in colder temperatures. I own a fairly new Trane ASHP that is trying to make heat at -5°F, but the COP is marginal. I am interested to know how much more efficient a modern ASHP can be than my Trane. I considered a GSHP, but the cost and complexity dissuaded me.
Mark Dischinger • via email
Several decades ago, ASHPs only made sense south of about 40° latitude—at temperatures below 30°F, the performance dropped significantly. They were little better than electric-resistance heat. But times have changed. The new generation of variable-refrigerant-flow (VRF) ASHPs, often called minisplits or ductless minisplits, now perform reasonably well at temperatures down to -10°F (or even lower). For example, in our well-insulated Vermont house, the 18,000 Btu/hour Mitsubishi model we installed kept us reasonably comfortable—even during last winter’s chilly polar vortex—with a COP that averaged about 2.0. On a few of the coldest evenings, with temperatures between -5°F and -10°F, we fired our small wood heater for supplemental heat.
Japanese companies, including Mitsubishi, Daikin, Fujitsu, and Sanyo (now part of Panasonic), have led the charge with these high-performance, cold-climate ASHPs. I hope that American companies, such as Trane and Carrier, will soon catch up with their Japanese competitors. Experts I’ve spoken to expect a long-term COP of 2.5 for these VRF ASHPs in cold climates. A COP of 2.5 means that for every one unit of energy consumed (as electricity), 2.5 units of energy (as heat) are supplied. That compares reasonably well with GSHPs. The best GSHPs might deliver an average COP of 3.5, but in a cold climate, the performance drops as the ground cools.
An in-depth study by Energy Saving Trust of residential GSHPs and ASHPs showed average GSHP performance to be only marginally better than that of ASHPs—and also showed a wide variation in actual performance. The measured COPs of the GSHP systems studied ranged from 1.55 to 3.47, with a median of 2.31. The COPs of the ASHP systems ranged from 1.2 to 2.2, with a median of 1.83. For systems in which the average “system performance factor” was determined (a metric that includes all electricity use for the components, as well as auxiliary or immersion heaters for space and water heating), GSHPs achieved an average COP of 2.82 while the ASHPs averaged a COP of 2.45.
While modern GSHPs and ASHPs have similar performance, there’s a huge disparity in installed cost. In New England, for example, GSHPs can cost five to seven times as much as VRF ASHPs.
Instead of spending the money on a GSHP, I recommend installing one or two minisplit VRF ASHPs and then investing the “savings” into building envelope improvements, such as more insulation, low-e storm windows, etc.
Alex Wilson • Home Power building technology editor