ASK THE EXPERTS: Air- vs. Ground-Source Heat Pumps

Heat Pump Comparison Table

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

Comments (2)

Fred Golden's picture

I was never sure of my 4 ton ASHP COP, but looked at the Goodman website and found this link. On page 21, it shows a chart with the heating COP at -10 to 65F in 5F steps, the COP for each temp, and guesstimate of the heat output for each outside air temp.

So at it's rated 47F outside air temp, the 48,000 Btu 14 SEER heat pump will consume about 3.71 KW, warm the 70F indoor air by 27.5F to about 97.5F, and have a COP of 3.63. EER of 12.5 and have a high pressure of around 353 PSI, low pressure of about 103 PSI. At 20F COP is 2.61. At 0F it is 1.82 and -10F is 1.34 COP. Will warm the air by 8.2F and consume 3KW, with 221 PSI high pressure and 22 PSI low pressure. At my home, I recently measured the output air at 95F with 68 indoor air, so that is 27F temp rise.

At my work, we have several R-22 ASHP installed in 2000. They will run about 40 PSI suction and 200 high side, produce about 15F warmer air, and require frequent use of the electric back up heat on a 35F day.

Newer units are much more energy efficient, and R-410 boils at a much higher pressure for a given air temperature. So even when it is 15F outside, the heat pump is blowing out 6,000 CFM of 0F air, and the boiling point is 51 PSI or coil temperature of 0F This warms the 1,500 CFM of indoor air by 17F.

Fred Golden's picture

I also have read of a study where the Ground Source heat pumps cool the ground so much that extracting heat becomes more difficult. I installed a 48,000 Btu 14 SEER heat pump in my 1,825 square foot well insulated home. I have 6" walls, and 8" floors, 8" ceiling insulation.

I find that at 32F outside (tonight) it will run about 30% of the time. This indicates that I should not need to run the electric back up heat, and I did not even hook up the electric heaters yet. (first winter with it in Portland OR).

If your newer Trane unit is not performing like it should, have someone service it, and check it for freon leaks. To test performance, on a 40 - 50F day, it should be warming the discharge air by about 30F to 40F. So 65F air into the return air duct should provide about 95 - 105F supply air. Anything less than 25F temperature increase on a 40F day indicates that the freon level might be really low, or outside airflow might be restricted (dirty coil or to much ice build up).

Make sure that you are changing your air filters. A 10% loss in airflow can increase the output temperature, but also will increase the compressor amp draw, and can lead to overheating the compressor motor windings. Use quality pleated air filters. Test the filter, if pepper will go through it, the quality is not high enough.

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