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Air Source vs. Geothermal Heat Pumps: Comparing efficiencies

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It used to be considered “common knowledge” that geothermal (or ground source) heat pumps are a lot more efficient than air source heat pumps.  During the winter,  temperatures are a lot higher under the ground than they are in the air, which means a geothermal system should be able to extract heat much more efficiently. The opposite will be true in the summer- a geothermal system should be able to move heat into the cooler ground more easily than an air source unit can expel it into the warmer outdoor air. However, you also have to remember that a geothermal system has to pump a heat transfer fluid (usually a mixture of water and antifreeze)  through the ground, and running this pump can take a lot of energy. Recently, “cold climate” air source heat pumps that specialize in extracting heat from cold winter air have been released. This has caused some to question whether geothermal systems are really much more efficient.

Comparing the efficiency ratings of geothermal and air source heat pumps

Geothermal heat pumps are given a COP (coefficient of performance) rating. This is the ratio of heat energy produced to the amount of electricity consumed. For example, a system with a COP of 4.0 is estimated to produce 4 watt-hours of heat for every watt-hour of electricity it consumes.

Air source heat pumps are given a Heating Seasonal Performance Factor rating, which is an estimate of the BTU’s of heat  that will be produced by each watt-hour of electricity. Different HSPF ratings are given for different regions, as the efficiency of an air source heat pump is highly dependent on the climate. The average COP (across all temperatures) of an air source heat pump can be roughly estimated by dividing the HSPF by 3.41 (the number of BTUs in a watt-hour). Note that the HSPF also takes into account the need for back-up heat (it assumes an electric furnace with a COP of 1 is being used on very cold days). Some manufactuters publish tables of the COPs their systems achieve at various temperatures, but others do not.

The pump reduces the efficiency of a geothermal heat pump

The COP (coefficient of performance) numbers that are published by geothermal heat pump manufacturers do not account for the electricity required to run the pump, as this will vary from case to case. However, this can be a substantial amount of energy. In an article on Green Building Advisor, energy consultant Henry Gifford estimates that pump usage will cause a typical geothermal system’s COP to drop from 3.5-4 down to less than 3.0- a level that can be achieved by many air source systems.

New air source systems deliver much higher COPs at lower temperatures

Until recently, air source heat pumps were just moderately modified central air conditioners. They were primarily engineered for maximum cooling efficiency. But now “cold climate” air source heat pumps are being released. Mitsubishi has been a leader in this regard, and their Zuba central heat pump maintains most of its heating capacity and efficiency down to -25C. Carrier recently released their Greenspeed heat pump which is rated at 13 HSPF, meaning it offers superior efficiency levels than many geothermal systems. Other manufacturers are expected to follow.

Are geothermal systems worth the extra expense?

Installing a geothermal system usually costs at least $20,000 more than an air source unit- and that doesn’t include the cost of repairing your lawn after it is dug up. With geothermal systems offering marginal or even no energy savings over cheaper air source units, we don’t feel they are suitable for many homes.

Real word performance comparison: UK study

A study done in the UK by the Energy Saving Trust measured the real world performance of 28 air source heat pumps and 53 geothermal systems. Although the geothermal systems performed slightly better on average, note that the best performing geothermal systems had similar performance to the best performing air source units. The wide range in performance within each category highlights the importance of a quality installation.

UK study on real world heat pump efficiencies: The geothermal systems did slightly better on average, but the best performing air source systems performed just as well as the best performing geothermal systems.

Learn more about air source heat pumps on our heat pumps page.

To schedule a free quote on an air source heat pump, call 604 GOOD GUY (604 466 3489), or use our online form.

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