FAIRBANKS — As the days get longer and sunnier, we’re storing more heat in the ground to tap into next winter.

Ground source heat pumps take free energy from the ground, add a little electricity, and then pump it into your home as heat. Because the ground temperature is warmer than the outside air in the winter, heat pumps have the potential to be more efficient than other types of heating appliances.

While they are gaining popularity in Alaska, there are still many unanswered questions about how they perform in cold soils. For example, how much heat do they remove from the ground and how does this affect their efficiency over time?

To help answer these questions, CCHRC installed a ground source heat pump in November 2013 and will monitor its heating efficiency and its impact on ground temperatures for a 10-year period.

During the first year, it exceeded our expectations — creating 3.6 units of heat for every 1 unit of electricity it consumed — and only requiring one maintenance visit (a warranty fix). The six-ton residential heat pump replaced a 76,000-Btu-per-hour oil-fired boiler. Our ground source heat saved us more than 400 gallons of heating oil last year.

The ground loop consists of six “slinky coils,” 100-foot-long loops of ¾-inch plastic tubing. The loop was buried nine feet deep in the ground outside the research and testing facility. The ground loop is filled with fluid that absorbs “heat” from the soil.

The heat pump appliance pulls this heat off the ground loop, raises the temperature using a refrigeration process (just like your refrigerator) and transfers the heat to our hydronic in-floor heating system. Overall, the heat pump heats 5,000 square feet of the building.

The Coefficient of Performance (or COP) during the first year was 3.62. That means for every 1 unit of electricity it used, it produced 3.62 units of energy. COP varies throughout the year, depending on several factors. The warmer the ground temperature, the higher the COP. Also, when it’s warmer outside, the temperature of the fluid delivered to the floors also can be warmer, which increases the COP.

Another research question is the long-term effect of the ground loop on the ground temperature. By harvesting heat from the ground during the winter, are we lowering the temperature of the ground, thereby reducing the efficiency of the heat pump? Starting at 34 degrees Fahrenheit in November 2013, the ground temperature had dropped 1 degree by the following November.

To maintain the efficiency of the heat pump, the ground has to re-absorb as much energy during the summer as we extracted during the winter.

We spread three different materials over the loop field — dark rocks, sand, and grass — to test how they affected the ground’s ability to recharge. The rocks absorbed slightly more heat, but the effect was only noticeable at the surface.

We’ll continue to monitor to heat pump for at least 10 years to see if there are appreciable changes in the ground temperature and thus in the efficiency of the heat pump.

View a report on the ground source heat pump’s performance at www.cchrc.org/sites/default/files/docs/CCHRC_GSHP_Final.pdf.

CCHRC also produced a decision-making model to help homeowners determine if a ground source heat pump is right for them at www.cchrc.org/sites/default/files/docs/GSHP_DecisionModelFinal.pdf.

The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.

 

Ask a Builder articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, contact us at info@cchrc.org or 457-3454.

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