Monitored energy use of homes with geothermal heat pumps: A compilation and analysis of performance. Final report

Abstract

The performance of residential geothermal heat pumps (GHPs) was assessed by comparing heating, ventilation, and air conditioning (HVAC) system and whole house energy use of GHP houses and control houses. Actual energy savings were calculated and compared to expected savings (based on ARI ratings and literature) and predicted savings (based on coefficient of performance - COP - measurements). Differences between GHP and control houses were normalized for heating degree days and floor area or total insulation value. Predicted savings were consistently slightly below expected savings but within the range of performance cited by the industry. Average rated COP was 3.4. Average measured COP was 3.1. Actual savings were inconsistent and sometimes significantly below predicted savings. No correlation was found between actual savings and actual energy use. This suggests that factors such as insulation and occupant behavior probably have greater impact on energy use than type of HVAC equipment. There was also no clear correlation between climate and actual savings or between climate and actual energy use. There was a trend between GHP installation date and savings. Newer units appear to have lower savings than some of the older units which is opposite of what one would expect given the increase in rated efficiencies of GHPs. There are a number of explanations for why actual savings are repeatedly below rated savings or predicted savings. Poor ground loop sizing or installation procedures could be an issue. Given that performance is good compared to ASHPs but poor compared to electric resistance homes, the shortfall in savings could be due to duct leakage. The takeback effect could also be a reason for lower than expected savings. Occupants of heat pump homes are likely to heat more rooms and to use more air-conditioning than occupants of electric resistance homes. 10 refs., 17 figs., 10 tabs.