One of the most confusing aspects of geothermal heat pump technology is equipment ratings. These heating and cooling performance values are useful for comparing units of the same type (i.e., ASHP to ASHP or GHP to GHP). Unfortunately, the ratings used for different types of equipment (furnaces, ASHP, GHP) are not generally consistant making comparisons difficult. As a result, it is useful to know what the ratings values include and what they don't.
All heat pumps are rated by the Air Conditioning and Refrigerant Institute (ARI). Results are published every six months in the Directory of Certified Applied Air Conditioning Products (for GHPs) and the Directory of Certified Unitary Products (for ASHPs).
For water-source heat pumps (the type of heat pump used in all GHP systems), cooling performance is defined by an index called EER (Energy Efficiency Ratio). This is the cooling affect produced by the unit (in Btu/hr) divided by the electrical input (in watts) resulting in units of Btu/watt*hr.
Heating performance is defined by the index called COP (Coefficient Of Performance). This is the heating affect produced by the unit (in Btu/hr) divided by the energy equivalent of the electrical input (in Btu/hr) resulting in a dimensionless (no units) value. For both COP and EER, the larger the numerical value, the less electricity required to operate it.
Both the COP and EER values for groundwater heat pumps are single point (valid only at the specific test conditions used in the rating) values only. This in contrast to the seasonal values (HSPF and SEER) published for air-source equipment. COP and EER are not the same as, or valid for use in comparison to, SEER and HSPF.
GHP Ratings
The ratings format for heat pump units used in these systems was recently changed. Up until June 2000, ratings were published under two different headings: ARI 325 (open loop or groundwater heat pumps) and ARI 330 (closed loop or ground-coupled heat pumps). Since much of the older manufacturers literature with the older ratings information included in it is still in circulation, the 325/330 rating system will be reviewed here. ARI 325 was intended for residential groundwater heat pump systems. Performance (EER and COP) was published at two water temperatures: 70° and 50°F. A pumping penalty (to reflect the power required by a well pump to supply water to the heat pump) was added to all units rated under ARI 325.
ARI 330 was intended for closed loop or ground-coupled GHPs and was based upon entering water temperature of 77° in the cooling mode and 32° in the heating mode. One of the limitations of this rating was that the temperatures used were reflective of a northern climate. Southern installations would see higher temperatures entering the heat pump and thus, have better winter and poorer summer performance than indicated. A much smaller pumping penalty (reflective of the power required by the small ground loop pump) was added to all units under ARI 330.
As of January 2000, the ARI 325 and 330 ratings were replaced with a single ISO 13256-1 rating system. Under the new system, performance values (EER and COP) are provided at different temperatures to reflect performance in different systems types. The ratings are divided in to three parts: WLHP, GWHP, GLHP of which the most useful for GSHP systems is the GLHP values. This information can be used to compare the products of one manufacturer to another for either open or closed loop systems. Like the older ARI 330 rating, the units are tested for performance at entering water temperatures of 32 °F in the heating mode and 77°F in the cooling mode. The major difference in the newer ISO rating is that the pumping penalties (used in the older ARI ratings) along with penalties for fan power (external to the unit) have been removed. This has had the result of raising the performance values compared to the older ARI system. Where an average unit in the old rating system may have had a 13.0 EER in cooling, under the ISO rating this same unit might show a 14.0 or 14.5 rating.
The ratings values (COP and EER) should be thought of as relative numbers. That is they are intended for the comparison of one manufactuer's products to another's. They DO NOT reflect actual performance in any installation.
ASHP Ratings
The major difference between ratings for ASHPs and GHPs is that the air source values are seasonal. They are intended to reflect the total heating or cooling output for the season divided by the total electrical input for the season. These ratings (HSPF - heating, SEER - cooling) cannot be directly compared to the GHP EER and COP numbers.
ASHPs are rated under ARI 210/240. In order to simplify the process, a number of assumptions are made regarding operation of the heat pump. The rating is based on a moderate climate (Washington, DC) and as a result, is not reflective of either very cold or very warm areas of the country.
Furnaces
Furnaces are rated by an index known as AFUE or annual fuel utilization efficiency. This is intended to reflect the annual heat energy supplied divided by the energy content of the fuel consumed to supply that heat. The major drawback is that the electricity required to operate the fan in the furnace ( and the combustion air fan if so equipped) is not included in the rating.