The coefficient of performance is used to measure the heating or cooling efficiency of equipment. COP is called a “dimensionless” value. What does that mean to a homeowner? It means it’s an accurate way of testing equipment in a specific moment.
For example, another way to rate the efficiency of a heat pump in heating mode is a Heating Seasonal Performance Factor (HSPF) rating. So what’s the difference? An HSPF rating is a seasonal rating or average that’s kinda like the MPG rating of your car. Meaning, there are things that can throw that rating off. Flooring the gas and breaking will hurt your gas mileage, and a super cold day will skew an HSPF rating.
The energy efficiency ratio (EER) is used to measure the cooling efficiency of an air conditioner or heat pump. It is the ratio of cooling that is output in BTUs an hour to the input electrical watts. And this measurement is at a given operating point, so it is similar to COP for heating. Geothermal heat pumps are rated with EER and COP ratings, while air source heat pumps are rated with SEER and HSPF ratings.
The Seasonal Energy Efficiency Ratio (SEER) rating of an air conditioner or heat pump averages the unit’s cooling performance for a typical season in two different operating conditions. This is to try and simulate higher and lower humidity levels, etc. The SEER rating of a unit attempts to predict how it will perform on a seasonal average in varying temps and humidity. Can you calculate the SEER rating of equipment from the EER value? No. It is impossible without accurate laboratory test data.
An HSPF rating is commonly used to rate the efficiency of a heat pump when in heating mode. And like the SEER rating, the Heating Seasonal Performance Factor of a heat pump is an estimated seasonal efficiency based on certain conditions. If you take your heat pump up to Minnesota and measure the efficiency over the frigid month of January, it’s not going to be at the labeled HSPF rating.
So why not use COP and EER all of the time? Again, it’s because a snapshot of the efficiency of an air conditioner when outside temp is 90° with 80% humidity will be radically different than the same unit’s efficiency at 80° outside with 70% humidity. Make sense? So in an effort to deliver seasonal averages, SEER and HSPF ratings are used.
Rating Note: As mentioned above, Geothermal heat pumps are rated with an EER rating as opposed to a SEER rating. Now do you understand why? It’s because a geothermal heat pump is exchanging energy with the constant thermal layer under ground. So fluctuating temperatures don’t affect it, which means attempting to predict a seasonal average isn’t needed.
We wouldn’t be done here without talking about the Annual Fuel Utilization Efficiency or AFUE rating. An AFUE rating is shown as a percentage, and is used to measure a seasonal average for fossil fuel based heating (whereas all of the above are based on electricity). An AFUE of 90% means that 90% of the fuel is turned into actual heat energy, and 10% is lost. The difference in AFUE vs. HSPF is that electric heating has no heat energy loss due to venting, while a gas furnace must vent to get rid of carbon monoxide, dioxide, etc. Venting and other issues cause there to be a loss in the conversion of fossil fuel to heat energy.