Heat pump heating efficiency

In my quest to educate the world on building science topics, I find that I often have to go back to basics. Whether it's the basic nature of heat, why we're using the wrong quantity to characterize infiltration rates, or the reason that the media's focus on caulking windows drives me crazy, I try to find the fundamental issues involved and explain them in terms that are easy to understand.

Today, the topic is one even some people in the HVAC industry don't seem to understand. In January, I wrote an article titled How NOT to Use Your Heat Pump Thermostat because of bad advice that HVAC techs had given to two people I know. Today, I dig a little deeper and explain a central concept in heat pump operation - the balance point.

In the world of heating, ventilating, and air conditioning (HVAC) design, there are two terms that often get confused - heating/cooling load and heating/cooling capacity. The load is how much cooling or heating a house needs. The capacity is how much cooling or heating an HVAC system can supply. Pretty simple, right?

With furnaces, boilers, and electric resistance heat, the capacity of the heating system is not affected by the outdoor conditions. Well, that's true in an ideal world anyway, where the system and all ducts are inside the building envelope. In homes where the heating system or ducts are outside the building envelope, there is some loss of capacity as the temperature drops.

In air source heat pumps, however, the source of heat is the outdoor air. Yes, as improbable as that sounds, cold air CAN heat your home. As it gets colder outside, there's less heat available to bring inside, though, so the heating capacity of a heat pump is highly dependent on the outdoor conditions. In fact, the capacity goes in the opposite direction from the load. The image below shows the story.

Every house and heat pump will have the capacity and load lines in different places, so don't take the numbers in this graph as absolute. The important points to note about the relationship between heat pump heating capacity and a home's heating load are:

• As the temperature goes down, the load increases and the capacity decreases.
• At a certain temperature - the balance point - the capacity is equal to the load.
• For temperatures below the balance point, the home will need supplemental heat.