Finding Balance - Heat Pump Heating Load vs. Capacity
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.
Balance points for many homes are in the mid-30s Fahrenheit. That's not a given, though. I was talking to a HERS rater in Nashville this week who did an energy retrofit on his house, and his balance point is 22° F. That's right. The heat pump supplies 100% of the heat all the down to 22° F. Only then does the supplemental heat kick on.
One way to get a lower balance point is to make the house more energy efficient. That moves the load curve down and the balance point to the left. The other would be to increase the equipment size. Of course, if you do either of these, it benefits your heating efficiency, but you'll probably have an oversized cooling system.
Note also that supplemental heat is only to make up the difference between the load and capacity when the temperature is below the balance point. For example, from the graph above, this home would need about 9000 Btu per hour of supplemental heat at 25° F, and the heat pump would still be able to supply about 23,000 Btu per hour.
The heat pump doesn't have to supply all the heat in the house...unless you do something crazy like turning the thermostat to Emergency Heat just because it's cold outside. Then, you're using all electric resistance heat, and that's much less efficient.
Heat pumps are great sources of heat, especially for high performance homes. We ran a couple of guest posts here in January about why you should avoid using a furnace in a high performance home and instead consider a heat pump and hydronic system. If you understand the fundamentals and get a good HVAC design, you'll be more comfortable and have an efficient heating system.
Cat in the window photo by Aunt Owwee from flickr.com, used under a Creative Commons license.