As the outdoor temperature drops on a cold day, there’s less heat in the air for the heat pump to pump indoors. And at one special temperature, the balance point, the capacity of a heat pump equals the heating load in the house. In my recent article about the three types of heat associated with heat pumps, I mentioned balance point but didn’t explain how you might go about finding it. Let’s do that today.
I’ll keep it simple. All we need are three numbers: the heating load of the house at the outdoor 99% design temperature, the heating capacity of the heat pump at 17°F, and the heating capacity of the heat pump at 47°F. Then we’ll make an assumption about a fourth number: the temperature at which the heating load of the house is zero. We’ll take that to occur at 65°F.
So now we have four numbers, two for heating load of the house and two for heating capacity of the heat pump. We can plot the two load numbers on a graph of load versus outdoor temperature. We’ll then put the two capacity numbers on the same graph.
Next, we simplify things and assume the relationship between load and temperature is linear and that the same linear relationship holds for capacity and temperature. Let’s see what such a graph might look like.
We recently did a load calculation for a client in New Jersey, and that example works well for illustrating how to find balance point. The outdoor 99% design temperature for their location is 17°F. The heating load for the main part of the house (~1,800 square feet) is a little over 15,000 BTU/hr. For this exercise, let’s look at a single-stage, fixed capacity heat pump with a…