A Simple Way to Calculate Heat Pump Balance Point
As the outdoor temperature drops on a cold day, a home's heating load increases. At the same time, decreasing temperature means less heat in the air for a heat pump to pump indoors. And at one special temperature, the capacity of a heat pump equals the heating load in the house. That temperature is called the balance point. 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
- 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.
A New Jersey example
We recently did a load calculation for a home in New Jersey that works well for illustrating how to find balance point with this method. The outdoor 99% design temperature for their location is 17 °F. The heating load for 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 nominal capacity of 18,000 BTU/hr.
There are two ways to find the capacities of this heat pump at the temperatures of 17 °F and 47 °F that I mentioned earlier. You can get them from the manufacturer's data or the AHRI Directory. In this case, I used the AHRI Directory and found the capacities to 10,700 BTU/hr and 17,800 BTU/hr at 17 °F and 47 °F, respectively.
Plotting those two linear relationships for load and capacity yields the following graph.
The orange line above is the heating load. As the outdoor temperature drops, the house needs more and more heat and the orange line rises. The blue line is the heating capacity of the heat pump. As the outdoor temperature drops, the capacity of the heat pump drops because there's less heat to extract and pump indoors.
Where the two lines cross is the balance point. At that temperature, the heat pump is just able to keep up with the how much heat the house needs. The balance point temperature is about 25 °F in this case.
The significance of balance point
What I've done here is shown a simple way to find an estimate of the balance point based on the heating load calculated for the house and the specifications for the heat pump installed or being considered. The actual balance point is probably a bit different.
But what it tells you is that when the outdoor temperature goes below the balance point, you'll need some kind of supplementary heat because the heat pump by itself won't be able to keep up. If your supplementary heat is the common and expensive electric resistance strip heat, you want a low balance point so you don't have to use the supplementary heat as much.
Now, you've also got to balance your heating capacity with your cooling needs, and especially in a humid climate, cooling loads generally determine the size of heat pump you install and thus where the balance point will be.
In my next article, I'll look at two ways to change the balance point. In the meantime, let's start the discussion of finding the balance point in the comments below.
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