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.
A New Jersey example
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 nominal capacity of 18,000 BTU/hr.
Both the manufacturer’s data and the AHRI Directory give the capacities of this heat pump at the temperatures of 17°F and 47°F that I mentioned earlier. In this case, the capacities are 10,700 BTU/hr and 17,800 BTU/hr.
Plotting those numbers 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 effect of equipment size on balance point
If instead of a heat pump with a capacity of 18,000 BTU/hr we put in a heat pump with a 30,000 BTU/hr capacity, the balance point will be different. Just thinking through this, you should be able to see that more capacity will result in a lower balance point.
The graph below shows how to find it.
The orange line here is identical to the one in the previous graph. We haven’t done anything to change the heating load. The blue line, however, has moved up as the capacity is higher at both of the temperatures we’re using.
For this setup, the heat pump can provide all the heat needs down to and even a little below the design temperature. To find the balance point, we have to extrapolate the two lines to find the intersection, which is at about 15°F.
What I’ve done here is shown a simple way to find 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.