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Building Science

A Simple Way to Calculate Heat Pump Balance Point

All you need are the design heating load and the heat pump capacity

A heat pump loses capacity and a house needs more heat as the outdoor temperature drops. At the balance point temperature, the heat pump can provide just enough heat for the house. [Image credit: Energy Vanguard]

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…

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7 Comments

  1. Guy Lazure | | #1

    Neat. Two points that need clarification:
    1)How do you get the "Heating load at 99% design temperature" for a home?
    How do you apply this to a home that is more than 50 years old?
    What happens if they added insulation to the roof?
    2) How do you account for the cost of electricity vs alternate fuel. In other words when should you stop using the heat pump and start up the gas furnace?
    Regards
    Guy Lazure
    [email protected]

    1. User avater GBA Editor
      Allison A. Bailes III, PhD | | #4

      Guy, those are great questions. First, the heating load comes from a Manual J load calculation. Here's an article I wrote about Manual J and Martin Holladay has written about it here as well:

      Manual J Load Calculations vs. Rules of Thumb
      http://www.greenbuildingadvisor.com/articles/dept/building-science/manual-j-load-calculations-vs-rules-thumb

      There's a lot that goes into a load calculation and the accuracy of the results is limited by the accuracy of the inputs. The more assumptions you make, the more uncertain the results.

      Your second question is about a different type of balance point: the economic balance point. The current article is about the balance of heating capacity and heating load but the economic balance point considers the fuel costs and tells you at what temperature it's best to switch over from one fuel to another. That's the subject of a future article.

  2. Walter Ahlgrim | | #2

    Maybe I missed it but what was the simple way to establish point A (heat load at 99% design temp) on the graph?

    Where did number come from? If it was a manual J calculation, getting that number is anything but simple for the average homeowner. An accurate calculation requires lots of data points be collected and enter into a computer program some of that data is little more than wild guesses. Finding someone to produce an accurate number is not easy or inexpensive.

    In the real world every homeowner should lock out there electric heat they will know in the morning if last nights temperature fell below their systems balance point. It is important to set their system to lock out electric heat above the balance point. I found my thermostat was designed to provide maximum comfort by using electric heat starting at about 25°. I have yet to find my balance point I know it is below 8° but I see so few hours a year below 8°.

    I do like the article my only problem is the word SIMPLE in the title.

    Walta

    1. User avater GBA Editor
      Allison A. Bailes III, PhD | | #5

      Walter, I didn't say finding the heating load for the house was simple. There's a lot that goes into that calculation, and the Related Articles section above has a couple of articles on that topic.

      Your point about locking out the supplemental heat, especially for temperatures above the balance point, is a good one. Since you've discovered that your actual heat pump balance point is below 8 °F, have you also discovered what the home's thermal balance point is? That's the temperature at which the heat first comes on. I assumed 65 °F in the article above, but that's higher than it really is in many homes, especially newer ones.

  3. Trevor Lambert | | #3

    So just to confirm, the second image is what we should be aiming for, the balance point below the design temp?

    I'd like to point out that the actual load and capacity lines won't be linear. Using five data points available for a Fujitsu, the line I got has a relatively flat part in the middle of the range (-5 to 17), with a higher slope at higher (17-47) and lower (-15 - -5) temperatures. So as long as your design temp is above 17F, your approximation is good. If your design temp is say 5F, it might not be appropriate to use the 17F and 47F data points and just extrapolate that line. In the case of the Fujitsu model I graphed, the extrapolation yields a value of about 12.5kBTU, whereas the actual spec for the unit at 5F is 15kBTU.

    The heating load will change slope as the load approaches zero, due to the increased proportion of waste heat relative to the total load. But this is outside of the useful area of the graph, so it's of academic interest only.

    1. User avater GBA Editor
      Allison A. Bailes III, PhD | | #6

      Trevor, ideally you'd like the heat pump balance point to be lower than the design temperature. If that's the case, you can get by with little or no supplemental heat. The problem is that with conventional heat pumps, you're likely to end up oversizing the cooling side to achieve such a low balance point.

      Mini-splits, however, are made to provide extra heat compared to cooling and they have variable capacity so it's easier to heat with a mini-split and not have any supplemental heat. In fact, I have a friend in Minneapolis who does just that. Yes, Minneapolis, Minnesota!

      I knew someone would question my assumption about a linear relationship between heat pump capacity and outdoor temperature. You're absolutely right. It's not always linear.

      My assumption about the home's heating load also isn't accurate, especially for new homes. By turning off the supplemental heat and seeing how well the home holds its setpoint on cold nights, you can get an idea of what the actual balance point is. Or you can monitor the supplemental heat and see at what temperature it comes on.

  4. Kevin Kircher | | #7

    Re: finding Point A, it doesn't need to be the design load. Any two points determine a line, and we've already got one from the assumption of zero load at 65 F. If you have a single-speed furnace, you can get a second point with a quick experiment. Here it is:

    1. Find the capacity of your furnace.
    2. Record how long it's ON over the course of a couple of hours.
    3. Load = (capacity) * (ON time) / (total time).

    That load estimate, along with the outdoor temperature during the experiment, gives you the point you need on the load vs. temperature line. The experiment can also be repeated with different outdoor temperatures to generate more points.

    For best results, do the experiment at night when heat gains from the sun, lights, appliances, etc. are small. If you do it on a really cold night, you're essentially measuring your design load. Don't adjust your thermostat during the experiment or for several hours beforehand.

    Step 1 can usually be done by reading the model number off of your furnace and looking it up on the manufacturer's website. Step 2 can be done by noting when air is whooshing through your ducts, or by listening to your thermostat (often they make a little click when they trip on).

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