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Building Science

Heat Pump Balance Point and the Building Enclosure

The quality of a home’s building enclosure can have a big effect on heating with a heat pump

Gary Nelson stands in front of his ducted minisplit heat pump in Minneapolis, Minnesota. Because of the heat pump capacity and the efficiency of his building enclosure, his balance point is below zero degrees Fahrenheit. [Photo credit: Energy Vanguard]

So we’ve looked at a simple way to estimate the heat pump balance point.  Then I showed how the capacity of the heat pump affects the balance point.  (A larger heat pump lowers the balance point and vice versa.)  Today, let’s look at the other major factor that can affect the balance point.

First, recall that the simple method to find the balance point involved plotting the heating load of the house and heat pump capacity on the same graph.  Using two data points for each and assuming a linear relationship between load/capacity and outdoor temperature, we had two straight lines.  The temperature where they cross is the heat pump balance point.  This point tells you at what outdoor temperature the heat pump is just able to keep the house at the indoor design temperature (normally 70°F).

Now we’re ready to take it further.

One way to change the balance point is to move the heat pump capacity on the graph.  Likewise, we could change the other part of the graph, the heating load, to get a different balance point (assuming we don’t make a compensating change in equipment size).  If you have an existing home and want make it more comfortable and energy-efficient by doing some air sealing and insulating, the house won’t need as much heat.  The house in our example initially had a heating load of about 15,000 BTU/hr.  Let’s say we add some insulation, make it more airtight, and seal the ducts.  Now the load drops to about 10,000 BTU/hr, and we use that as our design temperature heating load point on the graph.

But wait!  There’s more.  In the last article, I assumed that zero load happens at 65°F.  In many homes it may well be lower, especially…

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3 Comments

  1. Sandra Lynn | | #1

    You mention a system thats over sized may short cycle in the summer. Is there a problem with turning it off in the spring and fall? Thats what I do now with a standard gas furnace/ central air. I only use it when its either really hot or cold outside. Does a heat pump system malfunction if left off for a few months at a time? I am considering getting a split system for a new house I will be building. So I am trying to get as much info as I can before I decide what to do.

    1. User avater
      Stephen Sheehy | | #2

      We shut our minisplits off in April and don't turn them on until we get a few super hot days in summer, if we use them at all until Fall. I don't think it hurts anything to shut them off.

      1. Sandra Lynn | | #3

        Thanks!

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