Heat pump sizing calculator
This heat pump sizing calculator shows what a heat pump gives at each outdoor temperature, where it stops keeping up, and how much backup heat you need.
Why a heat pump needs a second look
A furnace gives the same heat on a mild day and a freezing one. A heat pump does not. The colder it gets outside, the less heat it moves. At the same time the house needs more heat. The two lines cross at the balance point.
Above the balance point the heat pump does the whole job. Below it you need backup heat, like electric strips or a furnace.
The math in plain words
We find the heating load of the house on the design night. The load then falls in a straight line to zero at the indoor temperature. The heat pump gives its rated heat at 47 F and less as it gets colder, with straight lines between the points at 47 F, 17 F and 5 F.
The shares are typical values. They are a start. For a real unit, use the maker's data at your design temperature.
How to read the chart
- The orange line is the heat the heat pump gives.
- The dashed line is the heat the house needs.
- The yellow dot is the balance point.
- The red band is the gap the backup heat must fill.
Get the full version in the app
The web page gives a fast estimate. The iPhone app does the whole job.
- Every window, wall and duct, not just six choices
- Load split by room with CFM and duct sizes
- A PDF report with your logo
- Works with no signal. Your jobs stay on your phone.
Questions people ask
What is the balance point of a heat pump?
It is the outdoor temperature where the heat pump gives exactly as much heat as the house needs. Below it, the heat pump cannot keep up alone and the backup heat turns on.
Why is a heat pump sized for heating and not just cooling?
A heat pump gives less heat as it gets colder outside. In a cold climate the heating side decides how well it works in winter. The cooling load alone can hide that.
What is a cold climate heat pump?
It is a model that keeps most of its heat at low outdoor temperatures. Here, a standard unit gives 62 percent of its 47 F heat at 17 F, and a cold climate unit gives 95 percent. These are typical shares. The maker data for your unit is better.
How big should the backup heat be?
It should cover the gap at the coldest design night: the house load minus what the heat pump gives at that temperature. The result shows that gap in kW and the next standard heater size.
More free tools
An estimate in the style of ACCA Manual J. It does not replace a certified room by room design. Manual J is a trademark of ACCA. HVAC Load is not affiliated with ACCA.