At what temperature will a heat pump become ineffective?

by Jeff Rubin | June 30, 2018

At what temperature will a heat pump become ineffective?

Heat pump balance point graph
Heat pump balance point. Image courtesy of John Siegenthaler/APPROPRIATE DESIGNS. Specialists in the use of hydronic comfort heating. All rights reserved.

This graph shows the balance point for a very efficient, state-of-the-art Air to Water CCHP. This example also assumes a tight, well-insulated conventionally-constructed building envelope. In this example you would begin to need supplemental heat at around 15º.

The two variables of heat pump heating capacity—the red line in the graph, and building heat loss*—the blue line in the graph.  This will determine the temperature at which your particular heat pump can no longer keep up with the demand for heat.

* (as measured by a heat loss calculation )

Auxiliary heat

Some heat pumps have built-in auxiliary heat. At the point that the heat pump can not keep up with the heat loss it will turn on the backup electric, resistance heater. Resistance heat is two to four times less efficient than the heat pump’s typical performance. This can lead to high electric bills during the coldest part of the winter. Consider these other forms of auxiliary heat:

Wall-vented gas heater

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Electric Baseboard

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For a more complete discussion of heat pump performance in different kinds of buildings see How Cold is too Cold for a Cold Climate Heat Pump?

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Heat Loss

The two variables of of heat loss are a) geographic location, and b) building performance. Together they will determine the quantity of heat needed to maintain comfort.

Geographic location drives the building's “design day." Your design day is the difference in temperature between the inside and outside of your house on the theoretical coldest day of the year; so in the northeast we calculate a day that is -10ºf outside and comfortable 68ºf inside. This is referred to as the “delta-T” value. A typical delta-T for the northeast is 78º.

Building performance is the big one because there’s nothing we can do about geographic location. Today we can build a really tight thermal envelope, but most of our heating pollution comes from the millions of homes built over the last 100-150 years. We are getting better at buttoning-up those old buildings with insulation and weather sealing but weatherization is still  never going to be enough to overcome the heat loss from a 78º delta-T. That means we need heating systems that can quickly put a whole lot of heat back into those old houses on the days that are really cold.

Figuring out the exact heat loss of a particular building is a complicated job for a heating professional.  Your contractor will complete a Manual J calculation, but for practical purposes, a three-year history of your heating bills will provide a good estimation.  If you want to drill down on heat loss calculations (hint: you don’t) here is a good article.