Heat Pump Balance Point

 

The Balance Point
 
When you plot the heating requirement of the house and the output of a heat pump you get a graph that looks like the figure below. As the outdoor temperature drops, the output of the heat pump decreases. At the same time the heating requirement of the house increases. At some temperature (31 degrees F in our example), the heat pump output and the home heating requirements match. This temperature is called the balance point. Below the balance point temperature, supplemental heat will be required.
The heat pump's heating capacity ("size") should be selected based on a desired balance point. In climates where heating is the main concern, sizing may be a compromise between heating and cooling requirements. The chosen balance point varies according to climate. Typical balance point temperatures are in the 27 degrees F to 35 degrees F range.

 

What do these terms mean?

The terminology for the electric resistance heating system that is part of most heat pump systems in the Northwest causes confusion even among people who regularly work with heat pumps. We'll use the following definitions:

Auxiliary heat

The total of all electric resistance heat in the system, regardless of its intended use.

Supplemental heat

The portion of electric resistance heat used to make up for the heat pump's inability to heat the house during very cold temperatures and during defrost.

Emergency heat

The portion of electric resistance heat that comes on when the heat pump shuts down due to a failure or due to operation of a safety switch. Emergency heat usually uses all of the auxiliary heat, so "auxiliary" heat is often used to mean "emergency" heat.

Back-up heat

Same as emergency heat.

Heat pump

The part of the heating/cooling system that uses refrigeration equipment. When this is operating alone (with no electric resistance heat operating), the system is at its most economical.

Ton

It can get a little confusing talking about the heating or cooling capacity of a heat pump. A common sizing measurement is the "ton." This is a holdover from the days when refrigeration was used mostly to make ice (to sell to people who had ice boxes). A "three ton" refrigeration unit could make three tons of ice from 32 degrees F water in a day.

We know you're not making ice, but people still talk about heat pumps in tons. One ton is roughly equivalent to 12,000 Btu per hour heat output when the air is 47 degrees F outdoors, or 12,000 Btu cooling at 95 degrees F.

So what's a Btu? This is the most common measure of heat energy in the American heating and cooling industry. It stands for British Thermal Unit and is a small amount of energy, roughly equivalent to the energy given off from burning a wooden match.



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