Thermal and economic balance points

A heat pump has two balance points, and they answer different questions.

• Thermal balance point: Natural Resources Canada defines it as "the temperature below which the heat pump does not have enough capacity to meet the heating needs of the building on its own."^{[Source 1]}
• Economic balance point: "the temperature below which the ratio of electricity to a supplemental fuel (e.g., natural gas) means that using the supplementary system is more cost effective."^{[Source 1]}

Why there is a thermal balance point

Two lines cross. As the outdoor temperature falls, the house loses heat faster, so the heating load goes up. At the same time, an air-source heat pump's output goes down, because there is less heat in the outdoor air to move (see Heat pumps in cold weather). The outdoor temperature where the heat pump's capacity equals the load is the thermal balance point. The DOE Building America Solution Center puts it this way: "When outdoor temperatures are below the balance point, the heat pump cannot fully meet the home's heating needs without a backup heat source."^{[Source 3]}

How it is found

DOE training material describes finding the balance point where the load line from the house's load calculation crosses the heat pump's capacity curve, using the manufacturer's extended performance data or the NEEP cold-climate heat pump product list.^{[Source 4]} ComEd's switchover guide describes the same method: the installer uses the load calculation and the heat pump's capacity curve.^{[Source 2]}

That makes the balance point specific to one house and one heat pump. It depends on:

• the house's heating load (Manual J), which depends on insulation, air leakage, windows and the local design temperature
• the capacity of the specific outdoor and indoor unit combination at each outdoor temperature
• for variable-speed units, the maximum capacity at each temperature, which manufacturers publish in extended performance tables

There is no single correct balance point for a climate or a size of house.

Why there is an economic balance point

Even above the thermal balance point, a heat pump may cost more to run than a gas or propane furnace at some outdoor temperatures. That happens when the heat pump's efficiency (COP (coefficient of performance)) at that temperature is low enough, relative to your electricity and fuel prices, that the furnace delivers heat more cheaply. ComEd's guide says the economic balance point "will vary greatly depending on the supplemental fuel source, efficiency of the equipment, and local utility rates."^{[Source 2]} Dual fuel and hybrid heat pump systems shows how to compute the breakeven COP from your rates.

With electric resistance backup heat, there is no meaningful economic balance point in this sense, because resistance heat has a COP of 1 and the heat pump's COP is higher whenever it runs. The question becomes only the thermal one.

Which one sets the controls

• Never switch to backup below the thermal balance point only. ComEd's guide is direct: "Systems should not be set up to switch over to supplemental heat below the thermal balance point!"^{[Source 2]} If the heat pump is the only source below that temperature, the house will not stay warm.
• Dual fuel: the switchover is usually set at the higher of the two balance points. A 2026 NEEA analysis recommends setting the switchover "close to the heat pump balance point" and "a few degrees above balance point."^{[Source 5]}
• Electric backup: NRCan recommends that electric backup be staged with a setpoint "2°C to 3°C below the heat pump heating temperature set-point," so the heat pump runs first and the backup only adds heat when needed.^{[Source 1]}

Thermostat settings for this are covered in Heat pump backup heat and lockout temperatures.

Balance point is not the cutoff temperature

The low-temperature cutoff is the outdoor temperature below which the heat pump stops running at all. NRCan notes that newer models' minimum operating temperature "can range from between -15°C to -25°C" (5°F to -13°F).^{[Source 1]} That is a property of the equipment. The thermal balance point is usually warmer, and it depends on the house. See Lockout temperature.