Heat Pump Balance Point: What It Is and How to Use It

"The heat pump can't keep up" is a complaint with two possible meanings. Either something is wrong with the equipment, or it is colder outside than the balance point and the auxiliary heat isn't doing its part. Knowing the balance point tells you which.

Two lines that cross

As it gets colder outside, two things happen at once:

• The house loses more heat.
• An air-source heat pump produces less heat, because there is less heat in the outdoor air to move.

Plot both against outdoor temperature. The house's heat loss is a line that rises as it gets colder. The heat pump's capacity is a line that falls. They cross at the thermal balance point.

Finding it

1. Heat loss. Take the design heat loss from the load calculation (Manual J) at the design outdoor temperature. Heat loss is about zero when it is as warm outside as inside. Draw a straight line between those two points.
2. Capacity. From the manufacturer's expanded performance data for the exact outdoor and indoor unit match, read heating capacity at two or more outdoor temperatures. Most tables list 47°F and 17°F. Draw that line.
3. Read the crossing. That outdoor temperature is the thermal balance point.

Use the manufacturer's data for the matched system. Nameplate tonnage is a cooling number and won't give you heating capacity at low temperatures.

Worked example

Example numbers, to show the method only.

Heat loss rises 600 Btu/h for each degree below 65°F. Capacity falls about 467 Btu/h per degree between 47°F and 17°F. Setting them equal gives a balance point of about 24°F. Below that, this system needs auxiliary heat to hold setpoint.

What you use it for

• Auxiliary heat lockout. Many thermostats and controls can lock out strip heat above a set outdoor temperature so it doesn't run when the heat pump can do the job alone. Set it at or a little above the balance point. See auxiliary and emergency heat.
• Sizing strip heat. At the design temperature, the gap between heat loss and heat pump capacity is what the auxiliary heat has to cover. In the example, at 5°F, that gap is the 36,000 Btu/h loss minus whatever the capacity table shows at 5°F.
• Diagnosing "can't keep up". Falling behind at 40°F with a 24°F balance point is an equipment or airflow problem. Falling behind at 15°F with no auxiliary heat running is an auxiliary heat problem.

Economic balance point (dual fuel)

With a heat pump on top of a gas or propane furnace, the question is cost, not capacity. The economic balance point is the outdoor temperature below which the furnace heats the house for less money than the heat pump.

It depends on the electric rate, the fuel price, the furnace efficiency and the heat pump's efficiency at each temperature (its COP, from the performance data). Since rates change, so does the switchover temperature. Set the dual-fuel changeover from current local prices, and tell the customer it may need revisiting when rates move.

Lowering the balance point

• Reduce the heat loss: air sealing, insulation, duct sealing.
• Fix what cuts capacity: low airflow, wrong charge, a dirty outdoor coil, defrost problems.
• Choose equipment that holds its capacity at low temperatures. The Department of Energy notes that newer air-source heat pumps are now a real option for space heating in colder regions.

Common mistakes

• Using cooling tonnage to estimate heating output in cold weather.
• Guessing the lockout temperature instead of working it out.
• Mixing up the two balance points. One is about capacity, the other about cost.
• Locking out auxiliary heat too low, so the house falls behind on the coldest days.
• Oversizing the heat pump to push the balance point down without checking what that does in cooling. See oversized AC problems.

FAQ

Sources

• ACCA Manual S: residential equipment selection
• ACCA Manual J: residential load calculation
• U.S. Department of Energy: Air-source heat pumps

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