How Oversizing Hurts Heat Pumps Performance

In the US, heat pump capacity is usually specified in British Thermal Units per hour (Btu for short), and sometimes tons (1 ton = 12,000 Btu/h).

A load calculation determines the system capacity required to heat and cool a home to desired indoor temperatures at 99% of expected outdoor conditions (the remaining 1% are rare enough and brief enough to not worry about). Load calculations are very accurate when done correctly.

So if the load calculation says a home needs 36,000 Btu for heating, that's the system output required to keep the home at 70° F at the coldest outdoor temperatures expected most of the time. The home will need less than this at other times, usually a lot less.

Traditional heat sources like boilers and furnaces are either on or off. Even multistage models will have only 2 or 3 output levels at most. The equipment turns on until it reaches the thermostat setpoint, then turns off until the temperature drops below it. Turning on and off isn't very energy efficient, and when it happens too frequently (like when a fixed output system has much more capacity than it needs) it's called short cycling. Short cycling is bad.

Modern, inverter-driven heat pumps have variable output. The indoor and outdoor fans and the compressor can speed up or slow down to exactly match the heating and cooling needs of the home. It is much more energy efficient to have a system constantly running at low output to precisely maintain a temperature than one that is constantly turning on and off as rooms heat up and then cool off again.

Many misguided HVAC contractors will say that since heat pumps can lower their output it doesn't matter if they are oversized. This is very, very wrong, and can lead to very expensive mistakes when specifying equipment.

While inverter heat pumps can lower their output, they can't lower it to anywhere near zero. Most equipment can only reduce output to around 25% of rated capacity. When heating loads are less than that they are forced to turn on and off instead. Some short cycling is inevitable at very mild temperatures, but when it happens on colder days it defeats the purpose of having a variable output system and operating costs go up.

Remember that a load calculation tells you the required output on the coldest days, but average heating days will need much less capacity.

Imagine a modest, energy efficient new home in a cold climate where the 99% design temperature is 0° F. The load calculation says it needs 24,000 Btu to maintain a 70° F indoor temperature when it is that cold outside.

But it usually isn't that cold outside. A location with a 0° F design temperature probably has average winter temperatures closer to 30°, and the home only needs 12,000 Btu to stay warm at that temperature. When it's 50° it only needs 6,000 Btu.

If the installed heat pump for this home was chosen correctly based on the load calculation, and it can turn down to 1/4th of its rated output, it will operate at maximum efficiency anywhere from 0° to 50°, almost the entire expected heating range.

Now imagine a heat pump for this same home was instead chosen without doing a load calculation, probably by somebody who has never done one and doesn't really know what the loads should look like. They want to make sure there's enough heat, and “more is better”, and the equipment can “modulate”, so they guess 48,000 Btu.

At 0° F this system is only running at half capacity. At 30° it will start to short cycle and it will be short cycling for at least half of the heating season, using far more electricity than a correctly sized system would.

The above isn't a contrived scenario. Those are real world numbers, and 100% oversizing is very common. Systems that are oversized by 200% or more aren't uncommon. When people complain that their electric bill with a heat pump is much higher than they expected, this is often the reason why. Minimum system capacity is critical for operating efficiency with inverter heat pumps.

All of the above holds true for heat pumps in cooling mode as well, but it is less of a concern in the Northeast since heating dominates energy use. In warmer climates it becomes more important to accurately match cooling loads, for both efficiency reasons and also to promote good dehumidification.