Heat Pump Sizing | Why Does it Matter?

Getting the right size heat pump for your home is a very important process. It makes as much difference to the comfort of your home and the durability of the system as getting the right heat pump model or finding the right installer for the job.

Why the Right Size Heat Pump Matters

Residential heat pumps are built in 1-ton and ½-ton increments. A typical lineup might be models with 1.5, 2,0, 3.0, 3.5, 4.0 and 5.0 ton models. Some 2.5 ton models are made but almost no 4.5 ton models are made.

If a heat pump is undersized, the negative effects are obvious. It won’t adequately heat or cool your home when temperatures are either very cold or very hot. It will work too hard during those times too, which may lead to mechanical problems and having to replace it prematurely.

If a heat pump is oversized, the most common complaints are significant temperature fluctuations and parts of the house that don’t get adequately heated or cooled. Here’s why these problems occur. An oversized unit creates more hot/cold air than is required. It pushes that air into the house too quickly. The area surrounding the thermostat might warm/cool too quickly, shutting down the cycle before areas furthest from the thermostat get sufficiently warmed or cooled.

In addition, this type of short cycling is often cited as a reason compressors break down. Finally, when a home is cooled too quickly, the system doesn’t run long enough to adequately remove humidity from the air. A house with an oversize heat pump will be cool and clammy in the summer – not very comfortable.

How to Get the Right Size Heat Pump for your Home

There’s only one way to get the right capacity heat pump. You must have a load calculation done on your home. This analysis is sometimes called a heat loss or gain calculation, or is commonly referred to as a Manual J after the name of the manual that contains the required calculations.

The load calculation is actually a complex set of calculations designed to determine how much heating and cooling your home requires based on how quickly it loses heat in winter and gains heat in summer.

There are many factors used in these equations. The square footage of your home is used as well as how that square footage is arranged. For example, all else being equal, a ranch home will lose/gain heat more quickly than the same size 2-story. Your home’s construction type, insulation type, window number and quality, and the climate in your area are all major factors that affect the heat gain/loss load calculation. The home should be evaluated room by room for the most accurate calculations.

Some contractors simply “eyeball” your home and factor in the square footage, and then add a ½ ton of capacity “just to be sure.” This method may lead to all the problems outlined above. Hire a competent contractor to do a complete Manual J load calculation in order to get the right size heat pump for your home and the comfort and durability that comes with it.