Sizing a Heat Pump Properly Impacts Your Home’s Cooling and Humidity

Published on: August 2, 2013

Unlike traditional cooling and heating methods that typically rely on separate units to condition your home’s air in both summer and winter, heat pumps can do both efficiently. This is especially the case in areas with moderate climates such as the Southeast. Even though heat pumps only remove moisture from your home when cooling, they can also act as a dehumidifier by switching between heating and cooling modes to remove moisture without changing your home’s temperature. Sizing a heat pump properly is important in order to get the most efficient operation and best humidity removal.

Heat Removal and Dehumidification

Heat pumps cool your home by removing heat from the air using the same refrigeration system as traditional air conditioners. In the indoor unit, an air handler blows air across the evaporator coil of the refrigerant system. The refrigerant flowing through the evaporator coil is maintained at a temperature cooler than the air in your home so heat energy in the air will be transferred to it, causing the refrigerant to change from a liquid into a gas. The refrigerant is then compressed to raise its temperature above that of the exterior air. It flows outside to the condensing unit where it passes through the condenser coil that has exterior air blowing across it. This allows the heat the refrigerant gained from the home to be expelled into the exterior air, and for the refrigerant to return to liquid form. It’s then allowed to expand so that it’s again cooler than the interior air and ready to be cycled through the system again.

Dehumidification happens during the cooling cycle as a result of the air in contact with the cold evaporator coil becoming saturated with water vapor. This occurs because colder air isn’t capable of holding as much water vapor as warmer air. The saturated air deposits condensation on the coils and the water is drained away, a process that results in drier indoor air.


The innovative thing about heat pumps is that the refrigeration system can be run in the opposite direction so that it’s transferring heat energy from the outside air into your home for efficient heating in the winter. While the highest efficiency that a modern gas furnace can achieve is around 99 percent, a well-maintained air-source heat pump can generate three to four times more heat than the electricity that goes into the process. This generally makes a heat pump a more inexpensive way to heat your home, except during times when natural gas prices are especially low.

An air-source heat pump also will struggle to adequately heat your home when outside temperatures fall below freezing for an extended period. They just can’t absorb enough heat from the outdoor cold air. For this reason, as well as providing the flexibility to take advantage of cheaper gas prices, many homeowners use a dual-fuel system that employs a gas furnace for so-called emergency heating. It can kick on when it gets especially cold outside, or you can manually switch to gas heating if that’s the cheaper way to heat your home as a result of changing energy prices. In our region with its moderate winter temperatures, your heat pump should be able to handle the coldest weather we’re likely to get.


For sizing a heat pump properly, your home’s cooling needs are a more important factor than its heating load. While you can compensate for not enough heating power with your backup, or emergency heating system, there’s not much a homeowner can do to counter the problems related to an oversized cooling system. However, you’ll still want to know your home’s heating load when considering the size of the backup heating system.

When it comes to cooling, an oversized heat pump can lead to energy inefficiency and decreased humidity removal. Both these problems stem from the fact that an oversized heat pump can quickly cool your home, so it only runs for a brief period before turning off. The amount of moisture that can be removed by a heat pump mostly depends on how long it runs, so the shorter run times lead to less humidity removal. This means you will have to run the heat pump in dehumidification mode to get as much humidity removal as you would during a normal cooling cycle of a properly sized heat pump. This is much less efficient since the heat pump is cycling between heating and cooling modes, but only removing moisture during cooling mode.

Oversizing also leads to less energy-efficient cooling. Even though a larger heat pump has to run for less time, it still uses more energy to cool your home than a properly sized heat pump would have. This is because it takes more energy to turn on and off than it does to run at a steady speed.

If you need help sizing a heat pump properly, please contact the pros at AirConditioningSouthEast.com. Our contractors serve residents of Mississippi, South Carolina, Georgia, Florida and Alabama.