Heat pumps use electricity to capture heat and move it from one place to another. They don't generate heat.

As the pool pump circulates the swimming pool's water, the water drawn from the pool passes through a filter and the heat pump heater. The heat pump heater has a fan that draws in the outside air and directs it over the evaporator coil. Liquid refrigerant within the evaporator coil absorbs the heat from the outside air and becomes a gas. The warm gas in the coil then passes through the compressor. The compressor increases the heat, creating a very hot gas that then passes through the condenser. The condenser transfers the heat from the hot gas to the cooler pool water circulating through the heater. The heated water then returns to the pool. The hot gas, as it flows through the condenser coil, returns to liquid form and back to the evaporator, where the whole process begins again.

Higher efficiency heat pump pool heaters usually use scroll compressors versus the reciprocal compressors of standard units.

Heat pump pool heaters work efficiently as long as the outside temperature remains above the 45ºF–50ºF range. The cooler the outside air they draw in, the more energy they use. However, since most people use outdoor swimming pools during warm and mild weather, this usually isn't an issue.

Heat pump pool heaters cost more than gas pool heaters, but they typically have much lower annual operating costs because of their higher efficiencies. With proper maintenance, heat pump pool heaters typically last longer than gas pool heaters. Therefore, you'll save more money in the long run.

When selecting a heat pump pool heater, you should consider its:

• Size
• Efficiency
• Costs.

You should have a trained pool professional perform a proper sizing analysis for your specific swimming pool to determine pool heater size.

Sizing a heat pump pool heater involves many factors. Basically, a heater is sized according to the surface area of the pool and the difference between the pool and the average air temperatures. Other factors also affect the heating load for outdoor pools, such as wind exposure, humidity levels, and cool night temperatures. Therefore, pools located in areas with higher average wind speeds at the pool surface, lower humidity, and cool nights will require a larger heater.

Heat pump pool heaters are rated by Btu output and horsepower (hp). Standard sizes include 3.5 hp/75,000 Btu, 5 hp/100,000 Btu, and 6 hp/125,000 Btu.

To calculate an approximate heater size for an outdoor swimming pool, follow these steps:

1. Determine your desired swimming pool temperature.
2. Determine the average temperature for the coldest month of pool use.
3. Subtract the average temperature for the coldest month from the desired pool temperature. This will give you the temperature rise needed.
4. Calculate the pool surface area in square feet.
5. Use the following formula to determine the Btu/hour output requirement of the heater:

Pool Area x Temperature Rise x 12

This formula is based on 1º to 1-1/4ºF temperature rise per hour and a 3-1/2 mile per hour average wind at the pool surface. For a 1-1/2ºF rise multiply by 1.5. For a 2ºF rise multiply by 2.0.

The energy efficiency of heat pump swimming pool heaters is measured by coefficient of performance (COP). The higher the COP number, the more efficient. However, there is no standard test for measuring the COP. Therefore, you really can't compare the COPs of different models unless you know that the manufacturers used the same test for each model. For example, the same heat pump will operate at a higher COP when the outside air temperature is higher.

Typically, manufacturers measure the COP by testing a heat pump pool heater with an outdoor temperature of 80ºF and pool temperature of 80ºF. COPs usually range from 3.0 to 7.0, which converts to an efficiency of 300%–700%. This means that for every unit of electricity it takes to runs the compressor, you get 3–7 units of heat out of the heat pump.

For an outdoor swimming pool, use the following tables to help estimate your annual heat pump pool heater costs and savings compared to using an electric resistance or a gas pool heater.

Table 1 estimates annual swimming heat pump pool heating costs by location, by water temperature, and with or without using a pool cover.

Table 1. Costs by Location of Heating Outdoor Pools with a Heat Pump*

*Figures based on a 1,000 square foot, outdoor pool heated with an air to water heat pump with an average COP of 5.0 at $.085/kwh.

Table 2 estimates the annual savings of using a heat pump pool heater compared to using an electric resistance or gas pool heater.

Table 2. Annual Savings Comparisons of
Gas and Electric Pool Heaters*

*Based on an electric resistance heated pool, which costs $1,000 per year at an electric cost of $.085/kwh, and using a natural gas cost 0f $.80/therm. A seasonal average COP of 5.0 was used to determine heat pump savings.

Proper installation and maintenance of your heat pump pool heater can optimize its efficiency. It's best to have a qualified pool professional install the heater, especially the electric hookup, and perform complicated maintenance or repair tasks.

Read your owner's manual for a maintenance schedule and/or recommendations. You'll probably need to tune up your pool heater annually. Because of a heat pump pool heater's many moving and electrical parts, it will probably require periodic service by an air conditioning technician.

With proper installation and maintenance, heat pump pool heaters can last 10 or more years.