According to the U.S. Department of Energy, heat pumps save both energy and money relative to gas heaters.
That is because heat pumps transfer heat from the outdoors into water, using heat that is already available and simply moving it from one place to another.
As the pool water circulates through the pool pump, it passes through the filter to the heat pump. A fan in the heat pump draws in the outside air and directs it over an evaporator coil. Liquid refrigerant within the evaporator coil absorbs the heat from the outside air, transforming it to a gas. The warm gas in the coil then passes through a compressor, which increases the heat, resulting in 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 rather than the reciprocal compressors of standard units. A scroll compressor uses two spiral-like vanes to compress gases and liquids. Using scroll compressors in heat pumps became common in the late 1980s and produced a quieter, smoother, and more reliable operation.
Because heat pumps use the heat from the outside air, they work efficiently as long as the outside temperature remains above the 45° F to 50° F range. The cooler the outside air they draw in, the less efficient they are, which results in higher energy bills. This generally isn’t an issue where they are installed appropriately because most people use outdoor pools during warm and mild weather.
Heat pump pool heaters generally have an upfront cost greater 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. Customers will save more money in the long run.
When selecting a heat pump pool heater, you should consider its:
• Size • Efficiency
• Costs Sizing a Heat Pump Pool Heater Heat pump sizing is not a do-ityourself job, and pool owners should be made aware that a trained pool professional should perform a proper sizing analysis for their specific pool to determine pool heater size.
That’s because 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: • Determine the desired swimming pool temperature.
• Determine the average temperature for the coldest month of pool use.
• Subtract the average temperature for the coldest month from the desired pool temperature. This will give the temperature rise needed.
• Calculate the pool surface area in square feet.
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 1° to 1 ¼ °F temperature rise per hour and a 3 ½ mile-per-hour average wind speed on the pool surface. For a 1 ½ °F rise, multiply by 1.5. For a 2° F rise, multiply by 2.0.
For example, for a 20 °F temperature rise and a 300-square-foot pool, a 72,000 Btu heater will be needed: 20 X 300 X 12 = 72,000.
Heat Pump Pool Heater Efficiency
The energy efficiency of heat pumps is measured by coefficient of performance, COP. The higher the COP, the more efficient the heater. The federal test procedure for heat pump pool heaters sets the test conditions at 80°F ambient dry bulb, 63% relative humidity, and 80 °F pool water.
The COP values range from 3 to 7, which means that for every 1 unit of electricity that goes into the heater, 3 to 7 units of heat come out of the heater.
Estimating Heat Pump Pool Heater Costs
The accompanying table from the U.S. Department of Energy estimates yearly heat pump pool heating operating costs by location, water temperature, with or without using a pool cover.
In the tables shown in this issue of Service Industry News, one can see the costs of heating pools with heat pumps and with gas heaters in different U.S. cities.
For example, a standard-size outdoor pool in Seattle operating an 80 percent efficient gas heater for 8 hours a day from the beginning of June until the end of August incurs an annual heating cost of $2,078. That same pool using a heat pump costs just more than $1,049 per year to heat.
Table 1: Annual Heat Pump Cost. 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 the national average energy cost of $.1301/kwh. Note that energy costs vary widely by region.