Heat pumps are an excellent choice for pools that experience mild seasonal variation because they don’t actually generate heat. Rather, they use electricity to transfer heat from one place to another. That is why they are a particularly good choice where the ambient air temperature is warm enough (above 40 ° F to 50 °F) to be useful in water heating. Most people swim outside when the weather is warm, so this isn’t always a great concern.
A heat pump works like a refrigerator operated in reverse. Refrigerant in the heat pump is used to capture heat from the outside air which is then transferred to water flowing through the unit.
More specifically, heat pumps work as follows: With the pool circulation system on, the water is drawn through the filter and the heat pump. Within the heat pump is a fan that draws in the outside air and directs it over the evaporator coil. Refrigerant inside the evaporator coil absorbs the heat from the outside air and converts from liquid into a gas. The warm gas in the coil then passes through the compressor. The compressor increases the heat, creating a hot gas that then passes through the condenser. The condenser transfers the heat from the hot gas to the pool water entering into the heater, which is then returned to the pool. As it flows through the condenser coil, the hot gas returns to liquid form and back to the evaporator, where the whole process begins again.
Standard heat pump units use reciprocal compressors while the newer higher efficiency heat pumps usually use scroll compressors.
Heat pump sizing involves several factors. Like other heaters, they are sized based on the surface area of the pool and the desired temperature rise..
However, wind exposure, humidity and night temperatures also affect the heating load. For example, a pool located in an area with high winds, low humidity and cold nights will need a larger heater.
Heat pumps are rated by their horse power and BTU output. Typical sizes are 3.5 hp/75,000 BTU, 5 hp/100,000 BTU, and 6 hp/125,000 BTU.
The proper way to size a heat pump begins with determining the desired temperature rise based on the coldest month that the pool will be in use. Next, calculate the pools surface area. Multiply the surface area by the temperature rise by twelve.
For example, for a 20 °F temperature rise and a 300 square foot pool, a 72,000 BTU heater will be needed The energy efficiency of heat pumps is measured by coefficient of performance, COP. The higher the COP, the more efficient the heater. Note that that there is no standard test for measuring COP, so it is not really possible to compare the COPs of heaters made by different manufacturers.
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.