Heat pumps use a small amount of driven electricity to absorb heat and transport it from one place to another. Indeed, they couldn’t generate heat.

As swimming pool’s water circulated by the pool pump, the water get from the pool passes through a filter and pool heater. The pool heater has a fan that brings in the outside air and guides it over the evaporator coil. Liquid refrigerant within the evaporator coil absorbs the heat from the outside and then evaporate gas. The evaporated gas in the coil then flows through the compressor. The compressor pump to increase the heat, generate a very hot gas that then flows through the condenser. The condenser moves the heat from the hot gas to heat the cooler pool water circulating through the heater. The heated water then flow into the pool. The hot gas, as cross flows through the condenser coil, back to liquid form and return the evaporator, where the whole process begins again.

Higher efficiency heat pump pool heaters usually use scroll compressors versus the rotary compressors of standard composition.
Heat pump pool heaters work efficiently as long as the outside temperature remains above the 7.2℃–10℃ range. The cooler the outside air they bring in, the more energy they exhaust. However, since most people adapt outdoor swimming pools during summer or mild weather, this usually isn't an issue