Gas Heater vs Heat Pump - Your first question might be one of cost. One alternative to a gas heater is a heat pump which runs on electricity and uses the outside air to heat your pool or spa. The data shown at left is based on a gas heater and comparable heat pump in the upper range of capabililty. A gas heater will heat your pool in the range of five to seven times faster than a heat pump and it costs half as much as a comparable heat pump. The main issue, however, is operational cost. Depending on energy costs, a heat pump can save you up to 75% over propane gas and 40% over natural gas. If you are in the northern states, you must be aware that heat pumps are only effective when the outside temperature is above 40 degrees. Below that temperature, the heat pump switches over to electric heating coils (resistive heating) which costs more than either gas.