Most pool circulation pumps are set up to run daily for a fixed period of time. This is controlled by a time-clock or a pool control system.
Heaters will only operate when water flow is present, which, in the above case is provided by a time controlled pump.
Within the timeframe of the pumps circulation, the heater attempts to raise the water temp to a set-point. Heaters are directed by temperature, but indirectly by water flow from the pump, and the pump is directly controlled by a time-clock or timer of a control system.
Years ago, before efficient methods of heat transfer and heater insulation were made, when your pool pump's operating cycle timed out, either from a control system or time clock, the heater would stop firing. The heater might still retain a great deal of heat since there was no water from the circulation pump to take it away from the heater and move it to the pool. Also, the heater remained hot to the touch, which could be unsafe. So, the question is, how do you either shut off the heater, even if it hasn't heated to the set-point, by 15 - 20 minutes before, or how do you extend the pump's run cycle by 15 - 20 minutes after turning off the heater?
A fireman switch ensures that the heater stops firing somewhere between 15-20 minutes before the pump stops circulating.
Hayward heaters do not require a fireman switch. With the UHS heaters, slightly more than 4/5's the heat is transferred to the water at the time the pump and heater shut off, based on their efficiency. The heater dissipates any heat remaining in the body of the heater in a brief period of time.
