Intermatic 156T4042A Fireman Switch for Pool/Spa Heater
This switch provides coordinated control for any pool/spa heater and any filter pump. It is designed to turn OFF a gas fired pool/spa heater approximately 20 minutes before the filter pump shuts down. This eliminates the problem of pounding and knocking due to overheated water, reduces scale and lime deposits and extends the service life of the heater.