All pool heaters have a series of safety switches inside that monitor if it’s okay for the heater to turn on. There is a pressure switch that senses if there is water coursing through the heater (heater MUST only engage if there is water flowing through the system). There are switches that sense if the heater is ‘too hot’ that will keep a heater from turning on. Sometimes the thermostat needs to be adjusted ‘higher’ to create a ‘demand for heat’. A dirty filter can prevent adequate water pressure from reaching the heater which will keep the heater from firing. The heater’s gas valve may be in the ‘off’ position. Please note that we provide instruction on lighting ‘standing pilot’ heaters in the event the pilot light has blown out in the heater. There are quite a few reasons that a heater may not fire, and if the problem involves mechanical repairs, we recommend that the unit be serviced by a trained mechanic. Please call us to schedule a inspection.