For the last two years my Hayward H300 starts up intermittently when the temp of the water is lower than the setting. Last year I found that taking a wire off one of any of the safety temp sensors and putting it back on made it work for a while and then it would stop again.
I have a Hayward H300 Millavolt system that is connected to a Jandy Aqua link.
1. When the Jandy calls for heat the heater does not fire.
2. With the Aqualink still connected to the Heater I jump the two TH terminals and she fires right up and stays running. If I turn the heater off with the Aqualink and try to start it again it wont fire. With the wires removed from the TH terminals a jumper across the terminals runs the heater until the jumper is removed.
3. With the two TH wires removed from the gas valve I have over 600mv's at the gas valve.(Across the TH terminals)
4. With the two TH wires removed from the gas valve and the Aqualink calling for heat the two wires have continuity between them.
6. With everything hooked up and the Aqualink calling for heat I tried jumping each safety sensor one at a time and she wont fire.
7. Thinking there might be a connection problem I removed all the blade connectors from the safety sensor wires and soldered the wires to the sensors. (Excluding the gas valve Connections)