Hayward H250 Millivolt Heater - Burner Won't Fire


I have a Hayward H250 Millivolt heater that is around 9 years old. I can get the pilot to light and stay lit with a nice blue flame and it is generating around 600 mV. When I turn the heater on and turn the thermostat up, the heater won't fire. If I jump across the TH terminals the heater will fire up and run.

I have taken the pilot assembly out, cleaned the orifices, and blown the lines, with no change in voltage output or behaviour. If I heat the thermopile with a torch it will go up to around 650 mV before going OL.

What should the voltage from the pilot/thermopile be to open the gas valve and run the heater?



TFP Guide
Jun 4, 2012
western NY
At 600 MV it should light. However, I have been trolled by thermopyles that seem to put out a good voltage but when under load seem to drop low enough to not allow them to light. The first thing I would do is to make sure all connections on all of the safety devices are good. These can build up resistance and they can be the cause of the system not firing. The jumpering that you did essentially took all of the safety's out of the circuit. Sometimes simply unplugging them and plugging them back in will be enough to establish a good conection.

  • Like
Reactions: dickiesw
Thread Status
Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.