My MasterTemp 250 heater works fine and heats the hot tub to the set point (104F). It cycles on and off as the temperature changes while it's being used. The problem I have is only when the jet pump is running.
When the jet pump is turned on, the temperature being reported slowly increases to around 109F, even though the water temperature slowly drops. Because it's reporting higher than the set point the heater doesn't come on and the water temperature slowly drops away. It's noticeably cooler than 104F.
Once the Jet Pump is turned off, the temperature correctly reports and the heater fires up again.
Unless someone has a better idea I think I'll replace the thermistor and the high limit switch.
When the jet pump is turned on, the temperature being reported slowly increases to around 109F, even though the water temperature slowly drops. Because it's reporting higher than the set point the heater doesn't come on and the water temperature slowly drops away. It's noticeably cooler than 104F.
Once the Jet Pump is turned off, the temperature correctly reports and the heater fires up again.
Unless someone has a better idea I think I'll replace the thermistor and the high limit switch.