Pool heater shut down, then restarted when pool filter turned off


Active member
Oct 17, 2018
Lake Oswego, Oregon
I feel like this must be some kind of electrical problem but the circuit breaker didn't trip. Fired up the heater (Pentair MasterTemp 300) this morning for the first time this season just like every season. No issues. Came home to 82 degree pool. Nice, but should have been 85. Checked the heater. It was off. No error code, no lights, no power. Dead. Still warm to the touch. No circuits tripped. Our pool filter is set to shut off for the night at 8pm and lo and behold, when it went off, the heater LED panel powered up! No error messages. Just like nothing happened. Any ideas?


Bronze Supporter
Sep 23, 2015
That sounds pretty bizarre. I think most heaters have flow sensors that will not allow them to turn on without adequate flow and/or pressure. Be sure your safety systems are working on it!


Feb 7, 2017
Sanford, NC
Question remains, what's the automation?
Sounds like the heater is hooked up to some sort of time clock or an automation panel.
If it just went dead, i'd suspect that the internal 24v 1.25amp fuse was blown, but coming back on like that? Makes me think the system is on some sort of a controller that is timed.


Bronze Supporter
Sep 23, 2015
My money is still on a flow issue. Maybe your filter is getting filled with debris and the flow and/or pressure to the heater is not enough to keep the flow sensor happy?


Gold Supporter
TFP Guide
Jun 7, 2017
Damascus, MD
Your explanation of the problem is confusing. For one, filters don't turn on/off, the pump does, however. Are you saying the pump was turned off but the heater remained on? Which can still happen although the heater's flow/pressure switch should prevent it from going into heat mode. Can you re-explain?