Sometimes from the moment I turn on the power to my 11 year old Max-E-Therm, the temperature value in the display randomly jumps around. It can be pretty stable at times, but when it’s really having this problem the value varies as much as 5 degrees up and down from the true temp with a new reading about every half second. This can happen even on a stone cold heater and before the burners fire. The heater will start up fine, but the moment the displayed value exceeds the temperature setting, it shuts off (as it should) even though the true water temp is nowhere near the target value. I can fool it for a while when heating my spa by setting the target temp to 106 (and it’ll heat without interruption for a long time), but once I set the target to something more reasonable, it’ll shut down once it gets close. Almost instantly after the shutdown, the heater will fire up again, presumably because it’s getting a lower-than-target value, run for a less than a minute (sometimes just seconds), shut down, get happy and fire up again, etc. Yes, this sort of shutoff cycle does sound sort of\kind of like a thermal regulator issue, but this cycling happens even when the intake water is chilly. Set the heater to 78 degrees, for example, and it’ll cycle like this once the water gets to 76.
I had hoped that replacing the thermistor (AKA temp sensor) with a new one would fix this, especially because it’s easy to do and the part is cheap, but I tried that today and nothing changed on the display. Still jumped around erratically, and eventually shows a temp so high that the heater shuts off. I thought that maybe the wires from the thermistor to the control board had issues, but I checked the resistance across the thermistor male electrical tabs and it was 8.88 something (micro ohms? I didn’t really pay attention to the unit display on the meter) and with the wires attached but pulled off of the control board, the resistance at the plug for connecting to the board was also 8.88. I guess it could be a bad wire or a loose plug to the control board despite my test, but doesn’t that usually just send the display to the max temp value when the connection to the thermistor is broken?
The membrane (the thin controller with the transparent window that allows the control board’s LCD numbers to show through) is a replacement Insync Engineering version that I put in back in 2015 and hasn't given me any problems since. But I assume that the membrane is simply an interface between humans and the heater, and couldn’t be the cause of the weird numbers. Is that a good assumption?
So maybe it’s the control board? Doesn’t it interpret the signal from the thermistor and translate those readings into what’s shown on its LCD? If the board was bad, it would explain why the processor thinks the water is at a temperature that it’s not.
Is a new control board in my future? I hate to drop $300 for a replacement if that’s not the case. Could it be some sort of wiring issue, where I’ve got an intermittent ground somewhere on the lines and as a result sending false sensor information to the control board? If so, any ideas of where I should look for such a problem?
Thanks for any advice!
Nick in Palm Springs
I had hoped that replacing the thermistor (AKA temp sensor) with a new one would fix this, especially because it’s easy to do and the part is cheap, but I tried that today and nothing changed on the display. Still jumped around erratically, and eventually shows a temp so high that the heater shuts off. I thought that maybe the wires from the thermistor to the control board had issues, but I checked the resistance across the thermistor male electrical tabs and it was 8.88 something (micro ohms? I didn’t really pay attention to the unit display on the meter) and with the wires attached but pulled off of the control board, the resistance at the plug for connecting to the board was also 8.88. I guess it could be a bad wire or a loose plug to the control board despite my test, but doesn’t that usually just send the display to the max temp value when the connection to the thermistor is broken?
The membrane (the thin controller with the transparent window that allows the control board’s LCD numbers to show through) is a replacement Insync Engineering version that I put in back in 2015 and hasn't given me any problems since. But I assume that the membrane is simply an interface between humans and the heater, and couldn’t be the cause of the weird numbers. Is that a good assumption?
So maybe it’s the control board? Doesn’t it interpret the signal from the thermistor and translate those readings into what’s shown on its LCD? If the board was bad, it would explain why the processor thinks the water is at a temperature that it’s not.
Is a new control board in my future? I hate to drop $300 for a replacement if that’s not the case. Could it be some sort of wiring issue, where I’ve got an intermittent ground somewhere on the lines and as a result sending false sensor information to the control board? If so, any ideas of where I should look for such a problem?
Thanks for any advice!
Nick in Palm Springs
