melting temperature of waste tanks

pianotuna wrote:
The bed heater comes with a thermostat that goes from 70 to 140 F. Clearly it was not getting any feedback. I do plan on an external thermostat set for 40 F. I know from previous testing that I'm good to -27 C (-17 f) with 600 watts of fan based heat, which was cycling off and on.

The risk is lack of feed back to the thermostat.

Your plan for an external thermostat would work for your application. The big advantage (in your idea) is that it can be mounted in direct contact with the tank. The “pseudo” feedback signal from the built-in thermostat is not telling the whole story in terms of utilizing it to control the heat pad. It is looking at the heat pad, not the tank that is supposed to be monitoring.

I understand the risk of failure and consequences should a malfunction come into play...but there is no really perfect design to address this. You can mitigate the resulting damage (should there be one) by using your existing water level sensor.

Use one of your level sensors (low, med or high) to control the heat pad. Use this to interface with the external thermostat. This will ensure that the tank is not empty before the heat pad gets a signal to turn on.

Camco makes eternal thermostat of different models to suit your need. Therm-O-Disc model 59T is sold by Home Depot. Last time I checked, it costs at about USD16.00.

It also comes with schematic diagram. PM if you need more details on hookup.

Requires a bit of technical skill, but at your level, this should be piece of cake.

Good Luck