Dimmer as backup to Thermostat?

[Gloryhound]

The people who insist that a rheostat won't work after a thermostat are mistaken.
If that were the case, a thermostat could only be used with one particular type of heat tape. By adding the rheostat, as far as the thermostat is concerned, you're just changing the characteristics of the heat tape.
Say the heat tape would normally get to 120F with the thermostat at 100%. You could set up initially without the thermostat and adjust the rheostat so the tape is 95F when the room is at the coldest. Then you add the thermostat before the rheostat and set it to the desired temp. If the thermostat fails 100% on, your heat tape won't be able to go over 95F. If it fails 100% off, your snakes will drop to room temp, but that's a lot better than cooking them.

The better solution is to use 4" flexwatt, which is designed to run cooler, to avoid the whole problem. I wouldn't use anything but a herpstat to regulate the temps.

The rheostat is a resister which limits current by using power! This means it will require the proportional T-stat to push more power to achieve the proper temps. This could result in overloading the proportional T-stat as most are only rated for between 400 Watts to 500 Watts. That equals a little less/over 4 amps at 120 Volts AC. In order to create a limiting point you will have to create a situation that limits based on its max Wattage. Running at max wattage is taxing on SCR's and transistors as they run hotter. This reduces the life of the proportional T-stat and also will create problems with the overload circuitry shutting down and no heat being created until it is reset. The only way I would do it is use a On/Off T-stat before the proportional controller if a back up was needed!

You can't hook over a 400 Watt heating element to a herpstat Pro! That is the most it will push!
Watts = Amps X Voltage
Voltage = Amps X Resistance

If you increase the resistance in the circuit voltage will go up to maintain a given amount of Amps through the circuit. If Voltage goes up to keep Amps the same then Watts go up! More Watts a proportional controller pushes out the hotter the internal circuitry will get. The hotter the internal circuitry gets the more likely failure becomes.