Dimmer as backup to Thermostat?

[Gloryhound]

If someone put a rheostat in parallel with the heat tape, a bad mistake, then your concerns would be well-founded.
In series, it's limiting the amount of current that could go through the thermostat. For example if you had 10W tape, with no rheostat and the probe came loose, you'd be pulling 10W through the thermostat at 110v. If you reduce the current 50% with a rheostat, the tape would only be able to pull 5W at 110v. As far as the thermostat knows you just replaced your 10w tape with 5w

The thermostat is going to run at a higher % at all times with the rheostat in place, but that current is going to be limited by the resistor in the circuit, and will use less power at max output than the tape alone. I'm not aware of any problem with a herpstat running around 100% for extended periods as long as you're not drawing too much current.

I don't think it's as good a plan as just using lower wattage heat tape, but it could work. A non-proportional thermostat before the proportional one may be a better idea, but that would increase your odds of a no-heat failure state.

I recommend 4" flexwatt + a herpstat, and no screwing around with rheostats or second thermostats at all.

Current is a constant in a series circuit and voltage is divided by the various loads.

Voltage is a constant in a Parallel circuit and the current is divided by the various loads.

This is why all household recepticles and lights are wired in parallel. If not each divice would not see 110V AC.