You have to use a thermostat, dimmers and rheostats are unreliable and can only work correctly if the room temps are 100% stable which is nearly impossible.

I am always surprised when folks cheap out on the most important part of the set up which is the thermostat. In the long run a $75 Ranco or Johnson Tstat is alot cheaper and less painful than a vet visit.

Heat rope is fine as I have used it and Flexxwatt for years but either one won't work correctly without a Tstat