Quote Originally Posted by kitedemon View Post
Cycling a resistive heater should do less harm that running it 100% 24/7.
if kept within proper voltage specs, a resistor should hold constant practically forever. Heating them up and cooling them down is what causes them to fail over time. However, a switch, manual or electronic, that holds a constant tend to fail.(from dirt build up, heat, corrosion etc).

Plastic holds a higher R value than glass.