Whoa, back heated racks work great! But the problem is usually the thermostat probe is placed on or next to the heat tape, where room temp changes still have an affect. Like i said if you place the probe in an empty tub you shouldnt see any problems. How are you heating your chondro cages?
NEVER SAID BACK HEAT WAS BAD. Its not the best in a house that cant keep steady temps. I have yet to see back heat hold a nice 90-94 temp in a room below 75.