Drain tube heaters work to keep frost from building up in the drain tube that leads from the evaporator trough to the condensate pan at the bottom of the unit. When frost builds up on the evaporator coils, it is melted off by the evaporator coil heater. This water then funnels into the drain tube and down to the condensate pan. If the drain tube heater fails to work, then the drain tube can become congested by ice build-up and can cause water to overflow inside the unit.