Abstract: A frost protection system is described for a condensate drain pipe for a boiler, air-conditioning unit etc to protect an outdoors run of the pipe. The system comprises an outdoors temperature sensor, a heater, and a controller for activating the heater in dependence upon the temperature sensed by the sensor. The heater is in the form of a flexible line (for example an electrical cable with a resistive filament). The heater (or a connection for it) passes through a fitting in the pipe indoors, and the heater extends along the inside of the outdoors run of the pipe. In cold weather, the controller activates the heater to produce heat inside the drain pipe and thus reduce the risk of the condensate freezing.