Why are my rads not heating but boiler appears to be working?

There's a valve - it directs the heated water to the rads or the taps (hot water tank) sometimes it gets stuck in the wrong position.

The "diverter valve" is an electrically operated valve which switches the hot water flowing from the boiler to the radiators or hot water cylinder depending on which thermostat is calling for heat. If both are calling for heat at the same time the valve will assume a 'mid-position', sharing the flow between cylinder and radiators. The diverter valve is sometimes called a 'mid-position valve' for this reason.

http://www.centralheating-diy.co.uk/images/3_port.jpg

Peccavi
January 2010