Boiler fires up for hot water but not heating why?

Hi, If you have a feed and expansion tank in the roof space, it may well be empty of water.
There is usually two tanks in the roof space, you need to look inside the smaller one.
Access inside the tank and push the ball valve lever down to allow the tank to fill, it should only part fill to give a head of water pressure to the system. Failing that try venting all your radiators then check the header tank again.

Graham F...........
January 2014