could be a blocked heater matrix ,try putting in a good coolant system flush and see if it clears blokage but you will then have to refill coolant system and antifreeze ,has anyone put something in the radiator by accident? such as oil or brake fluid? if so a detergent degreaser will need to be used as well as a flush . only use quality products most garages use forte products for this but there are other makes around which are good.follow refill proceedures and check for air locks
richard
January 2008