sounds like a circulation problem or low pressure, check the pump and the system pressure.
is it ok in hot water mode? if not you may have a blocked heat exchanger.
Ian
March 2010
with so little info it may be best to call the police to get next door to turn down the music or stop battering the wife
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.