it may have an air lock, some models have plastic bleed screws on the pipes to let air out or you may need to ease one of the heater pipes off. be careful not to scold yourself if doing it the latter way. or the heater matrix may be blocked. attach a hose pipe to the heater hoses & flush it through in both directions. old ones sometimes start to leak after flushing, then they need to be replaced.
jim, January 2010
