I noticed my heaters were not getting very warm so I flushed the heater pipes and fit a new thermastat, the temp gauge is just under half when hot and won't go any higher and the fan never comes on, checked the fan and wiring and it's fine, all hoses are hot bottom rad hose slightly cooler than top, heater is blowing warmer now since new thermastat but why no matter how long you leave it running it won't bring fan on? Any ideas.