I have a problem with my home heating system. The heat works fine when it decides to come on. I have a digital thermostat that is set for different temps throughout the day. At times the system works great and at other times it fails to come on when the house temperature drops below the minimum temperature the thermostat is set to. I've tried to manually turn the fan on the heater to the "on" position but it won't come on that way either. The heating system is close enough to the thermostat that I can hear the fan attempting to come on but it doesn't seem to be successful. I've replaced the thermostat thinking that it might be a problem with that. Also, the heater seems more likely to come on after it has been off for a while. Any help or knowledge would be greatly appreciated.