I have a 25 year old Trane Forced Air furnace, central air and a honeywell cronotherm 3 t'stat. Last Saturday morning, I lit the pilot light for the furnace and set the switch on the t'stat from OFF to HEAT. The fan switch on the t'stat is set to AUTO. As soon as the t'stat calls for heat, the burner and fan start at the same time. It has NEVER done that before. The unit continues to run for a few minutes, burner shuts off, fan runs for another couple minutes and shuts off - for about 5-10 seconds. Fan comes back on for 1-2 minutes and shuts off again. The fan limit switch appears to be set at 90 degrees off and 130 degrees on. I didn't have any trouble with the t'stat during the summer months with the AC. Could the fan limit switch or t'stat be bad?
If it's the old type fan/linit switch ( the type that rotates around) These have a spring & it rotates around by heat. Since it's had heat on it for 25 yrs it could have gone bad. If you jump it out at the t.stat r&w & it still does the same thing . It's probably that fan/limit switch