Hi there:
I have American Standard HEATING AND AIR , After set the temp, the heater is start running hot air, about 4-5 min is worm air, then become cold air and never stop, till manualy shot it off , keep blowing cold air , some one told me go and change the termostat controler but did not work, now what I'm: mad: ?
Thanks ,Rich

Your stat should have two fan settings. One setting is ato on and that lets the fan run only when heat is called for. The other setting runs the fan 24/7. 24/7 is actually not a bad thing. It is cost effective over the long hall and tends to make the temp in all rooms the same. When I move into a house I set the fan to run and it stays that way until I move out.

Furnaces have a high limit switch. If it trips, it will shut the gas off, and run the blower. It may trip because of inadequate air flow because of seriously dirty air filters or something, or because the switch is bad. If you seem to have good air flow, try replacing the switch. Remove it and take it to a HVAC or electrical supply and let them match it.

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Originally Posted by labman

Furnaces have a high limit switch. If it trips, it will shut the gas off, and run the blower. It may trip because of inadequate air flow because of seriously dirty air filters or something, or because the switch is bad. If you seem to have good air flow, try replacing the switch. Remove it and take it to a HVAC or electrical supply and let them match it.

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I agree with labman. Sounds like the main limit is opening but why, Lack of air flow or bad limit? Check return air filters. Evaporator coil may be dirty(cooling coil) restricting air flow or dirty blower wheel, both will make the main limit trip. Limits do go bad but over 90% of the time they are just doing their job.