Furnace Wont stop!!

Not sure if this is something common or not??
I have an American Standard Furnace from about the early-mid 90's
and i just noticed a day ago that the furnace in the middle of may was blowing air non-stop. i played with the temp controls, and then proceeded to switch it to the off position, to no avail.
At this point im thinking its the switch...i replaced that too, still no luck..same situation, the only way my furnace turns off is if i manually cut off the power to the unit.
Note: No heat is coming from the furnace, if knowing that helps any?
thanks in advance for any help provided...
Jay
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What's the Fan switch set to?
I don't know a lot about AS, but that may mean something is wrong. You may want to call a AC tech out to look at it.

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040519 2309 - JayP posted:

On the furnace there is a fan switch that turns the fan on when the heat exchanger gets hot enough, and shuts the fan off when the heat exchanger gets cool enough. While the furnace fan is operating in the present fault situation, tap this thermal switch with a screw driver handle and see of the fan stops running. It is possible that this thermal switch is stuck. If this works, and you don't have any more problems with it, then you have solved the problem. If the thermal switch continually acts up, you could take it apart and put a little lube on the swing arm of the switch, if it is accessible for repair. Some of these switches are sealed, and riveted closed. You could drill out the rivets and take the switch apart, then get some small screws and nuts and put it back together. Or, you might want to just get a new thermal switch.
If the fan control is part of an electronic circuit board, there is probably a relay on the board that is stuck. If this is the case, a tap with a screw driver handle should resolve this also.
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One of two things.
IF the unit has a ICB, or computer board that controls it, that is default to let you know the board is toast.
If the unit has a limit switch, the fan shut off setting is too low and its not going to cut off until the temp where the unit is located at drops, or the unit has gone bad, and the spring is sprung, and needs to have that replaced.
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Main blower on American Standard as well as most other newer furnaces are designed to come on and stay on if there is a problem with the furnace. Most likly cause is a limit switch not working. The high limit switch is designed to shutoff the gas but keep the fan blowing to cool down the furnace in the event of an overheating situation. Try looking above the burners to see if there is a high limit switch in this unit. Turn the power off to the furnace, pull the two wires off from the high limit switch and connect them together. Turn the power back on and see if the fan comes on. If the fan stays off the switch is defective and MUST BE replaced. Don't ever operate the furnace with the high limit switch jumped out. If you have access to a multimeter you can disconnect the wires to the limit switch and check the switch for continuety as opposed to doing the above method of testing.
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