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...
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.
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
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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