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QUESTION:To amplify, the control could be a brand other than Honeywell but will
likely have a round dial with several settings under the cover. Reading the
face of the dial you will see the warning "DO NOT ROTATE THE DIAL", If you
tap on the body of the control and it goes off you should schedule a service
call by a trained professional at a time that is convenient and economical
to have it set, checked or replaced. Setting them is not that difficult but
if it is the combination fan and limit switch type control the bi-metal also
controls the off safety of the gas valve in the event of overheating. You
could possibly adjust the control to cycle the fan but if the bi-metal is
relaxing with age you have no assurance that the high limit setting is
accurate or will work correctly. Replacing it requires a replacement that
is the proper insertion length. Furnaces must not exceed 200 in the plenum
by codes, yet the furnace manufacturer has no control over the plenum
because it is external to the furnace. The manufacturer is reduced to
testing in the R&D lab with a control that is placed on the furnace yet
prevents the magic 200 number in the plenum. Having worked in an R&D lab
you have my positive assurance that this is ridiculously tricky. While
replacement is definitely within the capability of some DIY'ers it is also
MOST DEFINITELY NOT within the capability of some who would lull themselves
into a false sense of security. Of course the particular furnace
manufacturer and vintage could mean it has an entirely different control
scheme rendering all of the above invalid except the part about calling a
professional. You should have enough information to phone for service and
cut out the inefficiency of not having the right part on the truck if it is
needed.
ANSWER: If this is a newer type furnace it has a control board and is running
the fan due to a fault with the system flame failure
pressure switch stuck open or closed
roll out switch open ect,
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