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QUESTION:I recently moved into the house we're living in, and have been having
problems with the gas furnace. I am an electronics technician, but have
not messed with furnaces.
My furnace appears to be made by Carrier. Model 395CAV036055, Serial #
3590A11001, Input 66,000 BUT/hr.
Here's my problem.....The electric ignitor (not sure if that's what it
is called) receives power (and glows, of course), then the gas is turned
on and it lights, then the electric ignitor begins to cool down and the
gas shuts off. Then the whole cycle repeats itself.
I have found that if I shut the gas and electricity off and clean
everything out, it will work fine for about a month, and then it will
start doing it again. Lately, that hasn't been working though, it only
works for a couple days. There are 3 gas burners on my furnace, and I
noticed that one of them has a temperature (or maybe flame?) sensor
that pokes up into the flame. It has one wire going to it. Is there a
way I can test this (I have a good multimeter)?
Any information would be helpful so I can get this problem licked.
ANSWER: The sensor you speak of is a flame sensor. It is a steel rod with a
pocelin insulator around it. To check this out you would need to hook
a milliamp meter is series with it and see what you get once the flame
is present. I dont know off hand what the exact measurement should be
for the particular Ignition control used on your furnace but I would
think .5 miliamps or better. Try cleaning it. I always do a before and
after measurement to confirm that cleaning actually made a differance.
Inspect the insulator for any small cracks, if no cracks are present
the sensor is ok. Also you may have a problem with the pressure
switch. To check for this TEMPORARALY jump out the switch and see if
the furnace runs properly then remove the jumper and check for a flue
blockage or possiby a bird inside either the flue, or inducer motor.
Pressure switch failures are fairly rare. It could also be a
possiblility that the inducer motor is not coming up to speed all the
time causeing the switch not to make. Other than that the problem may
be in the ignition control itself and may require replacement. If the
furnace is vented into a masonary chimney and was installed without a
flue liner, inspect the inside of the chimney to assure that no part
of it has deteriorated and fallen causeing a partial or full blockage
of the flue. If none of these suggestions helps I'm sure that there
will be others from this group.
Oh, one more thing, it may be a problem within the 3 wire pilot burner
which I will leave the explanition of how this works to someone else
or you can do a deja news search, I believe Ron (Sonofdawra) has
posted a very excellent explaination on the function of this not to
long ago.
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