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QUESTION:I'm kind of angry with myself. I have an extensive
background in electronics. I own a wide range of
test instruments and have tested or repaired many
electronic or electrical devices. My gas fired
induced draft furnance developed an intermittent
problem. Instead of getting the service manual
so I could at least try to narrow down the cause
of the problem I immediately called a very
reputable service technician. He couldn't do
anything, of course, because the intermittent
problem did not rear it's ugly head while he
was checking the furnace.
The following is a good description (copied from another
website) of how my furnace works:
If your gas furnace has a small motor which you hear running (not the
main blower motor) you most likely have a system that is loosely
called an induced draft furnace. That small motor is supposed to be
"inducing a draft through the furnace heat exchanger". Usually this is
the sequence of operation for these furnaces.
On a call for heat from the thermostat the first thing that happens is
that small inducer motor runs. After a couple of minutes either a
spark ignition takes place at an electronic pilot or a Hot Surface
Ignitor begins to glow " Red ".
Once a burning pilot takes place or the ignitor glows red, a signal is
then sent often times to a Printed Circuit Board (microprocessor) or
(minicomputer). Then and only then, will the burners try to light.
These printed circuit boards provide an extremely safe furnace in that
if the furnace doesn't light in (3) tries it will go into lock out
mode. To remove it from lock out turn the main power off to the
furnace and right back on. This will reset it and it will try 3 more
times to light.
My furnace is made by York (Model No. P3CCD12NO7601A).
After adjusting the thermostat that small noisy inducer
motor runs followed by the glowing red ignitor or electronic
pilot. The main burners light up as they should. The
problem is they turn off several second after they light
up. This process repeats a few times. If I turn my
thermostat off for a while and repeat the process the
furnace may or may not operate normally.
I'd like to be able to perform a series of tests so that
I can provide the service guy with some basic voltage,
current, or resistance readings. In other words, I've got
to provide the technician with enough information so he
might be able to quickly resolve the problem.
ANSWER: Many thermostats have a pre-act control which prevents the furnace from
running continuously until the setpoint house temperature is reached. If it
were not for the pre-act, the house temperature would overshoot to often
uncomfortable levels.
Therefore, a normal furnace with this type thermostat should "heat" the
house from temperature setpoint #1 to setpoint #2 by a series of often
relatively short burns.
WAG: Perhaps your thermostat is going bad. Happened to me.
If you short out the wires from the thermostat so that the furnace sees a
constant "on" signal, it should burn continuously until any internal high
temperature limit switches shut if off.
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