My furnace recently went out (on the coldest day in Chicago
nonetheless). It turns out that the intermittent pilot control (Johnson
Controls G779) failed from being overheated. We could see warping
around the circuit board, but it also turned out that the air filter
was also plugged.
After installing a new pilot control and air filter, I am very worried
that the heater may still be overheating. The heater has been on for
several hours to heat the house up from a warm 37 degrees. The burners
go on for 4-5 mintutes, and then shut off. After 1-2 mintutes they are
reignited. The fan stays on for the whole time. This process repeats
itself while the thermostat is still set.
Is this normal for an old gas furnace that has been on for a while? The
technician said that if it displayed behavior like this the A-coil may
be plugged. Is this likely? Any tips would be most appreciated. Thank
When you say the fan stays "on", I am assuming you do not have your
thermostat set at "fan on", in this case no, I don't think it's normal.
The burners are suppose to stay on until the thermostat is satisfied.
Something else is not working right. You should call someone to have a
look at it.
The fan stays on when the thermostat is set to "ON" and the desired
temperature has not been reached. If I turn the thermostat off,
everything turns off. My concern is if the burners should be cycling
like they do (on 5 minutes, off 1 minute) when the unit is "ON".
As other poster said sounds like a part changer.
Assuming your t-stat is not set for fan on, then the burner should come on
stay on and run until t-stat is satisfied. Your description is that the
burner is coming on then off fan stay on until it cools down. Fans usually
stay on to remove all of the heat from the exchanger. It sounds to me like
you do have another issue with the heater. Have you changed your filters
recently? Start there for sure. If that fails to correct the problem and I
would be very surprised if it did... (sorry) You need another service call
to clean the unit. Which with gas should be done every year anyway. At least
I did when I had gas heat. Maybe I am just more paranoid about gas than you
On 15 Dec 2004 21:23:52 -0800, email@example.com wrote:
Your problem is you have found a parts changer. Fixed the part, left
the problem. If it overheated, you need to find out why.
If you have a gas leak in your home and the house burns down, you have
to rebuild the house, right?
Do you just leave the gas leak for the house to burn down again?
Question? Was this the way your furnace was working before the part was
changed, only worse because of the clogged filter? Did your repair man
take a temperature rise from the return air to supply, or look at the
coil? I would bet that your problem is in the fan/limit located
directly above the burners, probably a honywell. You need a competant
hvac tech, not someone that says it probably is something, after he
already charged you to repair the unit.
It is not normal and is possibly due to dirt on the A-coil restricting the
airflow. This would cause overheating which would cause the safety limit
switch to cut off the burners. When it cools down, the safety switch would
then allow the burners to come on again and repeat the cycle. You definitely
need the problem diagnosed immediately and not continue to run the furnace.
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