Trouble shooting a gas burner
While there is a lot of mystery surrounding flame safety circuits, gas burners are actually quite easy to trouble shoot. They really only need three things to operate: a fuel supply, combustion air and an ignition source.
In a forced are furnace, the combustion fan will come on first to purge the chamber. After a predetermined time, the flame controller will open the gas valve and provide an ignition source. If the flame is not detected in a few seconds, the gas will shut off and a new cycle will be required to relight.
The gas will be controlled by an electric solenoid valve. It opens and closes with an audible click.
The first step in trouble shooting the system is to determine if the combustion fan is running. There is often a time delay before the fan begins to run after the system is energized. Check the time control relay if the fan will not start or does not start at the correct time.
Next there will be some kind of air proving mechanism. Often this is a simple as a paddle in the air steam connected to a switch. It may also be an air pressure switch. Make sure that the air proving signal is present before moving forward.
Once the controller is satisfied that there is sufficient air and the combustion chamber is purged, the gas valve will open and the ignition source will fire. You can usually tell if the gas valve is opening by listening for it to click or feeling for it to open.
The two most common types of ignition are the pilot light and the electronic ignition. The pilot light is a separate flame that runs all the time. The electronic ignition consists of a spark rod and a high voltage coil. The coil provides the voltage end to make a spark at the gap on the spark rod. A neon lamp can be held near the wire to ensure high voltage is present during the spark cycle.
If the controller does not sense a flame within a few seconds of opening the gas valve, it will close the valve again and purge the system.
If you are having trouble lighting the burner after the gas valve opens, verify the gas pressure and make sure the gas orifices are clean.
If the valve fails to open, suspect a problem with the gas control circuit. Often this is a printed circuit board. Filed repairs or modifications are not generally allowed to these boards for safety reasons. If the board is bad, you will have to replace it with an approved unit.
If the burner lights but goes out with in a few seconds, there may be a problem with the flame sensing mechanism. In many burners this is simply a rod that generates a voltage in the presence of a flame. If bad it will need to be cleaned or replaced. Some systems use an optical flame sensor. These need periodic cleaning and eventual replacement as the voltage from the flame eye decays with age.
If the flame is unstable check the gas source for proper pressure or contamination.
Understanding how a gas burner works can make trouble shooting relatively easy. However, it is often hard to access the components in an enclosed furnace. It may be possible to remove the burner form the duct work and bench test the unit. If you find that the air proving switch works o the bench but not with the unit installed, check for blockages in the duct work.
Never attempt to relight a burner that has not been properly purged. Never do anything that will shortcut or bypass the purge cycle. Using these steps you should be able to isolate the cause of a gas burner problem quickly and possible save an expensive service call.
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I was having similar problems–the furnace would work fine for awhile, then the house would get to 59 or 60 degrees even though the thermostat was set at 65. A couple of times i went down and checked the furnace and flipped the on off switch on and off and it’d fire right up. a week later I was having similar problems and did it again and again it worked for 1 heating cycle, then it wouldn’t come back on. I found the flashing code that indicated heat sensor issues and came to this web site. I pulled out my sensor and cleaned it with fine sandpaper and reinstalled it and viola–the flashing led lights went back to normal and the furnace is working fine so far! Thanks a million (and $145)
John
Titanic Controls
I do consider all of the ideas you have offered on your post. They are really convincing and will certainly work. Still, the posts are too quick for newbies. Could you please extend them a little from subsequent time? Thanks for the post.
Website is so nice surrounding flame safety circuits, gas burners are actually quite easy to trouble shoot and i like to visit on your site because your sharing the good detail about the gas burners.