How to troubleshoot a gas burner

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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A Process Improvement Story

A Process Improvement Story.

Back when I worked at Westvaco we had a need to change large heavy rollers called impression rollers. These impression rollers were changed about once a day and there were eight of them in a machine.

The process used for over 20 years was the one recommended by the printing press manufacturer. The process involved using a hand crank lifting table to raise and lower the rolls into place. Since the table lifted form directly under the roller, the paper web had to be cut in order to remove the roller. The web had to be taped back together afterwards. These tape splices were always difficult to get through the remainder of the printing press on start up.

We knew that on another brand of machine, they used a tray to catch the roller and the press had more travel in the impression roller mechanism so that the roller could be raised and lowered out of and into the tray. The tray was cantilevered so that it could slide between the roller and the web.

We decided to try to mimic the design of the other machine and provide a way to avoid cutting the paper. However, our machine was not capable of raising and lowering the rollers so we still had to have the capability of lifting the rollers.

We designed a hydraulic lifting table that was cantilevered from the operator end. The lifting bars rolled up and down on a post using roller bearings. The hydraulic cylinder was foot operated so that the operator could use his hands to align the roller into the bearing caps while he lifted it with his foot.

The lifting bars were narrow enough to slip between the web and the roller. We used webbing between the bars to support the roller so that it could be handled without damage.

The first prototype was very large and cumbersome as we were unsure of the strength needed to handle the rollers. Future versions were scaled down to make them more maneuverable and easier to handle.

These machines cost less than $2000 each to build. They were very simple to operate and completely reliable.

Down time on the machine cost about $1000 per hour so these machines paid for themselves almost every day they were used due to the time saved form not having to cut and splice the web.

I still have one of these machines in my shop and I use it for all sorts of lifting applications. It makes a great transmission jack. It is especially good for handling a heavy Jeep Transfer case transmission combination.