An oven's purge is designed to exchange four volumes of air, removing any combustible vapors from the unit before and attempt to light the burner is made. If the purge cycle is bypassed, combustible gases can build up in the oven, leading to an explosion.

Another person died recently. "So?" you say. "Thousands of people die every day." True, but this one died playing Russian roulette with the purge on an industrial oven. And it isn't the first time something like this has happened.

You're familiar with Russian roulette -- put one bullet in a revolver, spin the cylinder, point the gun at your head, pull the trigger and pray. With "oven roulette," the rules are a little different. If you don't shoot yourself the first time, you put a second bullet in the chamber. Each time you get lucky, you chamber another round until the cylinder's full. Fun, huh?

Of course, you don't have to play oven roulette. Oven and furnace control systems are designed so that the combustion air or exhaust fan purge the work chamber before the burner's pilot or igniter can be lit. The idea is to remove any gas or flammable vapors that may have accidentally accumulated in the oven since the last shutdown. Where can these gases come from? Perhaps the shutoff valves leaked over the weekend, the oven contains some unburned fuel from a startup attempt that failed. That's the situation I'll deal with here.

As a rule, at least four volume changes are required to drive out most of the flammable gases and dilute anything that remains below their lower limits of flammability. The fan damper is driven to the wide open position (if it's adjustable), and the fan is allowed to run long enough to provide the required number of air changes. Depending on the size of the fan and the volume of the oven, this can take several minutes. The whole operation is regulated by a timer in the combustion control panel.

The roulette game starts when somebody gets impatient -- usually when the burner fails to light after several tries. After repeatedly waiting for the purge timer to do its thing, someone gets the bright idea to dial its setting down to zero, or bypass it with a jumper. On the next ignition attempt, some gas is put into the oven. Round 1 is in the cylinder.

Skip the purge and try to light again. Click. Got lucky. Now bullet two is chambered.

Try again. Click. Phew! Bullet 3 is ready to go.

Sooner or later, the concentration of gas builds to a combustible concentration, and on the next lightoff attempt, the oven explodes. Now you understand why the purge is essential -- a purge would have flushed out the gas that was put into the oven after each failed lightoff attempt. Bypassing the purge each time allows the gas to gradually build to an explosive concentration.

So, here are the rules to follow to avoid becoming a victim of oven roulette:

  • Never dial down the purge timer or bypass it, no matter how aggravating the wait is. The poor little timer is only trying to protect your life. Even if there's a long wait between lightoff attempts (for example, while someone's trying to adjust the burner or pilot), don't assume the oven will purge itself.

  • If the burner won't light after two or three tries, quit trying.Something's obviously wrong, and continuing to press the start button will do you as much good as repeatedly stabbing the call button on an elevator or at a crosswalk. Get someone to check it out. Maybe the spark igniter is broken or a gas valve is not opening.

One last note: if you haven't done so recently, check the purge timers on your gas-fired ovens and furnaces. Make sure the times are set correctly and that there are no jumpers. Otherwise, the next person to start that oven will be picking up a loaded gun.

For Dick Bennett's followup piece on this column, "Russian Roulette Revisited," use the link at the bottom of the page.