What is the correct way to set the bypass on a FDO thermostat?

[ectofix]

That’s a good question to ask.  I’ve often encountered temperature-related service calls on those thermostats where I found they weren’t set up properly during installation (i.e., replaced by someone not knowing how).

 

MOST manufacturers of ovens & griddles employing those thermostats provide a step-by-step procedure in their service manuals.  You might want to check that out.  Otherwise, here’s MY step-by-step procedure:

 

Assuming the thermostat is GOOD:

Set it to an appropriately high set-point.  350° sounds good.  Let it get happy there.  Monitor the temperature with a quality digital thermometer to be sure.  If the thermostat is off by more than 50°, manufacturers will tell you to replace it.

-NOTE: If you’re working on an existing thermostat and was told it’s getting too hot, don’t assume it’s a bad thermostat yet.  Try adjusting the bypass DOWN first.  Somebody may have set it up wrong during replacement.

Once the thermostat has satisfied and throttled down, Turn its setting DOWN to something like 250°…WELL below set-point.  At this point, thermostat should only be passing gas through its bypass flame internal port. Inspect the flame.  It should around1/8″ high.  All burner ports should be participants throughout the perimeter of the of the burner and should show established but small flames each.  No flame fluttering or “walking” allowed. If the flame is ABOVE 1/8″, take a slot screwdriver and adjust the “B” screw (bypass) on the thermostat body IN – slowly and incrementally clockwise to gradually reduce gas flow to the burner. If the flame is BELOW 1/8″ or trying to go out, adjust it OUT – slowly and incrementally counter-clockwise. If you think you got it, check your work.  Briefly throttle the thermostat way above actual oven temperature to gain a full flame again, then crank it back down below actual oven temperature.

 

Did the bypass flame maintain your setting?  Very likely.  If not, then make further adjustments accordingly.

 

Final checks:

Let the thermostat ride at a set-point lower than actual oven temp.  The temp you’re reading on the digital thermometer should drop.  It will do so  s-l-o-w-l-y, but it should drop. Obviously when the thermostat is at bypass flame,the oven temperature should NEVER continue to rise. After all this is done, THEN check and adjust calibration of the thermostat.