I'm having problems with a 10-year-old Rheem Centurion forced-air furnace. The unit uses natural gas and is rated at 100K BTU.
I'm sure I'll use a lot of improper terminology trying to descibe this, but here goes:
Gas is fed to the combustion area through an electrically operated valve. The output from this valve goes to pipe that stands vertically. The pipe has four hex-shaped jets that each feeds into a horizontal venturi chamber where, I assume, the gas mixes with air for combustion. The flame from each venturi chamber is drawn into a horizontal, tubular combustion chamber about 12" long. Inside each combustion chamber, there is a pair of interlocking angle-brackets that, in effect, divide the length of the tube into four areas.
Here's what's happening with my unit: -- The ignition sequence goes fine. The impeller motor starts, the igniter glows, and the gas valve opens, all per the timing specs on the control-unit flowchart. -- During original combustion, with just the impeller motor running, I get blue flames racing neatly down the length of each combustion chamber. -- However, as soon as the main blower motor kicks in, the flame in the upper-most combustion tube starts losing its 'conformation.' A few seconds later, I'm seeing yellow flame mixed with the blue, and the flame starts to burble in the area between the venturi chamber and the combustion tube. If I don't shut the furnace down manually at this point, the out-of-chamber flame will trip a limit switch. -- This problem is unique to the top combustion tube; the other three continue to burn as expected after the blower motor starts.
Yesterday, I took out the venturi assembly and cleaned it. I also cleaned the angle-bracket 'separators' that run inside the combustion tubes. (I did not clean inside the tubes because I was afraid to poke something that didn't want to be poked.)
I noted that the bottom three 'separators' were entirely straight, while the top one (where my problem is) was bent about 6 degrees approximately four inches from the deepest end. This bend was somewhat crude; but I think it had to be intentional: there's no chance for something in this area to get whacked by mistake, and when I tried to put one of the straight separators into this tube, it didn't seem to fit.
Anyhow, after the cleaning, I put things back together and fired up -- but the problem was still there.
In the 10 years I've had this furnace, I've replaced one control unit (bad relay), one igniter, and one impeller motor (bad bearing). Other than that, the unit has worked pretty well.
Can anyone suggest why I'm getting this flame-out when the main blower motor kicks in? Solutions? Any hints on how to clean the inside of the combustion tubes? The combusion-tube 'separators' can be rotated about 45 degrees, to sit like the letter 'X' or like a horizontal-vertical cross; does this make a difference? Can anyone confirm that just one of the four separators on this model furnace is supposed to be bent? Also, the hex-headed jets on the gas feed pipe appear to be adjustable; any tricks-of-the-trade for these?
Thanks for any help.
Regards, Heatless (and clueless!)