DIYbanter - View Single Post - Gas Furnace Flame-Out

wrote:

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.

SNIP

Sounds like a hole has burnt thru the heat exchanger, allowing
the (main) blower to force air back into the combustion area.
Better crack open the piggy bank...

Jim