Ahh, just another ordinary day.
Installed in 99 (not by me). Problem is intermittent. Works fine, then
next time just the inducer runs and no heat. I got there the other
day. Customer says they have had off and on heating problems since
they had it installed. Previous company has been changing limits,
press switches, etc. Still same result. Sometimes it works. Other
times it dont. I checked everything and couldnt make it screw up.
Led 1 Led 2
Off Slow flash
*Press sw open or has opened after 5 times, clogged condensate lines,
vac hoses, flue, blah, blah (you know the drill.)*
I finally figure (guess) the press sw is bad as all the old metal ones
seem too. I took a shot and found the same day that I was wrong. No
harm. Im not billing them for incorrect diagnosis nor wrong part.
"Yes kids, even I have to take a stab at a diagnosis once in a great
while when i cant make the damn thing screw up in front of me."
I pull out the last resort and call Lennox to see if they have any
oddities that jump out at them. I describe the problem to them (no
heat and the vent motor just running and running). Seems they had a
lot of that. The guy tells me immediately it is the "Surelight"
control board causing it. The board has a relay that sticks. Kill the
board and it works again until next time. Change the board and all
will be well. Dont you just love "idiot lights"? I didnt even ask the
price. I know this one is going to "hurt bad". Customer is happy as
hell. I found the problem in one visit that the other company (can you
say, "Service Experts") and all there techs (10) couldnt find in five
I still hate Lennox. There parts are just too damn expensive.
Positive Pressure switches and problems... I know all about them.
I'm been living with a Lennox G24M3/4-100 furnace for 9 years and
fighting with problems that whole time. After having the board
replaced, the pressure switch replaced twice and a "high wind kit" put
in at my expense here is what I have learned:
1) The system is just plain flaky. The inducer fan never fails,
but the positive pressure sensor (aka pressure switch) often
does. It can take several iterations of the inducer fan before
the sensor registers. Sometimes it will fail 5 times in a row,
the furnace goes into "watchguard" mode and the house gets cold.
2) There are a lot of factors that affect this:
a) If the furnace gets hot inside, the problem is more likely
to happen. Keep the filter very clean! Don't close too
many vents or cold air returns. Set the main fan to run
as long as possible after the gas is turned off. The reason
for this, I believe, is that hot air is lighter and the switch
fails due to this. If you use a setback thermostat this can
accerbate the problem. When the temp is turned up in the
morning, the furnace runs for a long time and the inside of
the box gets hot.
b) The hose that connects the inducer to the switch must be clean
and not kinked. It shouldn't be too long. The connections
on each end must be airtight.
c) The connection to the inducer can get dusty. Take a drill bit
and clean out the connection. You may want to try to widen
the connection to the inducer.
Believe me, next Summer, if I can afford it, I will replace the
Any suggestions on a good model (preferably one without a positive
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