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Lennox furnace problem

6381 Views 17 Replies 4 Participants Last post by  trandothuy
Hello, I am new to this forum. I found the forum while searching for my Lennox G61MPV problem. I don't want to start a new thread and I don't want to hijack OP but I have the same unit. If I need to start a new thread, let me know. My diag code is flashing 2+3 which is low pressure failed open. My unit sits horizontally.

Exhaust vent to top of roof. Intake pipe is open to attic space. There seem to be some debris in the exhaust pipe. I took a 15' flexible electrical fish rod and shove it down the exhaust flue pipe from the roof. I had a small container on the other end to catch the debris. I removed the many rubber tubes inside the unit and blow it out with compressed air and brown gunk comes flying out of tee pipe.

I have many questions and hope someone will educate me on this unit. I am not HVAC guy so any explanation will be greatly appreciated.

While searching on the internet, I see many folks with Lennox high efficiency unit show a diag code of 2+4 (High pressure switch). Mine show a 2+3 code (Low pressure switch).

Could some one educate me on what the low pressure switch regulates and what the high pressure switch regulates?

Second question, the black box trap, does it have a switch or float inside this black box?

Third question, does the black box need to have some water in it for the heat furnace to run?

Btw, AC run fine Or (atleast that what it appears to me).

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You have a 2 stage furnace, low and high fire. Where the tubes from the pressure switches attach to the furnace ONE AT A TIME remove them and poke thru the holes they attach to on the collector box with a nail as they can get water mineral blockage. DO NOT mix them up when re-installing.
You need to use a nail or drill bit as it can be hard mineral.

If you mix up the hoses the switch won't work as there is a + and - pressure port. One of my techs did that. There is a +/- stamp if you look very closely beside the port.

On my G51 the black hose goes to the + however I am not sure about the G61.
The nail hits a baffle inside so no the length is different.

Only way to test for sure is to put a differential pressure manometer across the ports and see if the pressure/vacuum is correct. You may still have a blockage in your exhaust pipe. I VERY rarely have problems with gunk in the condensate traps of that furnace. Just checked my 11 yr old one and the water was crystal clear. I doubt it is a trap problem but probably blockage in the exhaust pipe still.

Lennox never seems to need to have water in the trap to start. Some brands you start and run for a couple minutes and then shut down to allow water to quickly fill the trap.
Jeez I would a NEVER thought of that.:surprise:

Not my first day at the Rodeo, Bud.:vs_no_no_no:

They don't need priming and very few do.
How old is the furnace. Probably 6-10 or more years and yeah they can fail after 5.

Failed open or failed to close means the same thing. If it is open then there is no draft/vacuum to close it as the inducer was not working.

Lots of companies use flat rate pricing which includes picking up the part and labor. Not sure if your guy does. It should have a 5 year warranty. Post the serial # and I can figure it out or you can do it yourself.

Lennox uses a 6396E type system. 96 is 1996 and E is May as E is the 5th month.

63 is the plant where it was made. If the vent was partly plugged it may have overloaded it and shortened it's life.
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