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  1. #1
    Join Date
    Apr 2005
    Posts
    333
    Had a call today on a Lennox Pulse. M# G21Q3-80-1. S# 5893F05132. HO said that furnace take long time to get going in the morning. Would lock out.(???). They would reset by t'stat and eventually fire and keep running. Of course when I arrived the system was running fine. Checked the diaphram, was dirty. System runs on LP and had 9"w.c. from a tap on the outlet side of the gas valve. Tap had been bored into a 90. The pressure would start at 9" then decrease to about 6.5" once it ignited. Unsure if this is OK with LP. Also pulled the flame sensor plug and ingnition plug with the special wrench. Real bugger since I'm right handed. Although the unit is from '93 the plugs were OK yet I cleaned them anyway. Unit lit off for me every time. Took a CO reading with my UEI meter outside at the termination. Read 9 ppm. Figure OK. Went back inside and took a reading in the plenum and got 1 (one) ppm. Plan on coming back in several days to do a pressure test on the hex. Although the system was igniting and running while I was there, could not figure out what was causing the lock out. ANY THOUGHTS ??? I have not worked on many Pulses.
    meanwhile, back at the ranch.....

  2. #2
    Join Date
    Jun 2002
    Location
    Bemidji, Mn
    Posts
    3,573
    They have had MANY issues witht hose Ign Modules locking out, if all checks out, replace the Module.
    You picked a fine time to leave me loose wheel...

    http://rapalaguy.spaces.live.com/

  3. #3
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,516
    should be 9in while running. if it is droppong to 6in you may have a problem with the diaphram comming into the house or at the tank. is the drat blower working. the trap need to be cleaned out once in a while also. i think it is a gas presure problem since it is colder in the am and the presure will be lower then. i assune you replced the air flapper metereial when you were there. being it was dirty can also affect the operation of the unit. first thing i do on all pules furnaces is replace the ari flaper meterail.

  4. #4
    Join Date
    Apr 2004
    Posts
    7,680
    I'd intermitant that POS to the curb.

  5. #5
    Join Date
    May 2005
    Posts
    9
    did you check your air flapper and what`s your out let and inlet pressure when the unit is on,the pluse furnace needs and equal amount of gas and air to work ,about the CO you might need to check your second heat exchanger.
    good luck

  6. #6
    Join Date
    May 2005
    Posts
    9
    just so you know in LP the manifold pressure should 9 and if the inlet pressure doesn`t change then you have a bad gas valve.

  7. #7
    Join Date
    Aug 2002
    Location
    manitowoc wisconsin
    Posts
    4,943
    The low manifold pressure or dirty air flapper can both cause intermittent ignition.With 1 ppm in the plenum I wolud say you need to sell them a new furnace.
    Take your time & do it right!

  8. #8
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    The G21 furnaces were never part of the recall for potential leaks in the connections to the secondary heat exchanger, but you should still do a pressure test to check it out.

    As others have said, the outlet pressure from the gas valve should stay at 9" WC while running. What was the input pressure?

    Did you get any CO readings in the return, or in the house in general? Measuring CO in the supply plenum alone is a relativly meaningless measurement without further checking to narrow down the source.
    1 ppm isn't much, most of my service area would register higher ambient CO than that most of the day if I could find a place with no CO at all to zero my analyzer.

    Someone mentioned checking the "draft blower". It is actually just a purge blower and should only run up until the furnace lights off, then it stops. Once the call for heat ends, the purge blower comes back on for 20ish seconds to clear out the HX.
    Once the combustion proccess is established, the pressures needed for air and gas intake, and exhaust of the combustion byproducts, is generated by the combustion "pulses" in the specially shaped combustion chamber.
    The furnace would run very rough if the purge blower didn't shut off, wich is the most common control module failure I used to find on the G21 furnaces.

    Don't be to quick to kick that thing into the gutter, those are great furnaces.
    If more government is the answer, then it's a really stupid question.

  9. #9
    Join Date
    Apr 2005
    Posts
    333
    Thanks for all the replies. I am not looking to condemn the unit, but definitely want to check/diagnose it properly. You guys gave me a wealth of info. Again, plan to pressure test unit friday. Will look into suggestions.
    meanwhile, back at the ranch.....

  10. #10
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,516
    the pulse is very gas air sensitive. if the air flapper is even a little dirty the mixture can be thrown off enough to shut it down. if the gas presure is not right the same thing. the condensate trap is pron the clodged up and should be cleaned. or blown out with a light flo of air.
    the exhaust should be checked for sags between supports. if water colects in sagging pipe it will not run. look down the pipes if you can and see is there is something blocking them. just the other day i pulled a plastic bag out of the purge blower compartment. these are the easyest furnaces to work on once you get past the thought they are complacated. right gas presure,clean air flapper for correct air mixture, and drainage, these are the most important things to look at. outside should have a inch and a half reduce with a 10 in piece of pipe straight out on the exhaust. and i put a 90 facing down on the intake but it isnt needed

  11. #11
    Join Date
    Apr 2005
    Location
    Boulder,CO.
    Posts
    559
    Check your condensate lines if a/c also runs in the same line. Iv'e found them plugged up several times unit runs fine on short run times but when it comes out of setback the longer run time will keep unit from draining and shut down.

  12. #12
    Join Date
    Apr 2005
    Posts
    333
    We (Me and a much more experienced tech) did a pressure test on the unit today. Did not hold. Leaking at secondary hex manifold. HO opted for new unit. They did not have any problems since I was there for intermittent heat. At this point consider myself lucky. Thanks for all the input!!!
    meanwhile, back at the ranch.....

  13. #13
    Join Date
    Jun 2002
    Location
    Bemidji, Mn
    Posts
    3,573
    Was this one under recall, pre 1988?
    You picked a fine time to leave me loose wheel...

    http://rapalaguy.spaces.live.com/

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