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Old 11-30-2008, 01:13 PM   #16
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Lennox Pulse 21 won't fire


Maybe at your company.
But at many other ones. They will still just change parts until it starts working again.

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Old 11-30-2008, 06:52 PM   #17
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Lennox Pulse 21 won't fire


Yuri,

I'm in the same boat. You are so right on about if the kid has a good attitude and will listen. It is a lot for newbies coming out to understand mvt systems and the like. I think the trade we are in is almost recession proof because we have seen all the changes as they happened and we understand the basics.
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Old 11-30-2008, 07:17 PM   #18
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No shortage of cold weather in the Great White North where I live so there is lots of work. The hardest part is getting them to understand the "logic" of how the machines work. Any fool can change parts but few people have the ability to truly think logically and understand circuitry etc.
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Old 11-30-2008, 08:53 PM   #19
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Lennox Pulse 21 won't fire


For CARRIERMAN:

Gas valve was bad, replaced it. Set pressure at 9" for LP. Replaced air flapper fabric. Checked spark output from module, jumps 3/4" at least. Took condensate trap apart and cleaned it. Cut inlet pipe in two and put it back together, thought maybe it was obstructed. Purge blower is shaded pole type but doesn't seem to run very fast to me. Should it? I'm going to try crows foot wrench on gas flapper on Monday, broke a band wrench on it last week. Wouldn't be so bad if it was easy to get to. I'm thinking of using a 3/4" plug socket driven by an end wrench to remove plug. I saw the tool in the service manual. It looks like it fits over the plug and lets you put a 3/8 drive extension in a square hole next to the plug in an offset fashion. Is that right? I have no idea of the history of this furnace, but its the first time I've been called on it. If I wasn't so stubborn, I'd tell the owner to find a Lennox pro.
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Old 12-01-2008, 12:59 PM   #20
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Lennox Pulse 21 won't fire


Dang.. that was quick ...Bye-Bye Aggatto
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Old 12-01-2008, 02:57 PM   #21
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Hi hoistdoctor

You usually have no need to remove gas inlet valve to the heat exchanger unless you suspect the metal reed is broken. The tool you saw in the manual is the one that is needed, yes it is that simple of tool. It sounds like to me that if you have done all the other stuff, all that needs to be done now is to change out the flame sensor and ignitor plugs. The inducer on these run alot slower than you are commonly used too, however the metal door covering the air inlet and inducer must be in place for yhis furnace to operate. They are alot quieter than we are used too also. If you are not sure if you have a restricted flue or air inlet, start furnace up and take a reading across presure switch to see if the switch closes. If you show no voltage than you have no obstruction after verifying there is voltage to the pressure switch. There were several different ignition modules used on these units, the original on yours should have been a black Robert Shaw with a gold label. But they very seldom loose ignition modules due to the simplicity of the furnace. On a call for heat the inducer comes on, the pressure switch closes, then the ignition module gets voltage via the limit circuit. The ignition module does a prepurge check, powers gas valve of check is ok and at the same time fires ignitor plug. If unit fires up, flame sensor reads a rectified a/c singal or what I like to call a contolled short to ground. If the module does not read this signal than the module is more than likely to be bad. The other problem in this scenario is called formicary corrosion, this will inhibit the ground completely or run the module out of its peramiteres. Either way it will not continue to run if this is the case. Let me know if I can help any further.

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Old 12-01-2008, 05:05 PM   #22
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Lennox Pulse 21 won't fire


"The other problem in this scenario is called formicary corrosion, this will inhibit the ground completely or run the module out of its peramiteres."

What is this "formicary corrosion"? I have never heard of it.
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Old 12-01-2008, 05:12 PM   #23
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Lennox Pulse 21 won't fire


It does everything it supposed to do except fire off. I have a spark (at least the module works) and I have air flow through it. I have tried to operate it with the condensate trap disassembled and the air inlet pipe cut in two as close to the inlet I could get and still couple it back together, and it still will not ignite the gas. I can feel the air flow through it and smell a little gas, not real strong odor. That's why I thought I'd look at the gas flapper before I removed the spark plug. I can hear a spark but maybe it was through a porcelain crack? The original problem was a bad valve. The original gas valve was a Robertshaw and Lennox supplied a White-Rodgers which is longer. I had to replace some nipples to make it fit. FYI, the ignition module is Johnson Controls. If Congress would bail me out, I'd take a six months vacation some where its always warm.

Last edited by hoistdoctor; 12-01-2008 at 05:14 PM. Reason: Poor spelling
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Old 12-03-2008, 04:25 PM   #24
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Lennox Pulse 21 won't fire


Hi yuri

I probably could have been a little more clear on that. Most furnace ignition modules and furnace manufacturers rely on the chassis for the ground path. When you place a furnace in a harsh condition, in this case the attic. If the space is not a climate controlled area it will start to electrostatically draw moisture and corrosive acids out of the air. It does not attack the painted cabinet as quick as it does the galvanizing on the screws that assemble the furnace. You can usually pull a screw out of the cabinet that has this problem and see a small amount of white stuff or a discoloration under the dead of the screw. This form of corrosion actually over time will wick itself around the threads of the screw and cause almost a silicone like insulator. I know this sounds crazy, but if you run into a problem similar to this you will understand. You will note since you are a Lennox person that the new GCS series combination units that have been built since the late 90's have a ground wire to the burner frame that extends to the ground screw on logic board. Call Lennox and asked them who suggested this to solve the misfire problems that were prevelent on the earlier GCS 16 & 21. You have my Dad and I to thank for that fix. Anyway hope this helps explain.

Hoistdoctor,

Pull the flame sensor and ignitor access door and look at the heat exchanger with a extension mirror and a good flashlight. You will see a cast steel football shaped piece that the ignitor and flame sensor screw into. On the bottom of that you will see a coiled piece of pipe that looks like an exhaust pipe. Look at where the football and the exhaust pipe are welded together and make sure it is not cracked or even completely busted. Follow the exhaust pipe down to the secondary heat exchanger, it looks like part of an evap coil. Make sure you don't see any cracks or breaks in the pipe. This furnace has had this problem when not properly serviced over its lifetime, this is sad when this happens. As I said there arent very many advocates for this furnace but I was one of the few that really understood it's operation well at the company I used to work for.

Good luck
Rusty
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Old 12-03-2008, 07:19 PM   #25
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Thanks for the reply. We don't have attic furnaces where I am as it is too cold up there. A few A/C airhandlers so I won't necessarily see your problem. Damp basements yes. Have seen circuit boards turn green and fail prematurely. Told one guy to buy a de-humidifier or I would void his warranty next time. I like the Pulse but hate all the horrible times I had with them re-circulating etc etc. Hard to kill them. Mom's unit is 25 yrs old because I can fix it for free. There apparently is another problem where corrosion builds up on the tip of the area where the gas enters the chamber and it will divert the flow of the gas away from the plug. Apparently you can remove the gas orifice and run a coat hanger down there and break off the deposit. A couple of old timers told me about this. Hoistdoctor will have a horrible time trying to do that procedure if he has to lay on his side to get at the unit etc. Most Pulse's in my area are darn expensive to repair (for what we charge) and a lot of people are replacing them with G61V or G71MPP units. The other interesting thing in our biz is how few people really understand flame rectification. I train a lot of newbies and they don't teach them much useful troubleshooting stuff in school. Codes etc yes. The newer circuit boards are also very grounding sensitive. Lennox grounds the secondary side of the transformer and the board does a self test and grounding test on startup. The G71MPP is sweet. Has alphanumeric diagnostic codes on the board etc etc. Pricey unit though. We sell lots of them. Tell the people it is the best and they buy into that. We are a very old well established company. 10 yrs p and l warranty helps a lot.

Cheers

Last edited by yuri; 12-03-2008 at 07:25 PM. Reason: add more info
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Old 12-05-2008, 07:09 PM   #26
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Lennox Pulse 21 won't fire


Been gone for a few days. I noticed one thing about the spark module, its a Johnson Controls and it has no neutral terminal though there is a neutral wire into the area where it is, its just not connected to anything. It looks as if the module has been replaced and the old one used the neutral. My plan is to attack this beast again on Saturday. I'll look inside for cracks, replace the plug (got the tool), pull the sensor and polish with Scotchbrite (don't have one), pull out the gas flapper valve and orifice and check for corrosion. I ordered the gas flapper material from Lennox and they sent me more air flapper stuff, should last me 200 years now.
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Old 12-05-2008, 07:12 PM   #27
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Lennox Pulse 21 won't fire


Dang. I hope you're getting paid TM.
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Old 12-05-2008, 07:43 PM   #28
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Lennox Pulse 21 won't fire


I would have given it a stick of dynamite by now. The extra neutral wire is indeed unused and unnecessary/fine the way it is now.

Last edited by yuri; 12-05-2008 at 07:45 PM. Reason: add more info
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Old 12-05-2008, 07:47 PM   #29
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Doesn't pulse technology live on in Hydrotherm and other Mfgs?
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Old 12-05-2008, 07:53 PM   #30
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I'm not sure. But my Plumber guys tell me that all the new high efficiency condensing boilers are not what they are made out to be. Problems with water hammer noises if they are not cleaned out properly at installation etc etc.

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