# Lennox Pulse 21 won't fire



## hoistdoctor (Jan 25, 2008)

I'm working on a Lennox GSR21Q-03-50-1 furnace. Found and replaced bad gas valve. Set pressure (LP) at 9". Won't ignite. Replaced air flapper (original?). Won't fire. Checked spark at module terminal...jumps 3/4" continuous. Plug wire looks ok. Haven't pulled out sparkplug yet. Tried to remove gas flapper and broke wrench! Cut inlet pipe to make sure was getting air, still no fire. Disassembled trap in exhaust to check for obstruction...no fire. Purge fan seems to me to be lazy, only "gentle breeze" at exhaust when it is running. Is this right? Should I pull off the air box to help break loose the gas flapper or have I missed something? This is frustrating!!! Will spark plug socket and imagination remove plug or do I have to have special Lennox tool? Would appreciate any advice short of explosives.


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## hvaclover (Oct 2, 2008)

hoistdoctor said:


> I'm working on a Lennox GSR21Q-03-50-1 furnace. Found and replaced bad gas valve. Set pressure (LP) at 9". Won't ignite. Replaced air flapper (original?). Won't fire. Checked spark at module terminal...jumps 3/4" continuous. Plug wire looks ok. Haven't pulled out sparkplug yet. Tried to remove gas flapper and broke wrench! Cut inlet pipe to make sure was getting air, still no fire. Disassembled trap in exhaust to check for obstruction...no fire. Purge fan seems to me to be lazy, only "gentle breeze" at exhaust when it is running. Is this right? Should I pull off the air box to help break loose the gas flapper or have I missed something? This is frustrating!!! Will spark plug socket and imagination remove plug or do I have to have special Lennox tool? Would appreciate any advice short of explosives.



Is 9" the right press for lp? You do know that certain of the pulses were safety recalled don't you?


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## beenthere (Oct 11, 2008)

You might want a complete check by a competent tech.


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## hoistdoctor (Jan 25, 2008)

9" wc is what the service manual and a tag on the furnace calls for. I know that you would expect 11", but that's not what it says. As far as "competent tech", I've only been in this business for 29 years and this has me thinking seriously of calling it quits! The recalls were for the earlier models, not the "21's."


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## hvaclover (Oct 2, 2008)

hoistdoctor said:


> 9" wc is what the service manual and a tag on the furnace calls for. I know that you would expect 11", but that's not what it says. As far as "competent tech", I've only been in this business for 29 years and this has me thinking seriously of calling it quits! The recalls were for the earlier models, not the "21's."



Might i suggest Hvac-talk since you are a pro?
Diy sites don't attract too many Lennox guys willing to give up pulse advice.


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## yuri (Nov 29, 2008)

I am a Lennox tech and got 1 yr on you. Been working on them since day 1 and hate them. The gas flapper is after the gas valve and not in the air box. You need a 5/8 and 3/4 deep socket (I believe) and flex head ratchet, a bottle of tylenol, safety gloves etc to remove the flame sensor and spark plug. The electronic fan timer board is almost obselete and I usually try sell the customer a new Lennox G61V or G71MPP unit as that one is very tricky to repair. I made custom wrenches/shortened some open/box head wrenches especially for those units. Good Luck

More info for you:http://tech.lennoxintl.com/PDFs/9815b.pdf


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## hvaclover (Oct 2, 2008)

LOL Reminds me of an incident.

A Lennox exec literally jumped in my **** (excuse lang) for criticizing the difficulty to access the spark plug and sensor. 

I remember when Lennox first unveiled the Pulse at our RSES chapter in Detroit. The place was packed to the rafters. I was still a young business owner then. 

The suit from Lennox made the the unit sound maintenance proof.

I didn't agree and told him getting to the spark plug was a bad design.

The Suit from Lennox asked me who I was to question this giant corp. called Lennox.

No surprise. They treat every body like dirt.


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## yuri (Nov 29, 2008)

I have no use for the "suits"/look at the big 3 car execs and their multi-million $$ salaries/results.Currently, Lennox builds a good product, has excellent tech support/training/DaveNet etc. which makes my life easier. Lots of the other brands have very little or no tech support/technical info. Cheers.


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## hoistdoctor (Jan 25, 2008)

I know where the gas flapper is, just broke band wrench trying to unscrew the gas line. I'm going to try a crows foot wrench next. I cn hear the spark so I don't think I'll try removing the plug unless I find nothing wrong with the gas flapper. The "suits" at Lennox out to be condemned to working on these the rest of their days!! To make matters worse, the brilliant installers put it in the attic with the business side facing the narrow side, just enough room for me to lay down beside it.


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## #CARRIERMAN (Oct 18, 2006)

Hi hoistdoctor

Just wanted to put my two cents in on this one. Its been a while for me, yuri is right the pulse furnace is a pain in the butt to service but still holds the highest efficency rating of its time. They are a very good furnace and I am betting that it has not been properly serviced since installation. That should tell you something of the potential quality. If you are going to tackle this yourself, grab your wallet and be ready. You are going to need a new flame sensor, and ignitor plug. The ignitor plug it uses is similar to a weed eater plug and in a pinch I have even used one. However the gap on the plug is about 80 thousanths of an inch. Not to easy to achieve on a weed eater plug, do yourself a favor and get the proper one. You can buy the service tool for about $25.00 if you go to a Lennox dealer. Don't laugh when you see how simple this tool is, you could easily make one for yourself. But I won't tell you how do to liability reasons. The next thing you need to do is pull the service cap off of the bottom of the flue pipe and clean all of the sludge out of the flue trap and the condensate drain. Last but not least take the six bolts out of the front cover and remove the plate. Take quarter inch nut driver and remove the intake flapper, its a cinch to do. You will need to replace this also when you are getting your other parts. All total you should be ready to spend about $100.00 including the service tool. After you have reasembled the furnace properly listen to how smooth this thing runs. One warning that I have is be carefull removing flame sensor and spark ignitor, there is a very good chance you will cut the back of your knuckles. Use extreme caution and take your time, I have also stripped out flame sensor threads trying to rush removing the sensor. If it feels like it does not want to unscrew easily, keep working it back and forth until it does. Thats about all there is too it, once again don't forget to clean the flue condensate trap, this is a must on these units. 

Good luck
Rusty


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## hvaclover (Oct 2, 2008)

If I was going thru all that crap I would be ready to do this.....


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## yuri (Nov 29, 2008)

I would send my junior techs and "talk" them thru it. Can get (2) 24 yr olds for the price of me.


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## hvaclover (Oct 2, 2008)

Hoist doctor

Let us know how this works out. Believe me when I tell you all the pros I know on the board are rooting for you.

Best of luck.


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## #CARRIERMAN (Oct 18, 2006)

Hi guys

Not sure if you had the same problems I have had with the junior techs. But most of the time all the money you should have made on a furnace like this one is gone when you have to go back and do it yourself anyway. I actually sent one of mine out when I was still a service manager with written instructions almost as detailed as these, he called me about 10 times and I finally gave up and went and fixed it myself. He unfortunately ended his carrer after this one, he wasn't quite as good as he could talk.

Rusty


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## yuri (Nov 29, 2008)

I am the senior tech in my company and have several of them "under my wing". Feel sorry for the young guys. We expect them to know how to work on everything from millivolt systems to ECM motors. If they have a good attitude I can get them up to speed, if not they will just be parts changers. The new Lennox G71MPP and other modulating units will put an end to parts changers. If a guy doesn't understand differential pressures/draft and manometers he is toast. Gives me more job security.


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## beenthere (Oct 11, 2008)

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


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## sgthvac (Apr 15, 2007)

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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## yuri (Nov 29, 2008)

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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## hoistdoctor (Jan 25, 2008)

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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## hvaclover (Oct 2, 2008)

Dang.. that was quick ...Bye-Bye Aggatto


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## #CARRIERMAN (Oct 18, 2006)

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.

Good luck
Rusty


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## yuri (Nov 29, 2008)

"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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## hoistdoctor (Jan 25, 2008)

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.


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## #CARRIERMAN (Oct 18, 2006)

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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## yuri (Nov 29, 2008)

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


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## hoistdoctor (Jan 25, 2008)

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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## hvaclover (Oct 2, 2008)

Dang. I hope you're getting paid TM.


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## yuri (Nov 29, 2008)

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.


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## hvaclover (Oct 2, 2008)

Doesn't pulse technology live on in Hydrotherm and other Mfgs?


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## yuri (Nov 29, 2008)

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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