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Old 02-12-2009, 08:42 AM   #1
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Lennox Limit Switch / Watchguard Issue


Hi all - first off, I have to state up front that unless things are realtively simple and idiot-proof, I likely won't be doing much DIY work - what I hope to get from members here is some guidance that helps me know what I'm talking about so if I do need to call a pro in, I know enought not to get gouged! Any specific pointers on that would be much appreciated!

I have a Lennox G43UF unit about 5 yrs old and am getting the red blinking light and steady greeen - this code is apprently to do with the limit switch / watchguard system. What's happening is that usually the furnace goes on, ignites, etc. but doesnt stay on too long and never reaches the temperature specified by the thermostat. Once it turns off, it seems to stay off for quite some time before coming on again (to me, I'm guessing it's triggering the high limit switch and the watchguard kicks in for 1 hour?). If i do an on/off to the furnace power it seems to "restart" this cycle and kicks the furnace back on (but it still switches off before getting up to temperature).

With the past few days being warmer though, I've noticed at least once that the furnace has NOT stayed off for an hour. i.e. I heard it come on for a while, it eventually went off for a while, but then it restarted itself not too long after this (i.e. it wasn't "forced" off for 1 hour).

Any thoughts?
(and FYI, one source I've checked was the HVACmechanic.com forum where a poster named marchvac sounded very familiar with this specific issue but I can't seem to find him anywhere - and leads would be appreciated - may be from a Yuri if you're out there as well?)

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Old 02-12-2009, 08:51 AM   #2
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Lennox Limit Switch / Watchguard Issue


Spammers killed that site unfortunately. Is your air filter clean, all vents in your rooms open, ductwork clean. That furnace is riding the limit control due to poor airflow. If you have A/C the underside of the coil may be plugged with dirt/blower dirty etc. Check the obvious things and then call a Lennox tech if necessary. Ask him to do a temp rise test of the furnace and find out why you have poor airflow. That is a good furnace and needs proper maintenance. The flame sensor is hard to get at and should be cleaned by a Lennox tech and the face of the burner it sits in front of. If not you will paying OT/extra in the middle of the night for a tech to do it.


Last edited by yuri; 02-12-2009 at 08:58 AM.
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Old 02-12-2009, 09:13 AM   #3
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Lennox Limit Switch / Watchguard Issue


Thanks Yuri - re: your comments, the air filter probably needs replacement so I'll do that and the ducts were cleaned a few months ago. Re: vents, the house is approx 3200 sq ft so we try to manage the flow by closing off most of the downstairs vents during the night and opening the upstiars vents (and reversing this for daytime). We don't touch the return vents. I wouldn't have thought this would be an issue as we did this in our old house (this is our first winter in the new house), but I'd value your comments.
I'll replace the filter and test opening all vents and if that doesn't solve things, I'll bring in a Lennox tech. I'm in the Toronto area and from what I recall, marchvac was as well - I'm weary of just contacting someone I don't know from the Yellow Pages so if you have any way of putting us in touch I'd appreciate it.
And really, thanks alot for replying and sharing your expertise. The internet's a wonderful thing but people like you who are willing to share make it that much more valuable!
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Old 02-12-2009, 01:09 PM   #4
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Lennox Limit Switch / Watchguard Issue


Glad to help. He is part owner of a commercial HVAC co in Toronto. Unfortunately Toronto is huge and there are many HVAC companies. Too bad the owner of that website doesn't care about it as it was very good until the **** spammers took over. You need all the vents open. If you are handy you can do a temp rise test on your own. Put a metal cooking stick type thermometer in the supply duct 1 foot downstream from the plenum in the horizontal position. Subtract the return air duct temp. Look at the rating sticker by the burners for the allowable range. If you snoop thru enough of those posts he offered a tech a job interview and gave out his ph # etc.

Good Luck

Last edited by yuri; 02-12-2009 at 01:11 PM.
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Old 02-13-2009, 06:16 AM   #5
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Lennox Limit Switch / Watchguard Issue


It also makes a difference on what kind of filter you are using, ask the contractor to check the static pressure.
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