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Old 09-18-2013, 07:41 AM   #1
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Flame Roll out on furnace


Hi Folks,

last year when I built my workshop I installed a used furnace i bought off of craigslist. It's a lenox natural gas powered forced hot air furnace. It is the type of furnace with a draft fan and exhausts to a 4" b-vent. It is NOT a high efficiency unit that vents through PVC...

Anyway, this particular unit is electronically controlled. The draft fan kicks on, the igniters start sparking, the gas valve opens and whoosh...there's a nice blue flame. The heat exchangers heat up and the blower motor kicks on. After several minutes of running the flame shuts off (as it should). Once the heat exchanger cools down and calls for heat, the igniters fire up and gas valve open. It is usually at this point I get a small flame roll-out. My assumption is that the heat exchangers are hot and that coupled with incoming fuel makes for a pre-detonation thus causing the roll-out.

I currently have the heater hanging from my ceiling. It's laid on it's side. According to the manufacturers instruction this is acceptable. Well maybe not hanging it but operating it on its side.

Once the desired temp is reached the t-stat shuts down the unit. Many minutes later when heat is called for and the furnace fires back up, it fires up beautifully, no roll-out. I am only seeing roll-out when the gas kicks off mid-operation.


Thoughts... Input... Suggestions on a 'solution'? I was thinking of maybe bringing a cold air input into the air return. I think the heat exchanger temperature needs to be cooler when the gas fires back up.... but that's my $0.02. I'm not a total moron, just not overly-experienced in HVAC setup!


Thanks,

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Old 09-18-2013, 08:34 AM   #2
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Flame Roll out on furnace


what is the complete model and serial # of it?

when it is cold the draft thru it and the chimney is different than when it has been running/warmed up and that will affect the way it lights up. I suspect the burners are dirty and the ignition points are dirty and may need to be reset and positioned according to the manufacturers specs. not an easy DIY job.

also the heat exchanger internals may be partly plugged up so it really should be checked by a pro. preferably a experienced Lennox one as some of their units are finicky to setup. I know, I do lots of them.

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Last edited by yuri; 09-18-2013 at 08:50 AM.
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Old 09-18-2013, 08:51 AM   #3
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Flame Roll out on furnace


Quote:
Originally Posted by yuri View Post
what is the complete model and serial # of it?

when it is cold the draft thru it is different than when it has been running/warmed up and that will affect the way it lights up. I suspect the burners are dirty and the ignition points are dirty and may need to be reset and positioned according to the manufacturers specs. not an easy DIY job.

also the heat exchanger internals may be partly plugged up so it really should be checked by a pro. preferably a experienced Lennox one as some of their units are finicky to setup. I know, I do lots of them.
I'll have to get the exact model number and s/n when I can find a ladder to climb up and find it. I pulled the manual out of the drawer so I can tell you at the least that it is an 80MGF series and either an 80MGF2-75 or 80MGF3(x)-75. Theres a chart with models numbers and their specs. It is definitely one of the these two! I've heard these Lennox units are finicky! I should have found a unit that's completely mechanical... but the 4" [direct] vent was what sold me on this. ha-ha

I had pulled the burner unit apart and cleaned everything. I did not check the heat exchangers internally. Is there a way to do this? I can access the heat exchangers from the blower output and it's not blocked by anything. When I first got the unit, the insulation on the inside had fallen and created a blockage.

While I'm not a professional furnace repairman I'm an advanced DIYer. That being said, I also don't want to burn my shop down or kill myself if the unit explodes! I have an old acquaintance in the HVAC business. I'll have to find his number and give him a ring. Just like to have my 'ducks in a row' so I can understand in detail whats going on!

Thanks!!!
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Old 09-18-2013, 09:46 AM   #4
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Flame Roll out on furnace


A very finicky unit that is. When you remove the burners you have to blow them out with compressed air and then clean the little slots and openings in the burners as they scale up. It has a tubular heat exchanger and there is no easy way to check it or clean it. I would get your Buddy to check it and check the gas manifold pressure too. If it was used and sat unused for awhile or is/was in a damp environment then the internals may be corroded/rusted up. The ventor fan can slow down also and affect the draft. Quite a few reasons it can be doing your problem and more than likely it will get a lot worse soon.
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Last edited by yuri; 09-18-2013 at 09:48 AM.
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Old 09-19-2013, 12:32 AM   #5
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Flame Roll out on furnace


cfm, of blowers ability. in other words there's not enough air movement thus the rollout, by all means, is doing it's job.

clean your evaporator coil and remove your return air filter to begin with.
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