3 out of 4 jets firing, gas smell in exhaust

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Old 01-15-09, 09:49 AM
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3 out of 4 jets firing, gas smell in exhaust

I have an 11-year-old Quatro True Blue 90, model # USA072NHR3. Actually, I have 2 of them, plus a carrier and a york. While renovating a 4-unit apartment building this fall, I've worked closely with an HVAC company to service all the furnaces. My crew and I did all the plumbing and electrical for them while they did the mechanical.

A couple weeks later, I noticed that in one of the Quatros, only 2 of 4 jets were firing. Sometimes 3, but never 4. I called the HVAC guys back out to take a look. They cleaned the jets and got everything working again.

This week, the other Quatro is having the same problem, plus the exhaust smelled strongly of natural gas. I figured I'd save $330 by cleaning out the jets myself. They did not appear to be dirty at all, but I blew them out with compressed air and blew out the manifold. I looked around the burner box and heat exchanger mouths (correct term?) for any other problems or obstructions, but did not see any.

Upon reassembly, little has changed. The 4th burner (farthest from the valve and manifold input) won't light. The gas smell seems to have gone away, though. I'll keep my eye (er nose) out for it and let you know if it's gone for good.

Thoughts or suggestions?

Thanks!
 
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Old 01-15-09, 10:21 AM
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Sounds like you did what you could. The subtlties of getting the burners to light are probably best addressed by someone who can observe them carefully with an experienced eye.

In the meantime, it sounds like you are wasting 25% of the gas you are using.
 
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Old 01-15-09, 10:50 AM
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the furnace isn't the only broke thing around here

Thanks for the quick reply.

Keeping in mind that I have already paid about $15,000 for HVAC services and I've incurred hundreds of thousands of dollars in debt during a severe economic downturn, what other things might I look for before calling the company back?

If gas is flowing out of the jet and through the heat exchanger, what might keep the flame next to it from igniting that gas? Is the induced air flow so strong that the unlit gas can't wander close enough to the other flame to ignite? Is my ignition system the main suspect?

Thanks again.
 
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Old 01-15-09, 07:58 PM
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Sometimes this is caused by bad burner alignment but that is something that probably would have been manufacturing defect and it wouldn't just started happening on it's own. Does your burner assembly look something like this?



A dirty or misaligned crossover (the flattened place between the burner elements) could cause this problem. If you feel you are up to it you could remove the burner assembly and check for alignment and foreign mater in the crossover impeding flame carry-over to the next burner. Be careful! If I were you I would take some pictures before disassembly to help getting everything back together correctly.

Originally Posted by lilfos View Post
Is the induced air flow so strong that the unlit gas can't wander close enough to the other flame to ignite? Is my ignition system the main suspect? Thanks again.
To much induced air flow can cause this problem but I cannot think of anything that would cause the furnace to just start doing this on it's own. You haven't changed out the inducer or removed any parts lately have you? The wrong inducer or a missing restrictor could definitely cause this problem.

It most likely is not a ignition system failure. It's doing its job by igniting the first burner. The fact that they all aren't lighting isn't the fault of the ignitor.

One other thing that can cause this problem is too little gas. Is your manifold pressure OK? I realize most do-it-yourself-ers probably don't have a way to check this but if you plan on doing much furnace repair you need a digital manometer or a magnahelic so you might consider purchasing one.
 
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Old 01-17-09, 05:23 PM
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xpogi, thanks for your help. It seems it was the crossover! I didn't realize how vital those "fins" are to propagating the flame from burner to burner. I thought they were just spacers.

I pulled out the burner assembly and removed the 3rd and 4th burners (it's pretty simple in this unit, just three screws holding in the 4 burners). The crossover part looked ok, but there was some deformity at the end that forms a ring around the jet. I straightened that out for good measure and then reassembled the burners, making sure to perfectly align the crossovers.

When I fired up the furnace, that fourth burner lit right up! There is no longer a mercaptan odor in the exhaust, either.

Thanks for steering me down the right path.
 
  #6  
Old 01-18-09, 06:38 AM
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Originally Posted by lilfos View Post
xpogi, thanks for your help...There is no longer a mercaptan odor in the exhaust....
Thanks for steering me down the right path.
Glad I could help and thanks for expanding my vocabulary. "Mercaptan" Had to look that one up.
 
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