Gas furnace pops and burn at the orifice


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Old 01-20-11, 04:51 PM
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Gas furnace pops and burn at the orifice

I have a Tempstar upflow condensing propane furnace. It will light normally, but sometimes during a run cycle, I will hear a pop followed by a wavering/whistling/vibrating/fluttering...very hard to describe, sound. The sound is caused by the gas burning at the orifice with the flame blowing into the manifold tube. The pop at the begining of this is when the orifice lights. The flame at the burner is only slightly smaller than it is normally. Usually, it will correct itself after a period of time. If I blow a gentle puff of air into the air intake of that burner, it returns to normal. This happens mostly to one burner. I have had all 3 burners replaced by local service guy. There is 10" W.C. pressure at the burner valve test port as specified in the manual.

What is going on and how do I fix it?

Thank You
Craig
 
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Old 01-20-11, 08:37 PM
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Was that pressure tested at the input or output side of the gas valve & was it taken while the furnace & all other gas appliances in the house were running?
 
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Old 01-20-11, 09:43 PM
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Originally Posted by Grady View Post
Was that pressure tested at the input or output side of the gas valve & was it taken while the furnace & all other gas appliances in the house were running?
Output side of valve, furnace running, no other gas appliances. The furnace is the only gas appliance.
 
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Old 01-21-11, 02:41 AM
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What you are describing is caused "flashback."


Fuel gas burns at a certain speed. If the gas flow is slow enough, the gas will flash back from the top of the burners to burn at the orifice. If the gas speed is fast enough, the gas burns at the surface of the burners where it should because the gas is traveling at too fast a speed for it to burn at the orifice.

That's pretty much always the cause of flashback.

Your 10" gas pressure implies that you use propane, which can have pressure issues that can reduce the gas speed and cause flashback.

Do you notice any patter to this problem? At night when it's cold for example? Long run times and cold weather might allow the liquid propane to get so cold that it can't produce enough gas from the liquid to provide sufficient gas pressure.
 
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Old 01-21-11, 01:33 PM
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It seems to happen more frequently when the weather is colder. It seems to happen most often (maybe 60%) in the first few minutes of a run cycle.

I killed some time in front of the furnace today, watching the manometer connected to the valve and waiting for a flashback. Took a while, but when it happened, the pressure didn't even twitch. I rechecked the pressure at that point. It was at 10.5" Just for kicks, I turned the gas valve regulator down, hoping to see what it took to make it flashback. I got down to 6.5 with no problem and quit at that point. It ran at 6.5" for maybe 30 seconds before I put it back at 10.5. I doubt that a momentary pressure drop of 4"+ would go unregistered by the manometer.

Your explanation of the cause makes perfect sense, and would point to the manifold pressure being low (regulator going bad) as the cause of it all. The burner that pops in 90% of events is the last one on the gas manifold, which reinforces your theory.


My experiment watching the pressure says that something else may be the problem, but I can't think of any other reasonable explanation. I was hoping for the manometer to confirm the pressure thing so that replacing the gas valve/regulator would be the obvious solution, but that hasn't happened.

Any other things to try before I roll the dice and replace the valve?

Craig
 
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Old 01-21-11, 03:36 PM
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I'd try removing and cleaning the burner orifices. A partially plugged orifice could cause the problem.
 
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Old 01-21-11, 03:45 PM
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When the problem first started, I had it serviced. the orifices were cleaned and burners replaced. I could't see the need to replace the burners, but agreed because I was not the 'professional' in the situation. It started doing it again almost immediatly. I may check and clean the orifices, it can't hurt, but wouldn't seem to be the problem.

Craig
 
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Old 01-21-11, 06:23 PM
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Do the burners have adjustable air shutters?
 
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Old 01-21-11, 06:47 PM
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Good work on using a manometer to monitor the burner manifold pressure.

Grady's question about a primary air shutter is a good one to ask.

Inspecting the burner orifice, especially the one feeding the offensing burner is worthwhile. Plugged burner orifices are rare, but insect can build nests in them and debris can accumulate, and that could reduce the speed of the gas.

Frankly, I wouldn't change out the gas valve based on what is known at this point. Your manometer testing eliminates that as a problem as far as I'm concerned, unless some other theory pops up.
 
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Old 01-21-11, 07:32 PM
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Yes, the burners have primary air shutters. They are all wide open as that seems to be where they make the proper flame. Do you have something in mind to try with them?

The orifices should be clean from the service call. but I'll try cleaning them over the next few days as time allows. While I'm at it, I'm going to swap the main offender to another burner and see if the problem follows it. I'll let you know the results.

How can such a simple thing be so complicated to diagnose?



Craig
 
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Old 01-22-11, 12:31 AM
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Primary air shutters wide open maximizes the gas velocity, minimizing the risk of flashback.


Primary air shutters closed would reduce the gas velocity, increasing the risk of flashback and might have contributed to your flashback problem.

Something worth checking, but not a problem for your furnace.
 
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Old 01-22-11, 08:12 AM
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Originally Posted by cm126 View Post
How can such a simple thing be so complicated to diagnose?
Ahhh...The challenge of service work.
 
 

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