Trane Nat. Vent furnace

Greetings:
I am hoping someone can offer me some assistance here. I have a customer that has a standing pilot Trane furnace. Some time ago it started failing to light the last burner once the heat exchanger was hot. The flame will come within about an 1 1/2 inches of the burner on the spreader. Of course, after a short period there will be a small explosion and the burner may light. This only happens when the exchanger is hot, it will work normally when it is cold, and don't happen all the time.
I replaced the burner assbly, gas valve, adjusted the manifold pressure above 3.5" and below, I adjusted the air up and down and yet the problem still presists. I think I could solve the problem by moving the pilot to the center or restricting the air flowing to the burner, but I don't want to modify the furnace. I am more interested in finding the solution. I'm sure it is something simple that I am overlooking. One thing I haven't done yet was check the reaction time or flow differences between the Honeywell gas valves and WR that normally comes with the furnace.
If anyone can help me I will greatly appreciated it and so will my customer.
Thanks in advance
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Certainly sounds like the hot heat exchanger is increasing the exhaust flow pulling the gas away from the pilot. Is it natural draft?
I would not restrict the airflow. But moving the pilot is a possibility. But it shouldn't be attempting to relight so soon after it has run either.
Dan Kohlman wrote:

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jamesgangnc wrote:

sets the stat to over 72, that is when the problem starts. She states that if she keeps the stat set at 68 or less, it doesn't happen. Strange! I have attempted everything I can think of short of installing a different gas valve, but this is only a 10> possibility. But it shouldn't be attempting to relight so soon after

Hi James: Thanks for your reply. It is a natural draft furnace. I did look to see if the heat exchanger was cracked in that segment, but there are no indications that is a problem. I hate to modify the furnace to solve a problem if I can find the right solution. Regarding relighting too soon, when the customer sets the stat to over 72, that is when the problem starts. She states that if she keeps the stat set at 68 or less, it doesn't happen. Strange! I have attempted everything I can think of short of installing a different gas valve, but this is only a 100K furnace and I am using a Honeywell which is rated up to 350K. If you can think of anything else, I will appreciate it. thanks again dan
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I havn't worked on any Trane 80 percenters, but have worked on other brands. The couple things that come to mind are:
1) set the lower setting on the fan limit switch down a couple degrees, so it blows the HX a bit cooler 2) Pull the flame spreader off, and remove the furthest two burners. Bump em on the floor a couple times (air intake end towards the ground) to clear what rust and soot you can. Wire brush the tops of the last two burners, at least the last couple inches toward the gas tube. Knock the soot out again. 3) make sure the flame spreader bar is back as far as possible, they tend to be rusty and sticky
Please let us know how things work out. These can be real hair pullers.
--

Christopher A. Young
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So Stormy, that means you are bald?
Bob

snipped for sanity
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"Dan Kohlman" wrote:

Dan you better have a mighty large liability insurance policy if you modify the ignition or relocate the pilot. You can convert a standing pilot to intermittant ignition, but anything else esp relocating the pilot voids the UL & GAMA certifications and you could be held liable for any fires etc.
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snipped-for-privacy@gonefishin.net wrote

Naaaahhhhh! He doesn't have to worry about that! His next of kin do.......
--
Respectfully, Bob

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prior to replacement, was the gas valve in the furnace the original oem valve?
you've inspected the flue & vent for any obstructions?
Is there adequate combustion and makeup air?
Orifaces not plugged or partially restricted?
you've did a complete & thorough inspection of the hx for cracks etc?
years ago I had to replace a 400,000 btu valve, only to find out the largest available then was 350,000. Same problem, not enuf gas to reach the last few burners. 5/16 brass plugs on the outer burners solved that problem. Over firing is not recommended.....
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snipped-for-privacy@gonefishin.net wrote:

Hi: Yes the valve was the orginal WR. I have checked the flue and we have plenty of combustion air. I also cleaned out the manifold and orfices but nothing of consequence there. There are no indications of a cracked HX. What your saying about the valve is my last thought. I have replaced many of the WR valves with the Honeywell and they all worked fine. To be on the safe side I even replaced the Honeywell valve with another just in case there was a problem in the valve, yet still the problem persists. Unfortunately I can't plug the outer burner, that is causing the problem since it is only a 100K furnace. If you think of anything else I will greatly appreciate it. Thanks again Dan
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wrote:

What are the gas pressures?
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Oscar_Lives wrote:

up to almost 4" and down to about 3" and still the problem continues.
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wrote:

What are they coming out of the valve?
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What do you mean you replaced the burner assembly? You replaced all 4 burners, or just the pilot assembly? I would change all 4 burners if that hasn't been done already. I smell a crack though. Analyze your flue gasses. O2 and CO2 will tell you for sure if the hx is sucking air. Sounds like this is impeding the flow of gas to the burner. Make sure you take a sample in your last cell, not just in the vent connector.
-Canadian Heat
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There ya go. That's what _should_ happen when the spreader bar does not reach a burner. Spreaders come in many designs. The "fit and finish", if you will, is paramount to its working properly. The spreader flame should spread evenly whether cold or hot. With that said, flame propagation behaves differently depending how it's fuel and air are mixed, and mixing environment. (still air vs. turbulent)
For example, on some Parker boilers/oil heaters, there is a separate orifice/air venturi just for the spreader that is lit-off by the pilot flame. You can remove, clean and adjust the assembly so its just right every time. (like a mini main burner)
If the spreader is a capture type devise running perpendicularly across the tops of tube burners, using the main fuel mix from each burner, the adjustment of each burners fuel/air becomes important. (possible flame diffusion interaction)
As the flame moves from over one burner to the next, it can be stopped in it's tracks by misalignment, (like when the burner has been running a while and may warp the spreader bar slightly) too lean of a mix in a particular burner (maybe right next to the burner that won't light) or even a crack or split in the combustion chamber section _next_ to the section that won't light (may open up more when the furnace has run for a long time). If the filter is plugged and the burner cycles while the blower is running (cracked exchanger, or a blower door improperly sealed after a service) causing turbulent air where it shouldn't be.
And the list goes on and on. (all these things, a home owner will never think even exist in this "simple" trade)
-zero
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wrote:

So is it just me or does this guy sound like a rank amateur do-it-yourselfer? Sounds like you have guessed at every part and replaced them all. Thats always a good way to solve a problem. Works well as long as you have lots of time and money. Bubba
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BINGO! Give Dat Man a CEE-GAR!
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