Heil LP furnace ignition problem model NULK075DG07

I have a issue going on with a furnace. When you turn the thermostat on the fresh air fan turns on like it should then the burner lights like it should after that is where I get the problem just before the fan kicks in or at the same time as it kicks in the flame suckes into the burner tube on one or all three burner tubes but may not do it in all three tubes at the same time and sometimes the flame returns to the burner and then the furnace works fine. I have had a heating guy out to look at it 2 years in a row and of coarse it didn't do it when he was here but " says he made adjustments and switched some wires around" which I know he didn't just charged me $200 each time. I'm tired of paying him for doing nothing. When he was here we did check the WC to see if that was right and it was. Tonight I took out the gas regulator and jet tube and pulled the jets out and blew them out but they were clean. I put it back together and nothing changed. Does anyone have anything i could try or know what's going on?
Thanks

 

I believe the 3 burners ignite in a random sequence. However the time interval between all three burners igniting should be small so gas does not accumulate and ignite with a small explosion' If the same burner ignites last, insure the burner is pulled into the cross burner lighter tube all the way. Good luck.

 

The burners light starting at the igniter..... and then in succession away from that burner.

He checked the gas regulator pressure at the furnace ?

If you have the problem after the blower starts you could be looking at heat exchanger issue.

 

Yes we checked the WC at the gas regulator. I think the problem is just before the fan starts up. I might have to double check that thought

 

The problem happening just before the blower starting doesn't make sense.
Nothing happens just before the blower starts.

Did he check the pressure with the furnace in standby and again with burners lit ?

 

I'm telling you that's when it happens. The burners are lit and just before or just as the fan kicks in the flame suckes inside the tube and makes a small explosion. It will also do it while the furnace is running. As far as your first question we checked it while it was running and when it wasn't.

 

I can only recommend a gas technician from your gas supplier at this time.

 

There nothing wrong with the gas lol

 

I didn't start servicing yesterday. If I CANNOT offer help and I think it's a safety issue and may be beyond a DIY's grasp I recommend professional help.

 

What you describe sounds like "flashback" a condition in which the gas switches from burning properly on top of a burner and starts burning at the burner orifice, inside the burner.

Is that what's happening?

If so, the usual cause is low gas pressure at the burner manifold, or in the gas pressure being supplied to the furnace in some way. You'd need to put a manometer on the burner manifold to verify this problem, or check upstream gas conditions that might be causing it.

Fuel gasses burner a more or less fixed velocity. If the velocity is correct, the gas is pushed up to the top of the burners where it should burn. If the velocity drops off, the gas "flashes back" to the orifice and burns there.

 

Yes that is what it does and then the flame will return to the top of the burner and then it may happen a few mins or few seconds later. This year is by far the worst it has ever acted up. It constantly does it this year.

 

Check to be sure that all the gas valves are fully on.

I'd be most suspicious of a defect in your propane regulator. If it's not operating properly, it may be starving the furnace of proper gas pressure, which can cause the flashback condition.

Could be a defect in the furnace electric gas valve if one of the valves in the control isn't opening and closing properly.

Also, do you have a good fuel supply at present? How large is your propane tank? Do you have other gas appliances, and if so, are these operating normally when the furnace isn't operating properly?

You need a skilled propane repairman who will put a manometer on your furnace and diagnose where the pressure problem is located, and correct it for you.

 

I "think" We checked it with a manometer before the gas valve and I know we did for sure after and if I'm remembering correctly it was 13" WC but I can't remb for sure. It was within the proper spec. As for the size of tank I had a 350 gallon and now this year I have a 1000 gallon tank. I have no other gas appliances just the furnace.

 

What should my pressure be at?

 

There is usually a 1/8" iron pipe plug in the burner manifold that can be removed, and a suitable 1/8" iron pipe fitting installed so that a manometer can be used to measure the burner manifold pressure.

It sounds like your incoming gas pressure is OK, but you need to measure the burner manifold pressure to see if that is dropping off when the flashback occurs.

If so, to be thorough you should check the incoming gas pressure to verify that that pressure is OK when the flashback occurs. If you find the pressure fall off at the burner manifold while the incoming gas pressure remains good, you very likely have a bad gas valve that is failing to maintain the correct operating pressure at the burner manifold.

 

Ok thanks. I am ordering a manometer off fleebay. I think I remember something. When we had the gauge hooked up to the gas valve I think I remember him turning the screw on the gas valve and it didn't do much or anything at all as far as changing the WC. Or it might of been when I was outside screwing in and out on the regulator on the house where he wasn't seeing much of a change in pressure. I just can't remb what it was. I will have to check it out and post my pressures.
So where do I check the incoming pressure at? Before it comes into the house? At the regulator on the house or what? Could you kinda do a step by step for me if that's possible? I sure would appreciate it
I think I have a gas valve laying around for another furnace I need to convert to LP. if I could swap it out just to see if there's any change???

 

So today I just checked the pressure on the burner side by pulling the pipe plug and hooking up my manometer and when the burner is lit it is just under 10"WC. When the flame back flashes the gauge does not fluctuate it stays just under 10"WC. I'm wondering since I now have a bigger tank than last winter and when we messed with the gas regulator on the outside of the house I wonder if that's what I should adjust just a hair to bring the pressure up into range?

UPDATE: I checked the pressure at the house regulator at it was 15"WC so I put it at 13"WC because that's what it says it's suppose to be on the tag. When we adjusted it last winter we never did recheck it outside to see if it were still in spec. I then rechecked the pressure at the burner and it remained the same, just under 10"WC. Bad gas valve?

 

Umm. Well, you've got me.


About the only other thing I can think of is to unscrew the burner orifices and inspect/clean them. But it's unlikely dirty burner orifices are the problem.

 

I've done that already. Shouldn't my WC be 10"-12" and not just under 10"?

 

Any thoughts on my last post?

 

Well I pulled my heat exchanger completely out today and I did t see any visible cracks. I called a different hvac place today also and the tech said more than likely the burners are bad. Idk what do you think?

 

I don't see any reason to suppose that the heat exchangers were the problem.

As long as you have them out, use a flashlight to shine a light on one side of the heat exchanger and look for any light on the other side. Cracks can be hard to see.

I think it was a mistake to change gas pressures without any real understanding of what you are doing.

At this point I think you should have a skilled propane repairman set the correct pressures at the tank, where the gas comes into the furnace and at the burner manifold.

You are past the point of DIY repairs in my opinion.


Sorry I can't be of help.

 

The pressures are now correct. They were not correct before because when the repair man was here we messed with the regulator outside and he never put the gauge back on it so that's how I know it was wrong. I put my manometer on it and put it where it states it should be which is 13" WC

 

So the cause of the flash back WAS low gas pressure?


That is really about the only cause I know of that problem, unless you are using the wrong fuel gas or something really exotic/screwed up.

 

No that's not the cause. The burners are the cause. When they heat up they widen out where the flame is causing to much gas and then flashing is what the heating guy I called said

 

O-K. I've never seen or heard of that one.

But if the burners are changing shape and getting too large, presumably that's reducing the velocity of the gas allowing flashback to occur.

If I were the manufacturer, I'd be GLAD to give you new burners in order to get the old burners to examine for defects. I wonder if they've heard of that one before?



So what did the repairman do about it?

 

It's a 22 year old furnace. I need to put it all back together then order burners ad try that I guess. He said if the pressures are all good and there's no cracks in the exchanger it's the burners. He said with a furnace this old it's not uncommon.

 

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Well, I never saw bad burners cause such a problem. My experience is that low gas pressure causes flashback, which is why I pursued that as a likely cause.

But, presumably the repairman saw something that was a problem, and if the burners are deformed enough that it slows the gas combustion enough, it sounds like that could cause flashback. So I'm inclined to respect his diagnosis and suggestion.

He sure gave you a lot of work to take out and inspect the heat exchangers, though....

 

I've never seen bad burners like that either.