Dunkirk quantum 90 Backfire daily

surritte

I have paid 2 service technicians to find the cause of my Dunkirk quantum 90 backfiring to no avail. They come and listen take things apart and leave shacking their heads but still charging me $500 for their service.
We are running propane and have had a technician replace the values at the house and the tank to eliminate that as a cause. This again cost me money but did not solve the problem.
It does not always backfire. Most of the day it runs just fine and operates as is should. It is also random when it backfires and scares the *&% out of us. I know my neighbors are tired of hear it as well.
Any help would be appreciated. I am at my wits end. I am hoping for a easy and yet inexpensive resolution.
We have verified all the outlet pipes have been cleared and saw no signs of water or debris. There are error codes.

mattmia2

If they didn't have a combustion analyzer and tools for measuring draft I would ask for my money back. You need to find someone that understands combustion and draft. Not only is that incompetence it is also very dangerous.

Intplm.

Are there any regulators on the LP lines feeding the boiler? Are there any other items using LP gas in the building?
I think that your fuel line needs to be checked for proper sizing as well as proper supply pressures.
You might want to have them come back (or another company come back) and have everything checked to the proper fuel-required specifications.

mattmia2

They also should have had a manometer to measure the gas pressure.

ZackR

An issue I had with my Q90, before I replaced the HX, was the sealant between the HX halves deteriorated and allowed air to enter the combustion area. I don't see how that would cause back-firing, but just something to note as that was missed by 2 different techs that came to look at my boiler before I got tired of their non-answers and took matters into my own hands.

We also had the gas orifice split down the center. It didn't back fire, just put tons of gas through, but maybe if there was a small crack that was sometimes letting puffs of gas into the combustion area?

Jamie Hall

The backfiring is delayed ignition -- and could be very dangerous. Get someone in there with the proper measuring and analysing tools and the knowledge to use them.

kcopp

Typically this is an indication of the mesh burner breaking down and/ or separating.

ZackR

Do you have a q90 series 2 or 3 with a spark ignitor or do you have a hot surface ignitor? I know there was a service bulletin about changing the spark ignitor to a hot surface ignitor on some(maybe all?) of the models that originally came with the spark ignitor. Make sure they figure out if your boiler has the most updated parts.

All the info about normal sequence of operations and service bulletins are linked below. Even if you're not doing any of the work, the more info you have the better off you'll be. I will echo what the others have said, don't let the next person near the boiler until they assure you that they'll be using a manometer and combustion analyzer.

https://dunkirk.com/product/condensing-gas-boiler-q90/

surritte

Thank you all for your feedback. I was able to verify that we have a hot surface ignitor.
The propane companies did not use a manometer. They used a low/high pressure gage.
I was watching the boiler waiting for it to make the back fire noise. The loud backfire sounds comes from the exhaust tubes outside the house. The boiler is in the house. I took the cover off and watched it fire up several time with no issue, Ignitor to blue flame. However on the time it created the backfire noise I noticed the ignitor light up - the gas flutter and create what appeared to be small explosions until it created a solid blue flame and worked as I would expect it.
I will try to find a new propane company to come do their part for sure. I will also find a competent propane boiler technician. I appreciate the diagrams and links.
If you know anyone in the Denver area as a recommendation I am open to any suggestions. I will update this forum as soon as I get more details and fingers crossed a resolution.

ZackR

Sounds like it might be a burner issue as @kcopp suggested. The burner has lots of small cutouts and could be cruded up. It's supposed to be checked and brushed off yearly during annual maintenance, as specified in the manual. Since the burner needs to be taken out for inspection, you should also wash out all the oxide dust that builds up in the HX. Make sure they are careful removing the ignitor, they are very fragile. I broke a new one trying to install it when I didn't follow the steps in the manual exactly.

surritte

I have an update. After getting a qualified person out with the right equipment he was able to reproduce the issue. He was also able to see it operate correctly. He verified the gas flow and the levels. The draft motor is failing. He noticed this right away by the slight noise it makes and the testing he did. It is on order and will be replaced. All other areas of the boiler checked out and looked good according the tech.
Fingers crossed this is the ultimate fix and the boiler lasts many more years.