PP Vent Explosion

Tom Olds

One of my techs called late yesterday ( a Friday of course, right?) to tell me that the second he cycled power on to a new mod-con boiler installation, the polypropylene vent exploded just above the boiler. Thankfully he wasn't hurt but I could tell he was shaken and understandably so. Afterwards, I sought advice from my best service tech and our local rep for the boiler and neither could conceive how this could happen. I then called my install tech back to confirm that the explosion happened immediately after he cycled power to the boiler. He said it was within a second or two at most. Normally, when these boilers are powered up, the combustion blower runs for a few seconds while the controls take additional time to boot up. If there is a call for heat or hot water after that, the blower will then go to pre-purge, ignition follows shortly thereafter, and hopefully it begins to satisfy whichever load is calling. I would think the normal time for that sequence is 30 seconds or so. He said he did not measure incoming gas pressure prior to startup so that's our first priority today. But even if it is excessive, it doesn't explain the immediate nature of the explosion. We'll check everything we can think of before trying again but this time power will be cycled from the breaker panel in another part of the building. Any thoughts from you pros would be appreciated. TO

DZoro

Did they have to convert to lp? If so everything tightened back up? Almost would think gas had to be leaking into the box during the install. Was it the intake pipe that blew?

Tom Olds

Nope, natural gas. It was a PP elbow that blew apart on the exhaust vent. The intake is PVC.

EBEBRATT-Ed

Maybe Teflon tape or pipe dope holding the gas valve open??

hvacfreak2

EBEBRATT-Ed said:

Maybe Teflon tape or pipe dope holding the gas valve open??

This or a faulty gas valve. Odd that it would ignite that soon after a power restart though . Faulty controller or program glitch maybe ? For some reason the system was stacked with fuel and a power restart provided ignition ?

hot_rod

Delayed ignition or the chamber full of gas for some reason. Usually the pre-purge clears out any excess gas from a missed ignition try. It maybe a tough condition to duplicate.

Good that your tech was not injured, an unusual event that we don't usually prepare for.

Did the PP just disassemble or did it fracture and come apart?

Mark Eatherton

I suspect that you are using PVC or ABS for the combustion air, and through the use of the cement and primers, when the appliance was first started, it drew the combustibles into the boiler, and saw a spark, and lit off the combustibles.

I had this happen with a spud gun in the front seat of my truck once... Almost filled my pants. I was looking down the barrel (empty) to see the spark, and when I sparked it, this fireball went whizzing past my head. It had been over an hour since I'd assembled.

Happy New Year Tom.

ME

JUGHNE

I have wondered about the glue/primer fumes and considered if I had to fire up on the same day, I would pull the spark plug wire off, shut off the gas and just let it pre and post purge thru a few cycles.......but then I seldom get anything done in one day.

ratio

Definitely had PVC primer/glue induced "ignition events", even after a normal pre-purge on a 90%er FAF. A bigger question in my mind is where did the source of ignition come from? ISTR that a prepurge is supposed to be like three air changes through the burner assembly, presumably the mfgr timed it for worst-case flue length—a few seconds is too short for the ignitor to be energized yet.

Once we're into the HX, we should be sealed: there shouldn't be any source of ignition. HOWEVER, if we were aspirating PVC primer fumes, we could have filled the HX all the way through to the flue, once we're in that position...

Still doesn't explain why the 90 blew apart, I've got no theory about how we could build enough pressure to cause that, since the flue should be open on the far end?

EBEBRATT-Ed

What @Mark Eatherton said is certainly possible.

A few years back when you could smoke on a job I worked for a sheet metal company. They installed some big duct and used solvent type duct sealer instead of water based.

Guy stuck a match to light a butt...boom. Nobody got hurt but the duct exploded.

If thy had just finished with the pvc intake then that is a good possibility

EBEBRATT-Ed

Had a guy build a potato gun out of pvc pipe. Squirted hair spray in it and light it off with a piezo ignitor. The potato went like a rocket.

Mark Eatherton

EBEBRATT-Ed said:

Had a guy build a potato gun out of pvc pipe. Squirted hair spray in it and light it off with a piezo ignitor. The potato went like a rocket.

I taught Dan Holohan all he knows about spud guns, and how to avoid being arrested when using it in a public park setting

ME

ratio

Mark Eatherton said:

... and how to avoid being arrested when using it in a public park setting

Would that be "run like the dickens"?

ChrisJ

Dan Holohan using a spud gun?

I believe that about as much as people who tell me steam heat is less dry than forced air.

Tom Olds

We have not determined the source of the explosion however the theory of PVC glue and primer fumes has been considered. My guys said they finished putting the intake pipe together about an hour before turning the system on for the first time. Both intake and exhaust go vertical so I could envision some of the glue and primer gaseous byproducts accumulating in the boiler but I too am mystified as to where the spark would've come from to create immediate ignition. The following day, with some trepidation, my tech restarted the boiler (after replacing the PP elbow) and while it started out rough, (lean on gas) he made adjustments and it has lit off fine ever since. BTW, the elbow did fracture apart so the explosion was quite violent. At this point, we're going with the glue and primer theory with a static discharge as the catalyst. We're just guessing tho.

JUGHNE

Dust collection systems in say a cabinet woodworking shop need a bare grounding wire inside the pipe. The air moving thru the PVC creates enough static electricity so as to cause the dust to cling to the pipe, (possibly prevent dust explosions but mainly to keep elbows from plugging up.)

The dry air flow thru the pipes could have created enough static charge to initiate the explosion?? Just a WAG.

Mark Eatherton

When Tom said explosion, he wasn't exagerating...

ratio

Wow.

BobZmuda

How long does everyone wait for the PVC glue/primer to clear out before firing the boiler? IMO it should be the first thing done after mounting the boiler.

Gordy

I'm thinking spud gun here made from pvc. Gas for ignition is contained in the chamber by a snugly fitting spud in the barrel.

A vent pipe is open so the explosion should be less violent. What am I missing?

Not saying it didn't happen it obviously did. Just saying same amount of explosive gas in an enclosed chamber will be a more violent explosion than an open chamber.

Edit: is PP more brittle than PVC?

DZoro

Wow, will be thinking twice with glue/cleaner. Never seen or heard of such. Have started furnaces very shortly after finishing piping. Maybe not so much anymore.

hot_rod

So the intake side was glued PVC and exhaust slip joint PP.

PP is a lot softer than PVC, surprised to see it shatter like that.

BobZmuda

I thought of a way to make sure this problem is avoided.

Run a radon fan after you set up the intake vent and push air outside. You could leave the last connection or last couple off so you can still work on the boiler.

Fernco the last connection together which is ok to use on the inlet.

https://www.menards.com/main/plumbing/rough-plumbing/radon-mitigation/homeaire-trade-radon-mitigating-fan-kit/rn104/p-1444444026100-c-19545.htm?tid=-4764323089261270610&ipos=2

kcopp

I have been using this for glue for a bunch of years for my drainage. Good stuff. No smell too!

https://www.flexiblepvc.net/Gorilla_PVC_Glue_Organic_PVC_Glue_p/gorilla-pvc-glue.htm