Flashover on start up, any ideas?

Mikey_333

hi I have an old peerless cast iron natural gas hot water boiler. Not sure of the BTUs but it serves a 1,000sf home in Long Island NY so it is not very big.

When trying to start it this year I heard the gas turn on, but nothing ignited. About 15 seconds later there was a pretty big flash (big enough to scare the you know what out of me) that came out of the front of the boiler housing. Seems that the gas was accumulating then all of a sudden it ignited for a second causing a large flash. The burners did not stay on and I quickly turned it off. I think the gas may have turned off automatically as well but I’m note sure as I turned the boiler off right away. Tried again a week later and same thing happened.

Over the summer I changed the expansion tank, relief valve and make up water valve myself. I bled the system and started it up and everything was working fine in May. Resting pressure is a little under 20.

I am going to calll a service company in to do a thourough cleaning as it has not been cleaned in a while, but any ideas as to what else may need to be checked? The igniter (hot surface ignigtor) must still be working as it did ignite (eventually) What worries me is why the gas did not turn off when there was no flame. Not sure if the unit has a thermopile or thermocoupler or if there is another safety mechanism built into the gas valve. I’m a DIY guy and this stuff really interests me, but I do know when to call in the professionals.
You just hope the guys you call really are professionals as some of the guys these companies send over now a days (around my areas at least) do not know as much as you would hope they do.

Thanks

mattmia2

Most likely something clogging the burner, probably spider webs.

Th ignition control will try for ignition for some period of time, usually tens of seconds, after it opens the valve, if it doesn't prove flame in that time it closes the valve. If the gas is accumulating outside the burner because it is clogged it will ignite in a small explosion before the proof of flame times out. Usually it uses flame rectification and a flame rod or the igniter itself. The flame may trigger a rollout switch outside the burner compartment.

Steamhead

@Mikey_333 , there are plenty of good heating contractors on LI. Go here:

https://heatinghelp.com/find-a-contractor/

then type in your zip code and you'll see the listings.

EdTheHeaterMan

@mattmia2 has the best explanation. Think of the ignitor being on burner #4 of a 8 burner rack. The spider web is blocking the orifice from burner #4 so no gas is there to ignite, but the other 7 burners are getting full gas pressure. Just as the trial for ignition is over the gas from the other burners finds its way to the ignitor and POOF you get a flash over. There are those of us that have experienced that in some fashion or another. I happen to have a mustache at the time of my first flash over. Not only did my mustache get a trim, I had no eyebrows for a couple of weeks also.

Once you learn about a flash over that way, you get more careful around that kind of stuff.

Mikey_333

thank you all. I’ll call one of the reccomended contractors in for a full cleaning and check up before the westher starts getting cold and they get too busy.