"Pooomf" is not a good sound
December 9, 2022 12:09 PM Subscribe
My home's natural gas boiler did something weird. What happened?
I have an older natural gas boiler - Slant Fin, "low fire", like this but a bit older. It was installed in 2006 and last serviced in 2017. It's in a small basement storage room that has a proper fresh air vent installed.
The pilot light had been going off every couple weeks, and needing to be manually re-lit. A month ago we cleaned the thermocouple, following a YouTube tutorial, and since then the pilot light has stayed properly lit.
The other night, there was a loud sound (Poomf!). The unlatched storage room door beside the boiler opened, as if there had been a small explosion / or a sudden spike in air pressure in the boiler room that pushed the door. The smoke detector went off. My relative saw orange light coming from under the boiler, as if from flames, which burned for 30-60 seconds then self-extinguished. The smoke detector went off but the carbon monoxide detector did not. There was a bit of a natural gas smell, but no strong burning "fire" type smell.
We called the fire department who said they didn't see any issue, didn't know what the problem had been, and the CO levels were fine.
We called the Gas Company and they sent a tech who inspected the boiler. He said it looked fine to him too, the CO levels were normal. He gave us an infraction notice only because it had been so long since the boiler's last service, and he shut off our gas.
We called a Furnace Guy who did a maintenance cleaning on the boiler, said the boiler looked fine and he also didn't know what caused the issue. He cleaned quite a bit of rust from inside that had flaked off the the heat exchanger, but he said that the rust wasn't the cause of the incident and it wasn't falling on the part of the tubes where the gas comes out.
Furnace Guy even drove away to get a more sensitive CO detector, to try to diagnose the problem, since he said certain CO concentration levels can help point to different problems. He said the CO, gas valve, gas burners, and pilot light all looked fine to him, he couldn't smell a gas leak when the boiler was on or off, and his soapy water test found no leaks. He reassured us that he was extra diligent in his inspection since his own license was on the line. He said he had no idea what caused the issue, but his data & assessment said the boiler is safe to operate, and that the Gas Company Guy's infraction note indicated that Gas Guy had also deemed the boiler safe and only shut off the gas because it's mandated for boilers that are overdue for maintenance.
Furnace Guy was from a reputable company and he himself seemed extremely professional and hyper-conscientious in his inspection and communication, and he says it's fine to run the boiler.
But... what happened and what do we do now?
The incident reminded us of when you let too much propane into an unlit barbecue, then light the flame, and the gas all explodes at once with a quick hot poof. That would explain the noise, the air pressure that opened the boiler room door, and the brief orange flame. BUT - there doesn't seem to be a way for that to happen, since the pilot light was burning consistently, so the gas shouldn't have been able to build up. Even if the pilot light did go out and somehow the gas continued to be released, the pilot on this furnace must be manually re-lit, so that would have just been a gas leak, not a mini-explosion.
We wondered if there could be a small leak in a gas line allowing the gas to leak out at a low concentration and build up until it ignited - but the CO readings and the inspection of the gas lines don't support that, and again the pilot light was on.
We wondered if something could have temporarily covered the part of the gas tube that lets the gas into the boiler - like a rust flake, dust bunny, leaf, mouse, anything. Could the gas have built up behind that obstruction then all been released at once. Furnace Guy and a Fire Fighter both said theoretically that was possible but they saw no evidence of it since the boiler was clean, the rust flakes weren't in the gas-release area, and there was no debris or ash.
So- we now have this boiler that's cleared to run for the next year - Furnace Guy thinks it's good for a few more years before it eventually fails due to age. If we had a clear explanation for what happened, and felt we could prevent a future incident, we would keep it for another couple of years. But we're nervous because the problem was such a mystery, so we don't know if it might happen again.
1. What do you think might have happened?
2. What should we do? Given the mystery, we're leaning towards replacing it. But is there any other test or explanation we could consider?
Thank you for any advice.
I have an older natural gas boiler - Slant Fin, "low fire", like this but a bit older. It was installed in 2006 and last serviced in 2017. It's in a small basement storage room that has a proper fresh air vent installed.
The pilot light had been going off every couple weeks, and needing to be manually re-lit. A month ago we cleaned the thermocouple, following a YouTube tutorial, and since then the pilot light has stayed properly lit.
The other night, there was a loud sound (Poomf!). The unlatched storage room door beside the boiler opened, as if there had been a small explosion / or a sudden spike in air pressure in the boiler room that pushed the door. The smoke detector went off. My relative saw orange light coming from under the boiler, as if from flames, which burned for 30-60 seconds then self-extinguished. The smoke detector went off but the carbon monoxide detector did not. There was a bit of a natural gas smell, but no strong burning "fire" type smell.
We called the fire department who said they didn't see any issue, didn't know what the problem had been, and the CO levels were fine.
We called the Gas Company and they sent a tech who inspected the boiler. He said it looked fine to him too, the CO levels were normal. He gave us an infraction notice only because it had been so long since the boiler's last service, and he shut off our gas.
We called a Furnace Guy who did a maintenance cleaning on the boiler, said the boiler looked fine and he also didn't know what caused the issue. He cleaned quite a bit of rust from inside that had flaked off the the heat exchanger, but he said that the rust wasn't the cause of the incident and it wasn't falling on the part of the tubes where the gas comes out.
Furnace Guy even drove away to get a more sensitive CO detector, to try to diagnose the problem, since he said certain CO concentration levels can help point to different problems. He said the CO, gas valve, gas burners, and pilot light all looked fine to him, he couldn't smell a gas leak when the boiler was on or off, and his soapy water test found no leaks. He reassured us that he was extra diligent in his inspection since his own license was on the line. He said he had no idea what caused the issue, but his data & assessment said the boiler is safe to operate, and that the Gas Company Guy's infraction note indicated that Gas Guy had also deemed the boiler safe and only shut off the gas because it's mandated for boilers that are overdue for maintenance.
Furnace Guy was from a reputable company and he himself seemed extremely professional and hyper-conscientious in his inspection and communication, and he says it's fine to run the boiler.
But... what happened and what do we do now?
The incident reminded us of when you let too much propane into an unlit barbecue, then light the flame, and the gas all explodes at once with a quick hot poof. That would explain the noise, the air pressure that opened the boiler room door, and the brief orange flame. BUT - there doesn't seem to be a way for that to happen, since the pilot light was burning consistently, so the gas shouldn't have been able to build up. Even if the pilot light did go out and somehow the gas continued to be released, the pilot on this furnace must be manually re-lit, so that would have just been a gas leak, not a mini-explosion.
We wondered if there could be a small leak in a gas line allowing the gas to leak out at a low concentration and build up until it ignited - but the CO readings and the inspection of the gas lines don't support that, and again the pilot light was on.
We wondered if something could have temporarily covered the part of the gas tube that lets the gas into the boiler - like a rust flake, dust bunny, leaf, mouse, anything. Could the gas have built up behind that obstruction then all been released at once. Furnace Guy and a Fire Fighter both said theoretically that was possible but they saw no evidence of it since the boiler was clean, the rust flakes weren't in the gas-release area, and there was no debris or ash.
So- we now have this boiler that's cleared to run for the next year - Furnace Guy thinks it's good for a few more years before it eventually fails due to age. If we had a clear explanation for what happened, and felt we could prevent a future incident, we would keep it for another couple of years. But we're nervous because the problem was such a mystery, so we don't know if it might happen again.
1. What do you think might have happened?
2. What should we do? Given the mystery, we're leaning towards replacing it. But is there any other test or explanation we could consider?
Thank you for any advice.
Not an expert, but I can tell you what caused this in my new apartment. The pilot light was staying on. The first time I turned the heater on, I did it gradually so that it (the heater) would come on at the lowest possible setting. When I got to that point, there was a click, a pause of a few seconds, and then a similar whoomp. Not as big a whoomp as yours, I think (I was standing right next to it and didn't have any singed hair), but big enough that I turned off the heater and the gas and called the landlord.
The problem turned out to be a partially blocked gas supply to the pilot. That caused the pilot flame to be too short to reach the main gas inlet, with the result that instead of immediate ignition when the thermostat turned on the gas, the gas had had to build up inside the heater until it reached the flame and then...whoomp.
posted by bricoleur at 1:03 PM on December 9, 2022 [2 favorites]
The problem turned out to be a partially blocked gas supply to the pilot. That caused the pilot flame to be too short to reach the main gas inlet, with the result that instead of immediate ignition when the thermostat turned on the gas, the gas had had to build up inside the heater until it reached the flame and then...whoomp.
posted by bricoleur at 1:03 PM on December 9, 2022 [2 favorites]
Our one does a lower-key version of this when the flame sensor needs cleaned. It runs for a couple of seconds, ignites, then the gas valve snaps off suddenly as it's not getting the signal from the flame sensor
posted by scruss at 1:04 PM on December 9, 2022 [1 favorite]
posted by scruss at 1:04 PM on December 9, 2022 [1 favorite]
30 to 60 seconds of flames is quite a bit of gas. Perhaps the light went out and the thermo couple didn't completely stop the flow and it built up for a while.
posted by jouke at 1:07 PM on December 9, 2022
posted by jouke at 1:07 PM on December 9, 2022
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Enough that the flame and pilot light went out, but then before the thermocouple had cooled off sufficiently to shut down gas flow, the pressure came back up and gas flowed out, accumulated until it reached the still hot burner region where it ignited and set off a small explosion.
Most pilot lights have an adjustable flame, and before you cleaned the thermocouple, either you or someone else might naturally have turned it up to the max to keep the boiler from shutting off. You might want to turn it down a bit now so that the thermocouple would cool down enough to shut off the gas more rapidly when the pilot light goes out.
posted by jamjam at 12:54 PM on December 9, 2022 [1 favorite]