Peerless boiler/Beckett CG15.2 explosion(delayed ignition)

cthvacguy860

Hello everyone I am grasping at straws here because I've been on this call for about a month on and off. I am a licensed S2 technician with experience in powerflame burners and commercial burners. It's a peerless boiler LC06 with a Beckett cg15.2 natural gas burner and a Honeywell 7800 series relay module. Initial call was for an "explosion" that blew off the flame sensor door and tripped boiler off. UV flame sensor was damaged in the explosion so I replaced it. Could not get boiler to fire off was still sensing flame with no flame present. Replaced whole relay module, flame amplifier, and pre purge card. After replacing the above mentioned parts I was able to get the boiler to fire off to test combustion, gas pressure in hi and lo and draft in fleu and over fire draft. All readings appear good and within acceptable range. Adjusted gas pressures .8 in wg for lo and 3.5 for hi as stated in burner manual. Then adjusted air damper settings (Siemens SQN70 damper motor assembly) test ran the boiler for a whole day. Never once did it explode or show any signs of delayed ignition. Issue is happening over night and we cannot replicate it. Burner also has new ignition electrodes and ignition transformer. Any help would be appreciated thanks in advance. All the pictures of combustion readings are on my work phone I'll start sending them to my personal phone to post if needed.

EBEBRATT-Ed

I am not familiar with that burner because I am retired. I do see that it apparently has direct spark ignition. I may be old school, but I am not an advocate of that on a gas burner of that size but it's what you have. Most of the commercial burners I worked on had a proven gas pilot. I worked for a Power Flame rep for 10 years back in the 70s

I can think of 2 clues that may help you.

#1 is as you probably know a UV detector can pick up a spark and think there is a flame when there is no flame.

#2 is that it happens at night. It shouldn't matter but the boiler probably lights better when it is warm and it sounds like delayed ignition during the night when the boiler is cold.

#3 I would check the 120 volt control circuit. Many forget that (and I don't know the ignition transformer voltage 6000??) if the 120volts is low the ig trans secondary voltage will be low

I would check to see if the UV scanner is picking up a flame signal from the spark….it should not. You can test this by shutting off the gas and trying to light the burner . Then I would concentrate on the low fire ignition settings air flow, gas pressure etc.

Sometimes you have to compromise between what the "book" tells you the low fire settings should be and what actually gives you better ignition. I am assuming that with the SQN70 this burner modulates or is at least low fire start. I would make sure the low fire proving switch is working. You don't want this burner lighting on anything but low fire..

Where in CT are you located?

Steamhead

Also could be a problem with the gas valve opening before it should.