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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