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LP Boiler backfiring

My house has a LP boiler that during the last three years has started backfiring when igniting or when the burners go off. Flames shoot out the burners and it ignites and goes off with a large boom. I've had two different companies check the boiler, burners were completely drilled out and cleaned, pressure is fine everywhere, no leaks. They are at a loss as to what is causing the backfiring. Lately the backfiring is so large it shakes the windows, doesn't happen every time, but frequently. Any ideas as to what is causing this? I know absolutely nothing about boilers, but I believe it was a natural gas boiler converted to LP (HP Smith G200 is the model). I've been in the house 16 years, and this just started a few years ago. Thanks.


  • Gordy
    Gordy Member Posts: 9,546
    That sounds like delayed ignition. Not good. You need to get a qualified tech involved. Not the ones that were there. I can't believe they left it with out fixing the delayed ignition. Did they try a new gas valve?
    JUGHNE Member Posts: 11,021
    I would start with the LP gas regulators. There are two of them. Should be one on your tank and another on the outside of the house. If you have not had them changed or inspected in the 16 years that you have lived there then it is time.

    Sometimes your LP supplier has someone competent enough to handle this. In my area the company will just change them after a certain number of years. Customer will pay for them.

    Someone needs to measure the gas pressure at the boiler during start-up. (That is why I always stand off to the side when lighting up)
  • lchmb
    lchmb Member Posts: 2,997
    if it is when the unit shuts down i would (after checking pressures to include manifold pressures) take a very close look at the burners..I have seen where they start to warp or crack and it only show's itself on occasion. As far as paying for regulators..most LP companies in my area own the equipment and our guys change them if there is any question...
  • TheRichFish
    TheRichFish Member Posts: 3
    I believe they replaced the gas valve two years ago. I'll have the regulators checked first thing and see if that does the trick, thanks everyone!
  • deadmansghost
    deadmansghost Member Posts: 32
    The boiler might have the natural gas regulator spring installed instead of the propane one. Hopefully the burner orifices are the sized for propane gas should have been changed at conversion.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
    The boiler is filling with gas before it ignites fully. Of that we can be sure. We have to take that information and begin diagnosis through a process of elimination.

    The first thing to check is make sure there is no gas leaking into the chamber while the burner is off.

    The next thing I would look at is the burner inshot tubes. That's the part where the gas and air mixes before entering the burner tube. The end that is attached to the burner tube is fan shaped and has little gas ports the whole way along the fan. This is what allows the flame to spread to all the tubes after being ignited at the ignition tube. With lp, the little ports on the fan get corroded shut over time and prevents the rest of the burner tubes from igniting right away. They will dump a mixture of gas and air into the chamber until the lit burner tube, by the igniter, ignites the whole chamber in a resounding fashion.

    Another thing to check is that the gas valve doesn't open before the ignitor is a brilliant orange color. If you have a bad wiring connection going to the igniter, it may take it longer to reach ignition temperature and if gas is already pouring into the chamber, when it does ignite it would do so in a fantastic way.

    The most important thing you can do is hire someone who knows what the heck is going on with this stuff and do it without delay.
  • Ironman
    Ironman Member Posts: 7,338
    Make sure the burners are sloped toward the manifold, not away from it. Otherwise, gas will flood the far end of the burner and pool back towards the manifold until it reaches the point of ignition - then: KABOOM! This is an LP phenomenon because it's heavier than air.

    Pretty much what Harvey said with a slightly different cause.

    Also, it needs to be confirmed that the flue is drafting properly.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    SWEIHarvey Ramer
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
    edited February 2016
    I just glanced at the manual for your boiler and it appears you have either a standing pilot or intermittent pilot.

    Make sure the pilot has a 2 pronged flame so it can ignite the burner tubes on either side.

    Make sure the burner tubes are all clean with all the holes open.

    Make sure the orfices on the manifold are all open and not partially clogged.

    The gas pressure at the inlet of the gas valve must be between 11" min. and 12" max wc. pressure. If it isn't, replace the regulators and have them properly set. As Jughne mentioned.

    When the burner is running, make sure the air shutters on the burner tubes are properly set for the right fuel/air mixture. An experienced tech will know what the flame is supposed to look like. Figure 3.2 in the boiler manual.

    If the air shutters are not properly set, or the gas pressures are to high or to low, this will cause an incorrect fuel/air mixture and will not ignite smoothly.


  • TheRichFish
    TheRichFish Member Posts: 3
    Success! (at least for now). Replaced the regulators, no change. Called in another company, they checked everything, turns out the gas valve was only at 8 pressure, they put a new one in. And a prior technician had put the pilot light at one edge of the burners, they moved it to where it was supposed to be. Everything is working much better now, can't even hear it ignite. Thanks again for everyone's help!
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
    Glad to hear it's working for you! Sounds like you have a new heat guy. Stick with him.