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The Case of the Cold Flue Pipe, This Friday's case

RayWohlfarth Member Posts: 1,528
I met the contractor for a small boiler project on a cold winter day. The building had one boiler with a gas power burner. Inside the room was a 40 gallon gas water heater. Looking around, the combustion air louver was blocked with a piece of plywood. I noticed the flue pipe for the water heater had white spots and several pin holes. I touched the water heater flue pipe as the boiler was running and it felt cold. The bottom of the water heater had black streaks by the burner. What was going on?

I'll let you know Friday morning when the case is posted on my you tube channel. have fun

Ray Wohlfarth
Boiler Lessons


  • retiredguy
    retiredguy Member Posts: 919
    edited April 2023
    With the combustion air louver partially blocked, the power burner used all the combustion air available from the outside air louver and may have even reversed the flue for the water heater. The water heater, I am assuming was a 40 gallon atmospheric type and did not get the proper amount of combustion air and became sooted. The soot partially blocked the center flue pipe and turbulator allowing just a limited amount of flue gasses to escape up the flue pipe. This limited amount of flue gasses condensed in the flue pipe causing the flue pipe to slowly disintegrate (rust) and show signs of it's deterioration by the white spots and several pin holes.

    As a side note, try starting up a couple power burners on a cold winter's day, that had the combustion air louvers installed at ground level right beside the boilers, made for an extremely cold day. I did this type of new boiler start-up and it is not fun. This may be why the combustion air louvers were partially blocked.
  • Steamhead
    Steamhead Member Posts: 16,942
    Looks like a great place for an automatic louver damper.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • captainco
    captainco Member Posts: 796
    Unless there are large exhaust fans in the room a power burner will find air. In this case it was using the water heater flue. But was it using it before the grille was blocked?
    Has anyone ever heard of a combustion air trap? Only lets air in when you need it and doesn't allow air out. Works better than a motorized damper.
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,528
    Thanks all @retiredguy you were correct. When the burner started, there wasn't enough combustion air. The air would come down the flue pipe from the outside and that was why the flue was cold. It was sucking the outside from down the chimney. This caused the flue gases to condense in the water heater flue and
    holes were in the flue. @Steamhead I prefer motorized dampers and they should have them in this instance.
    @captainco Combustion air trap? Never did. Will be looking that up for sure.
    Here is the link to the article

    Ray Wohlfarth
    Boiler Lessons
  • PC7060
    PC7060 Member Posts: 1,194
    Only combustion air traps I’m familiar with are PVC condensation traps on direct vent exhaust lines 
    JUGHNE Member Posts: 11,089
    That combustion air inlet louver is almost identical to one in a very crowded boiler room in a nursing home. It has a +1 million BTUH boiler and 2 commercial WH. This way since the 1960's.

    Fire Marshall noticed the closed louver and wrote them up with stern warning of home closure if the offense was repeated. (which it was, but he is only around perhaps once a year).

    The water main/meter is just below the opening, water lines would freeze up.

    I fashioned a hopper type baffle to prevent the cold air from coming in and dropping onto the water lines, but still have adequate required area. I would guess someone has removed it and back to the complete close up in the winter. I did document the issue on my invoices.
    This usually gets the "deer in headlights" attention.

    The "Fan in a Can" approach would have been the solution for this particular problem.

    The location of the compression tank will give you an idea of how small the room was.

  • captainco
    captainco Member Posts: 796
    What was the fix and how did you verify the burner wouldn't pull down the water heater flue?