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Chimney Cap Frost

Couldn't find anyone with a similar experience. -14 degrees gusting over 40mph. Center woodstove flue sent smoke down boiler flue. Both were running. Third flue is sealed. Boiler would not draft anymore and airtight woodstove burn slowed and sometimes stopped briefly (during a gust). Note: deflectors visible in photos were added over 5 years ago and greatly helped moderate the excessive draft here on the ocean.


  • Big Ed_4
    Big Ed_4 Member Posts: 2,831
    edited February 2023
    Well that cap is not working , but it looks nice ..
    , The third unused flue would have been used for air intake for a fire place.

    You can extend the wood burner or the boiler flue higher than the other .

    I am not keen on adding more mechanical but depends on your boiler , maybe a draft inducer will work ..

    There was an error rendering this rich post.

  • robinmunroe
    robinmunroe Member Posts: 6
    Thanks, don't know much about the third flue. Brick fireplace has no cleanout, just ventilator fans in side channels and metal firebox liner (heatilator?) We installed a woodstove insert. White frost easily brushed off giving back full function. My question is which appliance is the culprit. 150K propane vs woodstove. We used to need the wood on a frigid night, but with air sealing it is no long neccessary.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,167
    AIR SEALING... that is the culprit.

    Think about it for a minute. You have a box (your house) that has some exhaust fans in it. Like the fan in the bathroom that you should use whenever the shower is operating to remove the humidity, there is the one in the kitchen you use when you are cooking and there is the one that is connected to the clothes dryer. Add to that the exhaust of the heating system and water heater (if that is a fossil fuel appliance). Where is all that exhaust air coming from? If you answer "From inside the house" then I will ask where is the House getting the replacement air? If you were to suck all the air out of a plastic milk bottle (the way all those fans are sucking the air from your house) the milk bottle would collapse, Right?

    But your house doesn't collapse does it? That is because the air gets sucked in from outside thru all those places you just spent so much to seal up! Now your house exhaust fans and clothes dryer and heating boiler or furnace did not stop using that air. But there are no more holes to let the air in. Except that unused wood stove chimney. So guess what? The air is coming down the chimney faster than Santa on Christmas Eve. Your unused wood stove and perhaps your heater chimney becomes the air intake. The screen on the vent cap collects the snow, frost and other humidity as an air filter would.

    You need to provide make up air for all your exhaust air. Perhaps an air intake to the boiler room would be in order , or a heat recovery ventilator

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • kcopp
    kcopp Member Posts: 4,448
    Not comfortable w/ the overarching cap over the 2 flues. It leads to just what happened. Perhaps separate caps would be a safer solution.
  • gmcinnes
    gmcinnes Member Posts: 118
    Is this extreme weather the first time you've run the wood stove and the boiler hard together @robinmunroe ?
    EBEBRATT-Ed Member Posts: 15,839
    The propane boiler is probably the issue. Gas contains water vapor that will condense (and freeze) in extreme temperatures.

    The make up air issue that @EdTheHeaterMan pointed out is valid.

    I would put a chimney liner in for the boiler which will keep the exhaust gas from cooling as much.

    If you do that you must extend the liner through the existing chimney cap and then put the approved liner cap on it. I have done that and it works
  • robinmunroe
    robinmunroe Member Posts: 6
    Thanks for your comments. Air sealing was confined to attic and crawl space encapsulation. I have thousands of nails penetrating the wrap (cedar shingles) and a leftover first floor eave from a 2nd fl addition that is probably why the walls are cold. The draft was always more than enough and sometimes too much, until that night. I opened a window but saw no effect. I find the chimney cap and liner suggestions awesome, and coming from educated experience. I do believe it was the boiler. I'm hoping the cause (weather) won't be repeated for a long time, and if it is, I'm taking the top off the cap.
  • Bob Harper
    Bob Harper Member Posts: 1,049
    Both flues must be suitable for the class of service. If they don't have liners, they should. The woodstove liner must be insulated to meet the 2,100F UL listing. The woodstove should not be allowed to smolder. Modern stoves are designed to burn hot. The low burn rate will allow the ss cap and mesh to cool easily. Same with the boiler. If the woodstove is firing concurrently, the boiler may short cycle due to the alternative heat source. You should have combustion analysis performed on the boiler to ensure it is firing properly. An underfired boiler is a hazard not to mention inefficient. I agre with separating the flues. The heater liner can terminate under the big cap if you have about 5" clearance to the lid while using the UL listed top support plate. The woodstove liner can penetrate the big lid so the wind carries the smoke away. If the stove liner is extended above the boiler flue, the big lid will separate the flue gases. You could revert to individual caps but that big lid is protecting the crown better. As Ed pointed out, you may need MakeUp Air. Get an unlisted low level CO monitor. Test the firewood to ensure moisture content 20% or less temperature corrected as measured in the heartwood. Measure the draft and gross stack temperature on the stove then exit temperature on both liners. Adjust firing rate as needed. If your boiler is going through long standby periods you may need to adjust the thermostat setting, aquastat, pump speed or adding a bypass.
  • Waher
    Waher Member Posts: 257
    The cap might be too low over the top of the flues. When I had my chimney lined the chimney sweep company installed a new cap which was significantly taller.