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Furnace (and boiler) direct vents through roof

Chuck_17 Member Posts: 143
Snow blocking the air intake of a direct fire furnace (boiler) is a common issue. Care must be taken to not put the vent where the snow will drift.

A new problem - frost closing the exhaust vent in the very cold weather. The vents are extended up through the roof space and then combine to a concentric vent. It appears that the vents from lower floors may be worse as the flue gas has more time to cool. It also appears that the wind is as big an issue as the cold.

One of the reasons (among others) the vents are through the roof is concerns with condensate from wall vents dripping down the siding. Esp. form upper floors (2nd and 3rd floors). Should this really be a concern if installed correctly?


  • icesailor
    icesailor Member Posts: 7,265
    Heights of Vents Through Roof's:

    In plumbing, there is a reason that the minimum size of a vent through a roof is 2" and must be a minimum of 18" and not more than 24" above the roof piercing is to keep the opening from closing up in the cold. The bigger the size of the pipe, the less chance of it closing from ice. The 18" minimum is also to get the upper end of the termination from being warmer and getting that "Dew-Point" thing into play. In some really cold areas, minimum V TR size is 3".

    If you have a ground termination where the intake is covering with snow, I suggest that it is possible that you are getting "regurgitation" from the exhaust back into the intake because the outside winter air has a much lower humidity than the warmer water vapor laden exhaust. The water vapor will stick to the plastic and condense and freeze out. Then, the snow will stick to it.

    Some of you guys who aren't sailors and have never chased the elusive winds while racing, have no idea about wind turbulence and eddy's. You get the same ones in water pipes or flowing water streams. If you're going to put a PVC shower strainer on a power vented intake, increase the size of the intake to 3" or 4" to stop restriction of air flow. I'll bet you would bet a change in suction pressure between having the strainer on and them off.

    When sailing on a lake in the summer in light and variable winds, where do you often find the most winds? Right along the shore, where the air is heated by the dark trees and drops down along the water.
  • icesailor
    icesailor Member Posts: 7,265
    Direct Vents Through Roofs:

    If you understand how draft and chimney's work, those gas cents are not ever high enough above the roof unless they are very near the ridge of the roof. Flowing air has less pressure than static air. There is more pressure inside a building than there is outside when the wind is blowing. Hot air wants to flow to the cold air. If a vent is on the lee side of a roof, and the wind is blowing heavily from the opposite side, the wind will hit the windward side and flow over the peak. It will then roll, swirl, and sort itself out and go back on its merry turbulent way. This dead space has a lot to do with the speed of the wind, temperature, and how steep the pitch of the roof is. The pressure of the air goes up and down in this space. But the pressure is higher than the flowing wind above it. It can and will equal or overcome the pressure inside the house that depends on a power vent fan to overcome the problem. In the swirling, the wet gasses get sucked into the concentric. If you live in a really windy location like by the New England sea shore, you can see where paper shingles get ripped off roofs after 60+ MPH wind storms, the first or second courses down from the ridge cap. Because of swirling winds.

    There is an inherent problem with Power Venter's that use outside air for combustion and exhaust in high winds. Or high and osculating.

    Wind and draft is a science unto itself. Just like making fluids flow through pipes.