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  • Extending PVC furnace exhausts

    Never done it before but I get calls asking to do this.

    Usually it's not just a few feet, it usually involves numerous bends/turns, is most times 3" instead of 2" and the reasons for which it is being done is because it's rusting joist hangers or rotting the wood under the deck due to constant moisture.

    I try not to deal with HVAC whenever possible....even if it is a simple A-coil condensate drain clog. The clog usually dictates a bigger problem >> lack of maintenance which needs to be done to the A-coil to keep it from rotting the bottom.


    Anyway....I've been told there's a sensor on these that will sense an obstruction or partial obstruction on the outside opening....causing it to shut down.

    Of course, extending it would follow the same guidelines set forth on power vent modelled gas water heaters where you count every turn of direction as a friction loss over 10'. Otherwise you have to upsize to make up for application.


    Even if I "could" extend furnace exhausts....I choose not to for reason of liability of damage to the furnace, $$$$. Only time I get involved with furnaces is when I'm asked to rework a gas line and involves removing/reworking the lines feeding the furnace. Everything else is out of my jurisdiction.


    My question; Do HVAC guys make a habit of performing this task or do you all just tell the homeowners no, no way. Some of these will run a long long distance I've seen in 2".....but limited bends.

    And when there's pitch back to the furnace....why isn't there a drip leg to catch condensate? I've seen too many times when you remove the panel that condensate is making it back to the blower assembly.

    A.O. Smith PowerVent water heaters at one time required this to catch this and drain to a floor drain to protect the motor.

    AFAIK, condensation is almost impossible to avoid given the humidity level of combustion air, room temperature, whether or not the unit is set up for a sealed fresh air return from the outside. Humid outside <> Moisture level goes up at the point of combustion.
    Last edited by DUNBAR PLUMBING; 12-16-2007, 01:14 PM.
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  • #2
    Re: Extending PVC furnace exhausts

    Every manufacturer has different venting requirements for their equipment. So every scenario would be different. Some people might just want to extend the exhaust under the deck past the rim joists. No can do...condensate will freeze before exiting the pipe.

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    • #3
      Re: Extending PVC furnace exhausts

      Oh boy! Where to start.
      We frequently get calls for this type of thing.
      Usually though, we get no-heat calls because the deck or the stairway went up AFTER our installation. Besides joist hangers rusting, the biggest concern is flue gas being drawn into the intake pipe. (99% of our installations are two pipe systems) This causes the flame to be snuffed out; it can also cause frost and ice to build up in the intake, then the pressure switch will take the furnace off line. If you're within the piping specs for that furnace, extending the flue shouldn't be a problem. Pitched towards the furnace is correct, and it doesn't matter that the water gets back to the ID blower, it has a drain. Once it leaves the house, however, the pipe should be pitched down so that the condensation can drain while the unit is running and immediately after it shuts off. If it's piped like this, there will be no chance for the condensate to freeze in the exhaust pipe.

      There are two charts in the installation instructions, one for non-direct vent (single pipe) and one for direct vent (two pipe) installations, as most furnaces are dual certified. The charts list allowable vent lengths for either 2" or 3" pipe for each number of elbows.

      Maybe I'm just rambling..
      Anyway, I hope this helps.

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