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Access to backdraft damper

ken_o | Posted in Mechanicals on

I see a lot of discussion about bathroom fan venting, but nothing about where to locate a backdraft damper.  The new house will be in a cold climate.  I presume that in the lifetime of the house the damper may need service, cleaning, or replacement.  The bathroom on the first floor will vent straight out the wall, and the second floor bathroom vent will go inside the scissor trusses and out under the eave.  I want to have a rather airtight seal from the ceiling to the space inside the roof trusses, so I want to avoid a hatch.  Where could I locate a damper so that it can be serviced or replaced?  Or, “don’t worry about it?”  Any thoughts are appreciated.  Thanks.

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  1. Expert Member


    You may need a hatch either from the house or from outside to the truss space. Concealed spaces over a certain height (here it is 2'-0") are required by codes to have access to them. That will solve one of your two situations, as then the in-line damper can be anywhere on that duct.

    Most bathroom fans have dampers at the fan housing, and a second one at the exterior termination. If you want to put a third one on it makes sense to ensure it is accessible, but I would instead buy a good exterior termination with an effective back flow damper.

    That said, I've recently started to use another approach. To deal with this problem of concealed dampers and connections on bathroom fans, which make replacement of both the dampers and fan itself difficult, I now locate all the components in a box which is accessible from inside the room. You can see pictures of this at the end of Martin's blog:

  2. plumb_bob | | #2

    I would add that back drafting from bath and kitchen fans, dryers etc., is largely a function of negative pressure within the house. With balance mechanical ventilation it becomes much less of an issue.

    I built a small carriage house early in my career, with good attention to air tightness. I had the mechanical guys install a Hoyme damper interlocked with the bath fan, but for various reasons the damper never did work. On cold nights when the bath fan and dryer were on the stove hood vent would back draft freezing cold air. So I learned about balance ventilation the hard way.

  3. jberks | | #3

    You can try an internal backdraft damper like this:

    I like to do external or inline exhaust fans, so this is how I add a backdraft damper, I push it in the duct just above the ceiling. I can pull it out when I don't like it any more.


  4. ken_o | | #4

    Thanks for all of the ideas. Yes, I will be balancing ventilation and also providing some "make up" air.

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