Bathroom Vent and Ductwork

Discussion in 'HVAC Heating & Cooling' started by jadnashua, May 5, 2013.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,932
    Location:
    New England
    My mother wants a ventilation fan installed in the bathroom. It uses a 4" outlet. To minimize the resistance, I want to use 4" round metal as it will have to run about 20' and that plus an elbow puts it around the max for the Panasonic fan I'm looking at, but I'd like to insulate it to minimize condensation since it will be running through the attic. I'd thought about buying some insulated flexible duct and running the solid stuff through it and I think I'd probably need to use 6" stuff, since I do not think I could slide the 4" solid into a 4" flexible duct (I haven't found 5", if it exists but haven't searched everywhere yet). The fan can also take 6" ductwork. Should I just forget about running solid material and use a 6" flexible duct? What's the best way to minimize chances of condensation and moisture dripping back into the room in the winter with this?
  2. ntruro

    ntruro New Member

    Messages:
    1
    Location:
    Ohio
    I'm trying to solve a similar condensation problem.

    I live in a relatively cold area (Columbus, Ohio) and have R45 insulation in my attic. I installed an in-line bathroom vent fan (Fantech PB100) using 4" insulated flex-duct per Fantech's instructions. The insulated flex duct runs straight up through the attic insulation and then passes through the attic cavity to a roof cap. (The total run of the flex duct is approx. 10' with 8' of it being in the non-insulated attic cavity.) The fan runs on a timer with up to 60 minutes of run time. When it's cold out (20F), the warm, moist air pumped through the cold flex duct above the attic insulation creates condensation inside the flex duct. This condensation runs back down through the flex duct and drips onto the person showering.

    I've talked with Fantech tech support, but they've been unable to provide a solution. The well insulated attic causes the flex duct above the insulation to become unusually cold (esp. during the night) resulting in condensation when warm, moist air from the shower passes through it.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,932
    Location:
    New England
    I got the fan in today, but have to finish up the ductwork in the morning. There appears to be enough room in the attic, so I think I'll be blowing in some cellulose on top of it. I decided to go with a 6" duct, and got some insulation sleeves for it. Anything that gets added blowing in the cellulose should prevent condensation from happening, or at least enough to prevent it from being a problem. FWIW, the fan actually has a 4" opening, but the collar is stepped so that you can attach a 6" duct as well. I was very close to the limit at 4", and had trouble finding 4" sleeve insulation in stock anywhere, but with the 6", both the insulation and the limit on run length no longer becomes an issue. The fan is barely audible when on. I haven't tested the humidity controlled switch yet, but am about to take a shower after crawling around in the attic. We'll see if it turns itself on like it's supposed to...it does turn on manually. Question: the ceiling is 1/2" drywall (or whatever they used to support a plaster ceiling in the 50's - no lath in it) and about a 1/2" of plaster. The roof is so low that I'm sure I'll have a big hassle screwing the thing into the joists. They call for caulk to seal it to the ceiling, and I've got a good solid bead underneath the mounting flange all around. So, I don't think the thing is going anywhere. Do I really need to screw in the support arms? It doesn't rattle or make any noise. I'm leaning on leaving well enough alone! ANd, the full-width arm is towards the eaves, so it may be even lower.
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