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Bathroom Fans and Heat Loss?

Post in 'The Green Room' started by velvetfoot, Nov 28, 2008.

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  1. velvetfoot

    velvetfoot Minister of Fire

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    I haven't tried the incense test yet to check for air egress yet, as boekeham (sp?) and others suggested (bought the incense sticks - man it's been a long time since I did that). I did aim the IR gun at both upstairs fan/light combos and they had showed cold temps there. Last time I checked on one of them, the crappy little integrated flapper that prevents air ingress got disconnected and there was nothing preventing air from coming in or out for that matter.

    Are these things typically a source of heat loss? I imagine they would benefit from a flap at the outdoors end of the duct like a dryer vent, but damn, that'll be a non-trivial project for me, involving ladders, heights, and/or a messy crawl in the attic. One guy at work said something to me a while ago about having a separate, quiet exhaust fan, mounted in the attic, similar in design to a radon removal fan. That seems intriguing.

    My current arrangement has the ducts exhausting through the venting under the roof underhang, which probably isn't super ideal since it could therorectially be sucked back in by the vents.

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  2. Crabbypatty

    Crabbypatty New Member

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    you could allways invest in a energy recovery ventilator. ERV's take the heat or cooling out of the air being exhausted and recover it back into the home........i have a panasonic fan in my bathroom and it is top of the line.....not available at HD or Lowes, it is wisper quiet and it seems to be good at keeping backdrafts from entering when it is not operating. just my $.02
  3. velvetfoot

    velvetfoot Minister of Fire

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    I am not necessarily interested in an ERV for a bathroom fan.
    Seems expensive and potentially non effective for a little application like that.

    I just was thinking that the arrangement of a damper at the fan would cause the fan to be cold, since there would be no impediement to the cold air until it hit the damper, which seal may be less than perfect. Then, if the stack effect was sufficiently strong, the fan damper might move enough to let out heated air when the fan was not in operation.

    I have a positive damper on the end of the drier pipe. I was thinking something similar for the bathroom vent.

    So, DNJ, would your scenario produce with a typical 100 cfm fan produce a result of 60 btus per minute with a 50*F temp difference? So if the fan was run for 10 minutes that would be 600 btus?
    According to the nifty fuel cost comparison calculator at http://www.hearth.com/econtent/index.php/articles/fuel_cost_comparison_calculator/ , at $2.80 a gallon at 78% efficiency giving $26.56 per million BTU, that would mean running that fan for 10 minutes would cost 1.6 cents. hmmmmmmm....... Factored against medical costs resulting from falling off the roof, etc,.... hmmmmmm......
  4. d.n.f.

    d.n.f. New Member

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    I have one in a bathroom that doesn't get used at all.

    The draft up the fan is amazing. I have taken a piece of rigid foam and blocked it off. Obviously a temp solution.
    The fan howls like a mad dog and needs replacing. Am looking at a Panasonic model also.
  5. burnham

    burnham Member

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    Most of the new fans I install have that same cheap flapper, it often gets hung-up in the open position and does nothing for you. Some of the newer Nutone/Brone flappers actually have a counterweight on the back side that alllows the flapper to hang open at rest, I think the idea with them is they are supposed to close when the wind blows and air travels the wrong direction in the vent hose. Either way, both flappers are crap, especially when used with flex hose that's not pulled tight.

    I've bought a couple of in-line flappers from Home Depot and used them in unison with Nutone fan/flapper, positioning the in-line unit as close to the outside air as possible. This helps keep the whole vent hose from becoming filled with cold air (if you have some insulation over it) and makes a pretty big difference considering it only costs $6 or $7 for the part. The nicer in-line flappers are all metal, and the two-halves that seal the pipe are on a very light spring that does a good job closing off the air flow. The cheaper ones are all plastic, and use a one-piece flap that closes on gravity. They also usually have an open area, around 5-15% maybe, that is never blocked off when the flap is closed.

    I have a Panasonic fan in my house, it does do a much better job keeping the cold air where it belongs.
  6. velvetfoot

    velvetfoot Minister of Fire

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    Thanks much for the info.
  7. pybyr

    pybyr Minister of Fire

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    I went with Fantech in-line units that I was able to put in the attic (isolates the noise)- and they make separate one-way valves that seem less likely to get stuck than the usual built-in cheapos in cheapo fans. Another nice thing about the Fantechs is that they are OK to use with a decent speed control like a Lutron, so you can tailor the airflow for situations where you may not need a full tilt fan. Used SDR 35 4" pvc from bathroom ceiling vent up through ceiling/ into attic, then fiberglass insulated flex duct to fan (to avoid transmitting vibrations) and from fan to the damper on the side wall. got it all from HVACquick.com and found them very helpful.
  8. velvetfoot

    velvetfoot Minister of Fire

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    Thanks a lot for the info.
  9. Highbeam

    Highbeam Minister of Fire

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    I just installed new bathroom fans where there were none before. ABout 2 years ago I had my leky metal roof replaced and told them to cut in the little 4" roof vents with a nipple to be used for the fans. So here's the deal, these roof vents already include a flapper/dmper deal that opens vertical and shuts by gravity. Dang effective and I can check them from the roof. They look just like the dryer vent outlet but being vertical even better.

    The bathroom fans also included an attached plastic flapper that was mildly substantial but I used them since they were required to attach the 4" duct to the fixture. I get no leaks of cold air and when they run due to showering I get a plume of steam from the vent. Before blowing insulation in above them I caulked the snot out of any little hole.

    Seems to me that any leak from the fans would be allowing hot air to rise out of the vent and not to allow cold air down. Even the release of hot air would require a leak down below.

    Now the hood fan/microwave in the kitchen, no flapper on that one on the appliance end. Only the big 6" flapper on the roof vent.
  10. velvetfoot

    velvetfoot Minister of Fire

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    I'm kinda anti-metal-roof-penetrations, with the sliding snow and all.

    I did see one of those exhaust kits for a bathroom at HD today, only I think it was horizontal, and it too had a flapper at the end. I think it was a NuTone.

    I also saw a 4" in-line draft blocker, but it didn't seem to act as positive as the one with the kit.

    Thing is, my vent exhausts under the eaves, so I'm not sure what I have there, but I don't think there's another damper out there.

    I'm not relishing going up in the attic and splicing one in though.
  11. Sealcove

    Sealcove Member

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    I recently put a couple of Airking fan units in. They have a damper built in at the fan itself, and on the versions I got it seemed like a nice tight closure. These were quite a bit more expensive than your average fan though ($140 per fan).
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