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Old 02-26-2013, 04:02 PM   #1
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Bathroom exhaust fan ducting

I have a small ~70-80 square foot bathroom. My plan is to replace the central ceiling light with a combo fan/light. I have conflicting advice on where to vent the exhaust.

One option, which HVAC contractor is going to quote is to vent into the attic space. I thought that would be bad because temps drop below freezing and could cause damage to roof or mold.

Second option is to vent it out of the ceiling but I am not to excited about cutting a hole in the roof. Duct would run 3-4 feet.

Third option, which I liked, was to run duct approximately 15-20 feet and out the side of the house. Contractor for option one said that would not work due to "static pressure".

I am a scientist by training and I am not buying it. A fan blowing 100 CFM through a 4" duct should not have a problem with a 20' run.

Please any help and advice would be appreciated. Also if I should be looking for somebody other than a heating and a/c contractor for this job, let me know what type would be recommended. HVAC didn't really get me a lot of hits.


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Old 02-26-2013, 04:32 PM   #2
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Never vent a bathroom into an attic space.

Your average priced bathroom fan will not move much air with a long length of venting connected to it. What is considered "long length" is dependant on unit/manufacturer. The average units do not have a fan blade design that is capable of pushing against much if any resistence.

Check the specifications of units you like, they will list the static pressure or resistence they will work against while providing X amount of air movement. You may just need to oversize the unit with a 20' run while increasing the size of the pipe to move the amount of air you need.


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joecaption (02-26-2013)
Old 02-26-2013, 04:48 PM   #3
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I've been working on houses for a very long time and I've heard of any HVAC company having anything to do with installing a bathroom vent.
His suggestion of just running it into the attic which is 100% may be one reason why.
Run it out the roof.
Simple install and unless the guy is brain dead installing it and can follow simple roofing 101 rules it should never leak.
Any handy man, DIY, remodeler can put one in.
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bjames50 (02-26-2013)
Old 02-26-2013, 04:52 PM   #4
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I can't find specifics in the specs regarding the duct run. Here is one fan I was considering.
Broan 1.5-Sone 140 CFM White Bathroom Fan with Light ENERGY STAR model QT140LE.

It comes with 6" duct which I was planning to reduce to 4" before piping it out the wall. Probably reduce it right before the damper. I found a graph in spec sheet indicating that with 4 or 6 " duct I would still get 100 CFM with about 0.4 inches of static pressure. Any idea where I can look up the formula to determine static pressure?
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