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Old 09-12-2009, 07:11 PM   #1
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Bathroom Fan, or not?


Our master bedroom has a half bath. No tub no shower. It has a ventilation fan installed in the early eighties and I was planning to update it. The existing fan has an 8"x8" housing and the new fan has a 7-1/2"x7-1/4" housing. While I'm sure it would work I would have to do some patch work on the corners to be able to cover the opening with the vent cover.

The new fan is 70CFM. I could buy an 8"x8" fan for double the price that is 80CFM. I'm wondering which choice would be better...or do I even need a fan at all?

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Old 09-12-2009, 07:20 PM   #2
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Bathroom Fan, or not?


a vent fan should be used since you still have a sink and toilet both which will produce moisture that needs to be removed. Size of the fan depends on the size of the room

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Old 09-12-2009, 07:26 PM   #3
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Bathroom Fan, or not?


I always install a fan, especially with a toilet
70cfm is all you need
Depends upon if you want to patch
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Old 09-12-2009, 07:42 PM   #4
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Bathroom Fan, or not?


OK So what your saying is there is no issue with the fan box being a bit smaller. I have no problem patching, we need paint in there anyway.

Right after I posted this I read German House Rebuild and now I feel like a wimp!
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Old 09-13-2009, 01:27 AM   #5
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Bathroom Fan, or not?


The code requires a fan in all bathrooms, even half baths (an exception is made if the room has an operable window, but it is a stupid exception in my opinion). The only thing that will really be an issue in there is smells...No humidity from showers...So you don't need to put a huge fan in there. That being said, a larger fan will clear the room out faster.
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Old 09-13-2009, 10:29 AM   #6
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Bathroom Fan, or not?


70 CFM in a your 1/2 bath will be fine. Most important, be sure the fan is venting to the outside and there is a source for air to enter the room when the do is closed. 1 1/2" of space at the bottom of a 32" door should be enough.

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Old 09-13-2009, 01:17 PM   #7
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Bathroom Fan, or not?


I was concerned that, with the smaller fan box, I would lose heat through the spaces. I plan on installing a much larger fan in the full bath even though the room is not that big.
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Old 09-13-2009, 01:49 PM   #8
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Bathroom Fan, or not?


8 air changes per hour for the cubic feet of space in the room is the minimum per code. Who wants a minimum of anything? lol.
Be safe, Gary
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Old 09-13-2009, 01:50 PM   #9
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Bathroom Fan, or not?


Quote:
Originally Posted by GBR in WA View Post
8 air changes per hour for the cubic feet of space in the room is the minimum per code. Who wants a minimum of anything? lol.
Be safe, Gary
I'm not familiar with that one...Can you give a code section?
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Old 09-13-2009, 02:47 PM   #10
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Bathroom Fan, or not?


Actually one of my concerns is that the old system does not vent to the outside. They ran the pipe up to the false chimney. I guess it does go out but from what I've read it should vent out of the side of the house and not the roof.
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Old 09-13-2009, 09:36 PM   #11
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Bathroom Fan, or not?


I'm not familiar with that one...Can you give a code section?From Kc. Because I said it wrong...... Nothing gets by that guy...The UMC code is still 5 room air changes/hr. ? 1203.3 20 cfm minimum. That's even less! Thank you for catching that, Kc.

This article recommends 8 changes, from HVI: http://www.efi.org/factoids/bathroom_sizing.html Notice too, the use of corrugated pipe and increase suggested, as well as pipe size changes for higher cfm's.
I would vent through roof, not into chase. The roof mount exhaust is 5-10% more efficient at removing warm, moist air (hot air rises) than a side mount one. Roof terminal should have a pipe neck attached for a positive connection. Use straight pipe (metal or PVC) whenever possible, wrapped with insulation. Corrugated, flex, pipe creates turbulence and has almost double the surface area for water exhausted in baths to collect on. Use as few bends as possible, and 45s* instead of 90s*. Tape metal joints with silver tape, not duct tape (4 year life). Tape the individual joints of each elbow. Use 3 screws on each metal joint. Use HVAC black tape for plastic covered insulation. Insulate the whole pipe and the fan box. Use caulk to seal the small gaps at the fan to wallboard joint. Let no heated moist air escape into the attic which would cause mold, mildew, wet other insulation (defeating its purpose), possibly making frost in your attic and ice dams on the roof.

Be safe, G
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Old 09-14-2009, 12:58 AM   #12
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Bathroom Fan, or not?


All good advice there GBR!!!
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Old 09-19-2009, 12:28 PM   #13
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Bathroom Fan, or not?


This great information but at the same time I'm not sure if it is what I wanted to hear. The house was built in 1982 and as far as I can tell the configuration has not changed since then. I have found no mold or moisture damage. The fans look to me like contractor models and seem a little cheap. I also believe they are low CFM. The pipe is corrugated flexible tubing but it is short. The main bath is probably only 5-7 feet from the vent and the 1/2 bath might be ten. The concern is the vent. They fastened the pipe to the inside of the false chimney. To me it looks a bit lazy but I was hoping I can keep it like this because I am afraid to cut a hole in the roof. Not so much cutting the hole but reworking the shingles afterward. This turns what I thought would be a quick job into a big job. I have other things to do you know, the honey do list isn't getting any shorter!

Anyway, I am going to do away with the flex pipe. My wife is a penny pincher and I can't say I disagree, no sense throwing money out the window...or through the fan pipe.

Please tell me I can vent through the false chimney.
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Old 09-20-2009, 12:41 AM   #14
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Bathroom Fan, or not?


I was able to install both new fans today...just the fans. It was much more work than I expected. It was really hot in the attic and I spent most of the time either on my belly or crouching. My whole body hurts!

Since the half bath was smaller than the existing hole I took a piece of aluminum flashing and cut a piece that would fit the fan and cover the hole. I will tape it with silver tape tomorrow. I may add caulk from the ground side to fill in the space, do I need to do this?

I finally broke down and called my dad, he is the best DIY guy I know. He used to fix everything when he was younger. He told me the city would disapprove of the vents discharging into the false chimney. He encouraged me to make a hole in the roof (I'm very nervous about this)and use regular duct to vent the fans. Now I have more questions.

One fan has a 6 inch exit but it came with an optional 4 inch step down. I was thinking about running 4 inch duct straight up from that fan through the roof. Then I would run three inch from the 1/2 bath fan and splice it into the 4" pipe. Is that good thinking?
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Old 09-20-2009, 06:54 AM   #15
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Bathroom Fan, or not?


your plan will work fine.

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