I have a challenging bath ventilation issue. I'm on my second 50cfm fan from big box store and it's doing a lousy job of exhausting the humidity after showers. The louvere is moving when fan operates. It terminates through the gable wall. I'm nearing a big remodel for my bathroom and I've thinking about if it might help provide more fresh air to the bathroom if I install grates in the bottom of the bathroom doors. Two wood bathroom doors on opposite sides. I'd like to get some kind of stainable grate that doesn't look ugly. Think wood floor registers. Would that be a smart idea?

It would be faster to undercut the door a bit then install Louvers.

If you bought a 50 cfm fan, and then used aluminum slinky duct, or insulated flex duct to send it to the outside with a couple of turns, you may not be exhausting nearly enough air. I often test bath fans that are only have a flow rate of 10- 30 cfm. If you are remodeling anyway I recommend a 80 cfm Panasonic Whisper lite fan( or Whisper Green Lite Fan), and using 4” rigid galvanized duct or schedule 20 PVC for ducting. On the outside wall, use a heartland dryer vent with the low restriction spring installed and you will be in great shape.


Cheers,
Eric

Not for noise. I'd put in two fans, one for intake and one for exhaust. That will greatly increase air flow and keep the room pressure balanced (possibly a wall moisture concern when A/C is being used).

Posted By Eric Anderson on 11 Nov 2014 11:04 AM
It would be faster to undercut the door a bit then install Louvers.

If you bought a 50 cfm fan, and then used aluminum slinky duct, or insulated flex duct to send it to the outside with a couple of turns, you may not be exhausting nearly enough air. I often test bath fans that are only have a flow rate of 10- 30 cfm. If you are remodeling anyway I recommend a 80 cfm Panasonic Whisper lite fan( or Whisper Green Lite Fan), and using 4” rigid galvanized duct or schedule 20 PVC for ducting. On the outside wall, use a heartland dryer vent with the low restriction spring installed and you will be in great shape.


Cheers,
Eric

So if I indercut the door, how much do you think would be smart to do? A half inch? I wanted to install the grills in the door because I thought it would allow air to flow freely and without having to worry about leaving the door open. When you say schedule 20 pvc pipe, does that hold up well insulated in hot attic temps?

A bath fan is properly sized first by the intended use e.g. toilet, shower and/or tub. Then the size of the bathroom, sometimes a small bathroom with a shower means more fan. Finally the relative tightness of the house. I rarely use a bath fan in my regular design work preferring to use an ERV and wait 10 minutes for the mirror to clear, worst case. In winter this is even more rare.

If you can't tolerate a steamy mirror--then you don't appreciate the tension that wiping a mirror off can lend to a good bathroom thrill scene--there are many low-sone ventilators on the market. Unless you have a small powder room or exclusive bathtub a 100 cfm is my minimum choice without worry about pulling a negative in such an incidental load.

The only vent material for moist air is aluminum, insulated a minimum 5' in from the outside terminal in cold climates like our Minneapolis.

Schedule pvc piping seems to make for a better material to work with - flexible, tighter seal, fewer seams - how am I wrong?

Wish I could install an HRV or ERV, but this isn't new construction. This house isn't tight enough to justify it. I just want to make this simple bathroom fan perform better. Thank you.

You didn’t mention the size of your bathroom? The exhaust fan (and associated intake area) should be sized based on the bathroom size. You typically want 8 ACH (Air Changes per Hour) for bathroom ventilation. 60 minutes divided by 8 is 7.5 minutes. So if you divide the bathroom volume by 7.5, you will get the exhaust fan CFM that will provide this 8 ACH. For example, if you have 10’ long by 6’ wide by 8’ high bathroom, the volume is 480 CF. Dividing by 7.5 gives you 64 CFM for the minimum size exhaust fan.

I concur with Eric, a 3/4" gap between the floor and lower door should get the job done with minimal fuss…provided you have an adequate exhaust fan CFM.

Right, PVC is a good choice and ERV/HRV is not appropriate for your situation.

The basic point I was trying to make is that most fans are not putting out their rated airflow unless the duct system is near perfect. I often test fans that are producing a small fraction of their rated output. More often than not, the ductwork is the problem. When ever possible I use PVC for bath vents. Sometimes you have a long run to a gable end, PVC starts in 10 ft lengths and sections are glued together for an airtight fit. Easier than using mastic to seal aluminum or galvanized. You can buy duct insulation sleeve that fits over the pipe or just use insulated flex duct insulation ( after removing the inner liner) and slide it on. I will sometimes use a short section of Flex duct to connect to the rigid duct, but I avoid it if I can.
I recommend the Panasonics because they are quiet and dependable, I am sure there are others that are also, but I don’t have time to test them all. Panasonic is my go to fan. A hell of a lot of people won’t use their fan because it is too noisy. If you don’t use it, the cfm is not relevant.
The other thing that helps is a timer switch, I like the 5,10,15,30 min timer ones.

One of the stale air pickups for my erv is in the bathroom, but I still prefer using a dedicated bath fan when I am showering, vs running the erv on high for a while.
Cheers,
Eric

My bathroom ha about 8.5 x 6 feet with 8.5' ceilings. The fan is 50 com and it's above the shower.

I went to the Orange box store tonight looking for sch 20 pvc pipe and they not only didn't have it, they looked at me like I'm an alien. I'd rather use pvc because I feel it's more airtight and lightweight. Airsealing aluminum ducts is PITA.

What do you need scchedule 20 pvc for? Why not use scchedule 40 which is widely available if you want pvc?

ask for pvc sewer and drain pipe- which is usualy sch 20, not always. should have ~1/16" sidewall. The box stores are often staffed by well meaning idiots Cheers, Eric

Posted By Surfsup on 12 Nov 2014 07:52 AM
What do you need scchedule 20 pvc for? Why not use scchedule 40 which is widely available if you want pvc?

I think the reason is because sch 20 is bendable or at least more flexible. What kind of connectors do I use to attach to fan or exhaust.

Posted By BadgerBoilerMN on 11 Nov 2014 05:55 PM


If you can't tolerate a steamy mirror--then you don't appreciate the tension that wiping a mirror off can lend to a good bathroom thrill scene--there are many low-sone ventilators on the market.

Post of the Day.  :)

4" PVC sewer and drain pipe is lighter and costs much less than Schedule 40 PVC pipe. But I wouldn't consider it to be flexible.  It comes in 10' or 20' rigid straight sections that use glue-on fittings to make turns.  Note that Schedule 40 PVC pipe fittings will not fit sewer and drain pipe - the OD of the pipe is different - you must get the correct sewer and drain pipe fittings.  Another term often used for sewer and drain pipe is SDR 35 (this designates a ratio of the pipe wall thickness to the diameter). SDR 35 is a heavy duty version of sewer and drain pipe. Basic belled end 4" PVC sewer and drain pipe has a typical wall thickness of just over 1/16" which is more than suitable for bathroom exhaust venting. 4" SDR 35 has a typical wall thickness of ~1/8". 4" schedule 40 PVC has a wall thickness of ~1/4" which is overkill for air venting purposes.

Assuming your bath fan has a 4" diameter outlet, 4" sewer and drain pipe should slip over this outlet and would require aluminum foil tape to seal it to the housing.  Another way is to attach a section of 4" aluminum or galvanized metal snap-lock pipe to the exhaust fan and the crimped end of the metal pipe will slip inside the 4" sewer and drain pipe.  Ends can be secured with self-taping screws through the pipes and then sealed with aluminum foil tape.

http://www.tamtech.com/3-Pack-Perfect-Balance-In-Door-Return-Air-Pathway_p_106.html

I am considering these for bath as well as bedrooms that have supply only duct work.

Chris

I'm a big fan of the Panasonic fans as well. The Big Box fans don't even compare. I would suggest that you add the timer switch to control them though. They are so quiet that it is easy to forget they are on. Many the time I went in the bathroom and the fan was running because the wife didn't turn in off. Timer switch solves that.

Anyone have an opinion on whether it would be better to have a 6" semi-ridged exhaust duct with a reducer to accommodate fan's 4" connection vs a smooth walled schedule 20 pvc that's 4" all the way to the end? Would increasing the duct size to 6" make up for the loss of air flow from a 4"?