[Sequoia1321]

My bathroom is 8.5 x 5 ft, and I want to install a new bathroom ventilation fan. I was wondering how many CFM, and whether it matters if I go above the recommended CFM? Although my bathroom is small, I leave the door open when using it, so was wondering if this affects the CFM recommendation. Wouldn't mind going with a higher CFM if it doesn't hurt anything.

 

[user_12345a]

You can go higher cfm but the duct connected needs to be sized accordingly, otherwise it's a waste.

A 4" flex doesn't flow very much air.

Check local codes/rules to find out minimum cfm. It's probably done by square feet or volume.

If you have very high cfm, natural draft water heater may backdraft if you have one in the house.

 

[Old Thomas]

Generally a fan is required to move 50 CFM per sanitary fixture. Ducting has to be adequate to make it work. Contrary to popular belief, the fan is for venting sanitary fixtures (fart smellers) and not bathing facilities. You need makeup air. If you don’t want to leave the door open, undercut the bottom of it 3/4”.

 

[surferdude2]

Doubled

 

[surferdude2]

If you will leave the door open when you shower it will keep the wallpaper in the bathroom from falling off. Also help raise the humidity level.

If you will keep the door shut during and after pooing, it will keep your fellow house guests happier. Those little exhausters don't work very well with the door open or completely closed, unless undecut.

If not undercut, leave it slightly ajar after pooing and it will clear the room in about 5 minutes. A twist wall timer connected to it is money well spent for the convenience it provides.

 

[Old Thomas]

I will add that putting the fan opposite the door and undercutting the door will allow for the best air exchange. Air will enter under the door, sweep up and across the room, then out the exhauster.

 

[user_12345a]

Exhaust fans often don't move nearly as much air as rated to. They're tested at very low pressures and exhaust ducting/terminations can be very restrictive.

I would not get a fan rated below 70 cfm.

Yah leaving the door open is a bad idea.

Even a properly sized fan will not keep up with the moisture when showering, which is why you're supposed to run it for an extra 10 minutes after.

Contrary to popular belief, the fan is for venting sanitary fixtures (fart smellers) and not bathing facilities. You need makeup air. If you don’t want to leave the door open, undercut the bottom of it 3/4”.

Depends on local codes.

For tight houses, it's extremely important exhaust the moisture put off by showering to prevent major problems.

 

[3onthetree]

Bathroom exhaust is only required if you don't have a big enough window that can open. It is for the terlets, but a bonus is great for shower humidity so really should be utilized for that too.

The old days it was 1cfm/1sf, current code is 50cfm for a switched fan (unless your local has something more stringent) period. So a typical bath is 9x5 with 1 toilet is where they get the 50cfm.

An open door would defeat it, and they do not pull what they are rated for anyway. Many bump up to 70-100 for bigger rooms taking the shower into account and because it is usually not located right over the toilet. Keep in mind that the larger cfm the louder it is, and the duct outlet should be pre-sized for that fan's rating.

 

[user_12345a]

Bathroom exhaust is only required if you don't have a big enough window that can open. It is for the terlets, but a bonus is great for shower humidity so really should be utilized for that too.

The fan is mandatory in ontario even if there's a window, as it should be.

Building codes vary a lot.

Even if a fan is not mandatory in the op's area, it's unwise not to have one.
Open a window and cold air leaks in, sinks to the floor and hot/moist air gets pushed upwards. A window alone is not effective especially in cold climates.

 

[Old Thomas]

International Residential Code 2018 version Table M1505.4.4 says bathrooms and toilet rooms mechanical ventilation minimum is 50 CFM intermittent or 20 CFM continuous. Most states are based on the IRC but some modify it.

 

[3onthetree]

International Residential Code 2018 version Table M1505.4.4 says bathrooms and toilet rooms mechanical ventilation minimum is 50 CFM intermittent or 20 CFM continuous. Most states are based on the IRC but some modify it.

And just to pre-empt that IRC R303.3 states that a window of 3sf, with half operable is to be provided. If not, then mechanical ventilation needs to be provided via M1505.

 

[huesmann]

Yah leaving the door open is a bad idea.

Why is that? :confused1:

 

[3onthetree]

Yah leaving the door open is a bad idea.

Why is that? :confused1:

Well if you have a toddler instead of forcing them to peek under the door they will be standing next to you trying to see what's going on down in the water. :biggrin2:

 

[jmanhvac]

check out the Panasonic fv-0511vk2 fan. selectable CFM 50/80/110

 

[F250]

Total bathroom volume is the basis for properly sizing a fan.

https://www.wikihow.com/Calculate-CFM-for-Bathroom-Fan

 

[snic]

Keep in mind that the larger cfm the louder it is, and the duct outlet should be pre-sized for that fan's rating.

If you have an unfinished attic above the bathroom, one option is to install an in-line fan. Basically, in order from the ceiling: grille, plenum, insulated flexible duct, fan, insulated flexible duct, gable wall, vent cap, exterior.

I installed this system last year and it's so silent you don't even know the fan is on unless you really listen. So a timer switch is essential.

The company I linked to above will make custom plenum boxes so they fit right where your existing fan is, and you then attach a round duct to the box in the attic.

I installed a 200 cfm fan for a 5 x 10 bathroom, and it works beautifully.

 

[Old Thomas]

And just to pre-empt that IRC R303.3 states that a window of 3sf, with half operable is to be provided. If not, then mechanical ventilation needs to be provided via M1505.

You are correct. Had he asked about an operable window alternative I would have mentioned it. Good info.