[jhnmath]

Hi - first-time poster here. I searched my question and didn't find the topic addressed previously.

My bathroom has a 70 CFM fan, but it's 122 cubic feet (10'10" ceilings in a 8.5'x10' room). Is it acceptable to add a second 70 CFM fan, or do we need one 130+ CFM unit?

Thanks much!

 

[Bud9051]

Well, isn't that actually 920 ft³?
The more frequent limitation on a bath fan is incoming air. Where is that coming from?
The only drawback I can see would be double the noise, assuming the same fan. But modern fans can be very quiet so upgrading to a larger fan should also result in a quieter fan.
Then there is the issue of duct size and the need for a second duct should you go with a second fan.

Maybe a simpler solution would be a delayed off timer that would keep the fan running for a set period of time, maybe with the door open.

Just rambling. See anything there as options?

Bud

 

[jhnmath]

Well, isn't that actually 920 ft³?

Yup, it is! I goofed - I should have said that what I've seen recommended is 122 CFM, minimum.

Would the existing duct need to be enlarged whether I go with a second fan or a bigger, single fan?

The contractor installed a 70 CFM fan and is proposing to install a second identical unit at his cost, so I'm really just wondering whether that scenario is as good as replacing the 70 with a 130+. I should have noted that first.

Thanks!

 

[Bud9051]

I just installed a larger fan in my bathroom renovation and they recommended a 6" duct, although they provides a 6 to 4 reducer as an option. I went with the 6 to keep the noise down. Connecting 2 fans to one duct should involve a larger duct anyway.

What size duct is there now?

Bud

 

[jhnmath]

I wish I knew. It's probably the smallest size that a 70 CFM fan would require.

 

[Bud9051]

Could easily be a 3" duct. If so it won't work well with a second fan added to it or with a larger fan. Where is the current fan vented, roof, side, gable? Hopefully not to the attic of into the soffit.

Is there an attic directly above this bathroom?

Bud

 

[jhnmath]

I'm not sure where it's vented; it's brand new and was inspected by our local building department, so hopefully not into the attic (yes, there is an attic directly above it). I'd imagine it vents either through the roof or the side; both are pretty close to the fan's location.

Sounds like we'd need a bigger duct either way. Is a 2x70 CFM system going to be have the same effects as a 1x140 CFM fan (assuming both are equally well vented)?

Thanks again!

 

[ddawg16]

As Bud mentioned, how much air can get in? Have you tried leaving the door partially open to see if it pulls the air out better?

Think about it....I've seen a lot of bathroom doors with a gap of less than 1/2". At 1/2" and a 24" wide door...that is at best 12" sq inches.

A 4" duct has an area of a little over 12 sq in. With the space under the door, the fan is being starved for air. Open up the door a bit and airflow will improve.

 

[Bud9051]

Yes, 2 fans would perform similar to one larger, but it's a patch job.

The real question is, will a larger fan perform that much better than the 70 cfm and that depends upon the supply of air into that room. It is also dependent upon how steamy the showers are. Some folks like to wash the walls with every shower, then you need a 200 cfm fan and a window open (just kidding about the window).

Your judgement as to how much the builder will bend, but I would shoot for a single fan properly sized, and I would want the delayed off switch, standard option these days.

Bud

 

[Bret86844]

Two fans just complicates things - electrical, ductwork, more holes in the ceiling...

Jhnmath, what is the reason you want a larger fan?

 

[jhnmath]

Some good questions that I can actually answer:

1. How much air is getting into the bathroom?
A decent amount; there's space for a transom window above the door, but it hasn't yet been glassed-in, so it's totally open (probably ~2sf).

2. What kind of showers are happening?
One of us takes what I'd call normal temp showers, the other takes very hot ones!

3. What is the reason I want a larger fan?
It's very steamy in the bathroom for a long while after any kind of shower.

Thank you guys!

 

[ddawg16]

Have you gone up on the roof and checked the air coming out the exhaust? Are you sure you don't have a kink somewhere?

Mine uses a 4" duct...short run of metal flex and then smooth metal duct.

 

[jhnmath]

No, I haven't. I did the calculation I found on Home Depot and another site, and both recommended at least 122 CFM for our 920 cf room (10'10" ceilings), which made me think our 70 CFM fan is very inadequate. Do you think it should be doing the job?

 

[jhnmath]

Also, the fan was only installed a month or so ago.

 

[user_12345a]

I wonder why you think 70 cfm isn't enough.

The problem may be the type or size of duct being used. u won't get 70 cfm through a 4" flex pipe for example.

If you get a 100+ cfm fan, the ducting will have to be redone for sure.

You're looking at 5 or 6" hard pipe.

All bathrooms even with fans get very humid if someone's showering. the trick is to run the fan for 10 to 15 minutes after the shower.

ask your contractor to put a timer instead of switch.

 

[jhnmath]

I think 70 isn't enough because the bathroom is very steamy for a long time after a shower (an hour?). That's a pretty general statement so I'll add that it's much steamier for much longer than any other bathroom I can recall having showered in.

Installing a timer might be problematic: the fan is within a can light unit that's controlled by one switch, so the timer would also leave the light on/turn it off at potentially inconvenient times.

Thanks!

 

[user_12345a]

Your bathroom will stay wet with any size fan if you shut the fan right after showering. There's a lot of water to evaporate from shower or tub surfaces.

The fan should be really be on a separate switch.

If not you can actually do two light switches for the fan/light both feeding the same hot; one a switch, and the second one being a timer.

If you use led bulb, the extra electricity use of running for an extra 15 minutes will be minimal.

 

[ddawg16]

I have 2 lights in my bathroom. One is the vent vent fan/light combination. The other is the light over the sink.

The vanity light is slaved off the vent fan light. I'm also using a motion sensor for the vent fan. (the people on my house have not figured out how to turn off lights)

Hence, when you go in the bathroom....you turn on the light....that turns on the fan...but it's quiet enough you don't notice it.

 

[RRH]

Start with a Leviton 6260M-W push button timer.
Turn it on when you get in the shower. Also when you get out you can press it again for 10 or 20 minutes since that is how long it is recommended to fan a fan after you have finished a shower. It is important to also run it after.
Light or no steam press 10 min or heavy steam press 20 min and walk away.

Next , two fans are a bad idea for a few reasons and could work against each other sometimes.

So you a new higher cfm fan. Or I really like the very quiet Panasonic inline fans.
You could leave the current fan and add the inline fan any place in the ductwork.
Then with your current fan you can remove the cover and see the motor and the plug for it.
You will then have the option to just unplug this motor since the panasonic will be pretty powerful.
Or another option if you wanted, is to leave it to add extra cfm's as a boster fan.
Either way it should take only a couple of minutes and you can play with it and decide which is best for you.

 

[Pern]

Is the current 70cfm actually working? Don't assume it is because the motor makes noise. A 70 cfm fan will clear the 1000cf room in 14 minutes, if all things are correct.