Bathroom vent doesn't exhaust enough moisture

[Slik Geek]

Your exhaust fan needs to be sized for the room its installed in.

This is the conventional wisdom, like what is recommended by the Home Ventilating Institute. They say, "For bathrooms up to 100 square feet in area, HVI recommends that an exhaust fan provide 1 CFM per square foot (approximately eight air changes per hour) to properly ventilate the bathroom. For bathrooms above 100 square feet in area, HVI recommends a ventilation rate based on the number and type of fixtures present."

After pondering this for awhile, I've concluded that the ventilation rate should always be based upon the number and type of fixtures present. If you have the same shower in a tiny bathroom or a large bathroom, the amount of moisture added to the air is the same (up until the point of saturation of the air). The smaller room will have a more rapid increase in humidity, and a higher risk of saturation because the same water evaporation is entering a smaller space. To achieve a similar limit for the maximum relative humidities in the two sizes of rooms, the same volume of air exchange is required. The end result will be a more rapid air exchange rate in the small room.

Based upon my experience, HVI's recommendation of 50 CFM for a shower is too low. So what is required? I'm curious what CFM rating the new fans have in crokett's, Cheeky's and steve-norrell's installations.

[Crash2510]

In bathrooms the size you described we typically install 70 to 100 cfm fans which is usually architect spec in our area.

[Slik Geek]

In bathrooms the size you described we typically install 70 to 100 cfm fans which is usually architect spec in our area.

Those would have a better chance than the "recommended" value of 50 CFM. Perhaps the value specified is based upon the experience of the architects rather than a rule of thumb. Thanks for the data point.

[Garasaki]

I do HVAC design for commercial applications. Follows are some of the exhaust air guidelines we use:

Toilet rooms: 10 air changes per hour
Shower Rooms: 15 air changes per hour

General toilet rooms: 2 cfm/sq ft

Residential bathrooms are obviously different, as they are required to to serve double duty, toilet and shower, and typically are much smaller (no ADA codes). If I were designing residential, I don't believe I would exhaust any less then 10 ac/hr.

Food for thought: There are many advantages to providing more exhaust in a shower/restroom. What are the disadvantages?

Arguably higher energy costs (as you are removing additional air you have payed to condition in the first place). Potential pressure problems, doors slamming, etc…but the amount of air you'd have to exhaust to cause these problems would be huge.

So, IMO, it's better to oversize your exhaust…I might even go as far as to say you can't provide too much exhaust in your bathroom.

[ExYankee]

My solution was to install a PIR (passive infrared reciever) switch on the line to the fan. I set it for 15 min and then it goes out. Since it ALWAYs turns ons whenever someone is in the room, it keeps the air nice and fresh

[Black wallnut]

You may remove the cover and see if your fan has a mesh screen. Mine did and it was plugged. I cleaned it and it is just amazing how much air mine moves now. Just a quick free fix if this is your problem.

[Cheeky]

Based upon my experience, HVI's recommendation of 50 CFM for a shower is too low. So what is required? I'm curious what CFM rating the new fans have in crokett's, Cheeky's and steve-norrell's installations.

I have a 110cfm Panasonic in my bathroom (similar size room). There is no way I would go less, considering the elbow in the attic duct run and the lenghth ~12 feet.

The best advice I received prior to I installed my kitchen exhaust, was to compensate for any sharp changes in the direction of the duct work, and also longer duct runs.

Calculating the room size just doesn't provide a perfect sollution (you must account for the aforementioned variables), which is why I upgraded one size.

BTW, the timers are a great idea. I had family from Europe visiting last week, and no matter how many times I asked them to leave the fan on for ~15-20 minutes after the shower, they forgot (closed the door when they left the bathroom too....and they were adults). I'm installing one before I have any more guests.

[onedash]

I noticed that the finer the spray in the shower the more steam it makes too. When I switched the head on my shower I have to use some cold water now. Before I only used hot and it was just about right. With this new head on a heavier stream there is much less steam on the window and mirror. Im sure more water is being used too......

[Slik Geek]

Shower Rooms: 15 air changes per hour
General toilet rooms: 2 cfm/sq ft
If I were designing residential, I don't believe I would exhaust any less then 10 ac/hr.
So, IMO, it's better to oversize your exhaust…I might even go as far as to say you can't provide too much exhaust in your bathroom.

Good information. I ordered a new fan last week, and I decided to oversize. I picked a 150 cfm Panasonic. (It actually arrived yesterday, but it might be a couple days before I can install it).

For my room, 15 acph = 90 cfm
2 cfm/sq ft = 90 cfm

I was tempted to go for the 100 cfm model, but given the run length of the exhaust, I decided to error on the safe side.

I'll provide the results when the fan is installed and the data is in.

[Cheeky]

Good information. I ordered a new fan last week, and I decided to oversize. I picked a 150 cfm Panasonic. (It actually arrived yesterday, but it might be a couple days before I can install it).

For my room, 15 acph = 90 cfm
2 cfm/sq ft = 90 cfm

I was tempted to go for the 100 cfm model, but given the run length of the exhaust, I decided to error on the safe side.

I'll provide the results when the fan is installed and the data is in.

Slik,

Were you aware that the 150 cfm model has a 6" duct opening? You probably have 4" ducts in the attic.

[crokett]

Based upon my experience, HVI's recommendation of 50 CFM for a shower is too low. So what is required? I'm curious what CFM rating the new fans have in crokett's, Cheeky's and steve-norrell's installations.

Not sure. We don't own the house any more. The bathroom was small only about 45sqft. IIRC the fan I put in was 70CFM or so.

[Slik Geek]

Slik,
Were you aware that the 150 cfm model has a 6" duct opening? You probably have 4" ducts in the attic.

Yep, I noticed that when I was shopping. I was on the 100/150 fence, and decided to go for the higher flow and deal with the ducting. I may initially install it with a reducer at the output just to see if how well that works. (Then deal with the duct when the temperatures are a wee bit warmer).

Thanks for the heads up, that would have been a less than great surprise when I went to install it! I suspect that you are correct that the ducts are 4". That's what I'm guessing they are.

[Cheeky]

Yep, I noticed that when I was shopping. I was on the 100/150 fence, and decided to go for the higher flow and deal with the ducting. I may initially install it with a reducer at the output just to see if how well that works. (Then deal with the duct when the temperatures are a wee bit warmer).

Thanks for the heads up, that would have been a less than great surprise when I went to install it! I suspect that you are correct that the ducts are 4". That's what I'm guessing they are.

good deal. i helped my brother install them, and he failed to let me know this before we stared

i think you are going to be really happy with the results. good luck and let us know after the install (ps, the night light feature really isn't that cool).

[Slik Geek]

The new fan is installed! Time will tell if it fixes the problem. I purchased a Panasonic FV-15VQ4 Whisper Ceiling 150 CFM exhaust fan online from www.ventdirect.com, a site recommended earlier in this thread, as they had the best price of a few places that I searched. (Thanks gabedad). Delivery was prompt.

I fired the vent up to make sure it worked. It was whisper quiet on the bench.

When I got up to the attic, I discovered that my guess of the existing duct size was wrong. The builder had installed uninsulated 3" duct, which matched the original fan. That couldn't have helped the ventilation performance. (The lack of insulation may have accounted for the dripping vent we had during the first winter in this home. The warm moist exhaust air, when it met the cold metal duct in the attic, resulted in condensation which dripped back down into the fan. Additional insulation that we added on top of the fan enclosure in the attic stopped the condensation drip).

I Installed a 6" insulated flexible duct. (I know that rigid ducting would have higher performance, but the additional cost, labor and time wasn't in sync with my current plans). I left the existing vent exhaust port as it was. It was a 4" port, and the installer had cut slits in the duct to squeeze it down to receive the 3" duct tubing. I installed a 6" to 4" reducer at the vent.

With it installed, it isn't as quiet as it was on the bench. I suspect some of the noise is duct induced. Some of it may be related to resonance in the mounting hardware, or perhaps the grill mounting springs. Nevertheless, it is quieter than its lower flow predecessor.

I wasn't enamored with the installation instructions. There were four installation options, and since my scenario required installation #2, I had to frequently refer back to #1 for information that wasn't repeated. A few of the steps were lacking detailed explanations, so I spent some time sitting in the attic sorting out the installation.

I was really disappointed with two aspects of the design as they related to installation. The six inch port is mounted low on the enclosure, such that when it is installed as a retrofit, there is virtually no clearance between the bottom of the port and the ceiling drywall. Getting the duct onto the fan as difficult, and getting duct tape around it was impossible.

The electrical junction box was lame as well. It was mounted too low on the enclosure, which made wiring in place with drywall installed more difficult than it really should be.

[Cheeky]

I wasn't enamored with the installation instructions. There were four installation options, and since my scenario required installation #2, I had to frequently refer back to #1 for information that wasn't repeated. A few of the steps were lacking detailed explanations, so I spent some time sitting in the attic sorting out the installation.

I was really disappointed with two aspects of the design as they related to installation. The six inch port is mounted low on the enclosure, such that when it is installed as a retrofit, there is virtually no clearance between the bottom of the port and the ceiling drywall. Getting the duct onto the fan as difficult, and getting duct tape around it was impossible.

The electrical junction box was lame as well. It was mounted too low on the enclosure, which made wiring in place with drywall installed more difficult than it really should be.

Congrats Dude!

I should've warned you about the brief instructions....apologies

The 6" port is obscenely low, and I came up with something that works really well, if you want to go back up and make sure it's air tight. Go to Lowes or HD and get a minimum 20" zip tie (circumference = pi X diameter (roughly 18.9")). Loop it around starting at the bottom, and then zip it on top.

In reality, it's best to hook up the electrical before you fit the fan (I learned that the hard way too )

All in all, my toughest install was the Pan. 4" fan in a downstairs bath...my knuckles and wrists hurt for about a week after.

For the 6", I basically had to take a little from each of the instructions...but it was frustrating to say the least. But wait until it is time to see it in action....well worth it!