Bathrooms should have four complete air exchanges per hour.

First you have to determine how much air you need to move. To do so compute the volume of the room in cubic feet then multiply room volume by the number of air exchanges desired per hour.

(Length x width x height ) x 4 equal total cubic feet of air per hour.

example: for a room that is 10 x 10 with an 8ft ceiling it would be 10 x 10 x 8 = 800cu.ft of room volume.

for four exchanges per hour is would be 800cu.ft x 4= 3200cf/hr.

Most fans are rated in CFM

To convert the total cubic feet per hour to CFM divide by 60

example.. 3200cubic feet hour / 60min/hr = 53.3cfm (54cfm).

You would then need a fan that is rated equal to or slightly greater than 54cfm.

lazypup,

I was anticipating a need for 8 ach (air changes/hour) and the fan we currently have is rated at 120 cfm @ 0.2" static pressure (138 cfm @ 0.0" static)...it's the Panasonic FV-10NLF1.

All the calculations I run indicate this is the appropriate size for our need, yet it just doesn't *seem* to be adequate in real life. It's fine when running one showerhead but when we turn on the second one the wetroom becomes quite the steambath (which is certainly comfortable for us heat-seeking types but not so good for inhibiting mildew growth).

When we have the house open (Spring and Autumn), we just open the 30"x18" awning window in the room and don't use the fan at all...but then we're not running the water as hot either.

This has become quite the conundrum for us.

Here is a link that might be useful: Panasonic WhisperLine� 120 CFM In-Line Fan

Perhaps your looking in the wrong direction....LOL

Far too often when the exhaust fan fails to eliminate the steam ppl instantly think the fan is not big enough, but that may not be the problem at all.

Nature abhors a vacuum therefore no matter how big the fan and duct system are it will not discharge air from the room unless there is a provision for an equal amount of air to enter the room.

Typically the only provision for air to enter the room is the gap under the door and that may not be sufficient to your need. Try an experiment. Begin with the door closed in the normal manner and turn one both the fan and your showers. Once the steam begins to build up open the door slightly to allow a bit more air to enter around the door. If that seems to solve the problem then the permanent solution would be to cut a bit more off the bottom of the door or install a transfer air duct through the wall to an adjacent room.

lazypup,

That's an interesting thought, and one I understand, but we do have an ~10"x34" "gap" at the top of the door. Cutting some off the bottom of the door is not an option and a x-fer air duct would be difficult at best. But your idea would certainly be easy enough to try. I'll report back after the attempt (tomorrow at the earliest).

"just doesn't *seem* to be adequate in real life"

that's because 0.0" static pressure doesn't exist in the real world. 0.2" is pretty hard to come by too especially with flexduct.

antss,

You are *so* right but the vent runs are all hardpipe now and even at 82cfm at 0.4" static, we're well within the supposed calculated need.

lazypup,

Despite my cynicism, cracking the glass door by about 1/2" seems to have worked. I had also upped the temp of the floor in the wetroom so there may be a combination effect (less steam/condensation produced hitting a cooler floor). We're going to keep playing with this idea to see if it bears fruit.

Also, I figured out that we have a natural air x-fer in the way our linen closet is set up, the back side has doors into the wetroom so we could try opening one side of the door to see what happens...but that's on the same side as the exhaust fan so it may be for naught.

Thanks for the idea on a lack of make-up air, that may be the whole problem!!

Are there remote dampers on each trunk , or does the fan pull from both rooms when turned on???

If no dampers, then 84 cfm is no where near enough for both rooms especially one with two shower heads. You might also check how the system is balanced, it could be drawing more from the pwdr. room than it should be!

antss,

Given the length of the 2 runs, our belief was that there was more pull from the powder room with the shorter run. The butterflies are located *at* the register covers so we taped half of the powder room butterfly closed to try to gain additional pull from the wetroom.

The two showerheads in the wetroom is exactly where my concern lies but I can't find any answers regarding venting needs with the second showerhead operating.

Cracking our frameless glass door seems to have alleviated the problem, as near as we can tell.