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Bathroom fan venting issue

15 years ago

I commented on someone else's thread a while back about layout of bath fan ventilation and what tradesperson handles that. I am finally taking my own advice -- I have photos that I'd like to share of what is going on in our attic.

As a bit of background, there are two bathrooms that back up on one another. Originally, only bath #1 (an interior bathroom) had a fan. This was not good because although bath #2 had a window for ventilation (and I'm sure this was code at the time of construction,) it really wasn't enough with the small jacuzzi tub in there. In the process of having some other HVAC work done, we had the contractor add a fan for bath #2. They are connected to the same original vent shaft so there is only one roof penetration.

The problem: the fan in bathroom #1 really isn't very effective, and I think it has to do with the convoluted bath of the vent. It looks like two sharp angles.

The question(s) are:

1) What should be done to correct this?; and

2) Who should we call to do this, or is this a DYI?

All suggestions are welcome.

Here is a link that might be useful: bathroom vent pix

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Comments (4)

User
15 years ago
In my opinion, it's a DIY project. I did three of my own bathroom fans, and one was in between levels of the house. The first thing I noticed is that you have missing insulation between these two fans. Another missing item is insulation that should wrap those ducts for the fans.

I don't know what your area is, but I've heard around here that it's not generally recommended to join two fans to one vent, but if it must be done that the joint should be close to the exhaust vent itself.

Yes, those bends are certainly going to be one source of problem. The vent on the left of your picture also looks like it still has an old 90Â° elbow attached to the new duct.

If properly done, you would have a roof vent for each fan. The ducts would be composed of rigid ducting. This is available at Home Depot and Lowe's. You should use foil tape to seal the connections. Lastly you would need to wrap the ducts with insulation (foil backed insulation rolls are easy to find). Don't forget to locate your missing insulation that should be laying between those fans!
pepperidge_farm
15 years ago
Just to add, that there are fans specifically designed to vent 2 spaces, rather than 2 fans into one vent. Panasonic makes one very good one, as well as others.
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A bathroom exhaust fan should produce 4 air exchanges per hour. Your bathroom is 7 x 7 which is 49sq.ft Assuming an 8' ceiling the total volume would be 49 x 8 = 392cu.ft We want four complete air exchanges per hour so that would be 392cu.ft x 4 = 1568cu.ft per hour. There are 60 minutes in an hour so we divide 1568 by 60min and it equals 26.13cfm. You currently have a 50cfm fan, which is twice the size you really require yet your having problems with the mirror fogging up. I suspect your problem is not the fan. Nature abhors a vacuum therefore a fan cannot push air out of a room unless there is provision for an equal amount of air to enter the room. You can test the theory by running the shower and leaving the bathroom door slightly ajar to allow more air to enter the room. Generally they make a slightly higer gap under the bottom of a bathroom door to allow more space for makeup air to enter the room but in some circumstances that does not provide enough makeup air. The solution then is to install a duct through the wall to an adjacent room and put a grille on either side. That will allow the additional air.
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Our very tiny master bath (40 square feet) doesn't have one and never did. There is a big viga running across the ceiling in this bath, so I'm not sure where a fan would go if I wanted one (I would consider one of those remote units like pal has, but we have a flat roof with no real space between roof and ceilings). The room has a window, though, which helps. There is a newer combo heat/fan/light unit in the main bath. Neither bath has ceiling light fixtures, so there is no competition.
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kelpmermaid
Original Author
15 years ago
Pepperidge farm (love those cookies!), I had looked at a Fantech system, but was persuaded at the time that it was overkill. Perhaps I should look again, but who would I call to install it?

Braytonak, I will check out the missing insulation -- good catch. I hate going up there because I know there are spiders which I loathe. About the duct insulation, though...does my location in coastal Socal matter? I suppose condensation could happen, though...
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15 years ago
Good question about the insulation. I would think that the amount of heat generated inside your attic *could* make it worthwhile. However, I've never lived in a warm climate, so I don't know what the benefit would actually be.

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