Bathroom exhaust fan: what's wrong with these pictures?

7 years ago

I had 80 cfm exhaust fans installed in 2 bathrooms (no more than 7x8x8 each). Although we run the fan while the shower is on, the mirror remains fogged up for another 10-15 minutes after shower is off. Based on what you see in the photos, what can we do to improve the functioning? Advice would be appreciated.

2nd floor bathroom · More Info

2nd floor bathroom · More Info

2nd floor bathroom · More Info

2nd floor bathroom · More Info

Featured Answer

kats737

7 years ago

Is there some gap under the door of the bathroom so fresh air can move through the space?

Also, those duct runs look really long and swirly. Elbows and long runs have an effect on how well your fan performs and if you get the CFM advertised. There is some kind of equation for it. I am not an HVAC engineer, but I believe your runs should be shorter and straighter. Worth looking into.

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Comments (8)

User
7 years ago
Ditto what kats says above. Short ducts are preferable. You also have the wrong type of duct installed.

This type of duct creates less air turbulence, and therefore makes your fan work better:

I'd try to re-install the fan in the first pic to eliminate the multiple bends and shorten the duct length. Make sure to avoid sharp bends.

The second fan's duct is way too long. I'd have it rerouted to exit the roof closer to the fan. If you live in a cold climate be sure to use a generous size of "Ice shield" underneath the roofing shingles to install the roof vent.

Be sure to use HVAC foil tape to seal all connections (NOT duct tape!).

Also, it's really important that, if you live in a cold climate area you have the fan housing and the whole length of duct insulated as well.
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From an energy standpoint, as well as a moisture standpoint, your husband is doing the wrong things. First, you are losing more heat with the window open than you would by simply running the fan; even if the door is closed, all the solid surfaces of the bathroom are losing heat out the window, and then will draw heat from the rest of the house to come back up to ambient temperature once the door is open again. Second, the cold air coming in the window condenses the warm, steamy water in the air and makes a mess, as you've observed; further, since the vapor drops out of the air as it's condensed, it can't be evacuated and makes the exhaust fan ineffective. Keeping the window open is fighting against everything you want to do. Tell your husband he's been outvoted and to close the window in the winter.
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Your answers have been very helpful and we are contacting the HVAC company to ensure they are aware they will be the ones installing the duct work for these along with his system.
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Suzi B
Original Author
7 years ago
Thank you!!
Suzi B
Original Author
7 years ago
Pippa, how should I describe the type of duct you posted when I ask for it at the store? Is it aluminum? And re HVAC foil tape, yes! On it now.
homepro01
7 years ago
I don't see why you need flexible duct. You should be able to use rigid duct with elbows.

Good luck!
Suzi B thanked homepro01
PRO
Cabot & Rowe
7 years ago
Solid piping is required in my fair city as of this year. Might wanna check with your local building authority.
MongoCT
7 years ago
Some of my info will repeat what has already been written.

In the first photo:
1. It looks like the duct is looped around to go behind the fan and then it goes up to the roof to exhaust. I would have rotated the installation of the fan housing itself to give a shorter path to the exhaust boot.
2) Rigid duct is better than flex duct. The wire coil in the flex offers more resistance to air movement through the duct resulting in your fan moving less air.

3) Insulate the duct for less chance of moisture vapor in the duct condensing and pooling in the duct, or running back into the fan housing.

4) It looks like light can be seen coming from the bathroom below. If so, the fan housing installation is poorly sealed to the bathroom below. Seal those gaps/holes, either with canned foam or aluminum tape, etc. Free air movement between conditioned living space and unconditioned space is bad for myriad reasons. See the dark/discolored/dirty fiberglass insulation? That discoloration is usually a result of the fiberglass insulation acting as an air filter. The discoloration is from the fiberglass filtering pollutants put of the air as air flows through the fiberglass.

The second fan:

Comments remain the same, with one addition. Since the duct is longer, when you replace it with rigid duct, have the duct leave the fan and run it vertical to a height a couple of inches above the height of the gable wall's exhaust boot. Then as it runs horizontal to the exhaust boot, pitch the duct downward a bit. That way if there is any condensation in the duct, it'll run out versus pooling in the duct.

Bathroom exhaust fan: what's wrong with these pictures?

kats737

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Bathroom exhaust fan: what's wrong with these pictures?

kats737

Comments (8)

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