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New bath fan - condensation drip

Hi, I'm a long time troller but a first time poster.

We are in CZ 3 - San Francisco Bay Area. Mild year-round, dry summers, wet winters.

As pat of a larger renovation, we installed a Panasonic WhisperGreen fan/light combo in the bathroom. The homeowner later complained that when they take long showers, or sequential showers, they get a slow drip out of the bath fan. A single 5-10 minute shower poses no problem. I have not observed the actual dripping, but did see the drip stain.

When I arrived to check out the problem, I found the installer had installed the fan duct with a minor dip and a sharp flexible turn in the duct. The dip was due to a cross brace being installed atop the joists - the duct had to dip below the brace.

We re-worked the duct to take a 90 degree turn straight off the fan and run with constant slop to the roof exhaust termination. The duct is insulated to R-8.

I understand that a 90 straight off the fan is not ideal, but the 16" OC, 2x8 joists do not allow us to orient the fan exhaust port North-South. We can't go West because the rafters contact the ceiling joists. Our only option appears to be East, either dipping the duct under the cross brace and then turning, or doing the 90 straight off the fan. The 90 seems better than the dip to me.

When the problem surfaced, we had not yet re-insulated the attic, and I was hoping that the moisture was due to the hot, moist air hitting the cold metal of the fan exposed to attic conditions. However, the fan is now buried in cellulose and the problem persists.

Here's the airflow data as measured with an Alnor balometer:

Rated: 80 CFM
Actual output with no duct attached: 66 CFM
Actual output with original duct configuration: 46 CFM
Actual output with modified duct configuration: 56 CFM

Panasonic recommends up-sizing to a 130 CFM fan. I'm dubious of this solution, as I have always heard 50 CFM is a good target for a bath fan.

Thanks in advance for your insight!

Asked by John Kidda
Posted Jan 16, 2013 2:40 PM ET


4 Answers

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The condensation is inside the duct, not so much inside the fan. The up-and-out orientation of the duct means that any condensation forming inside it will run back down to the fan. Better to go minimally up and then laterally out, but maybe that couldn't happen. Anyway, they need to run the fan for longer after showers. Install a 10-20-30-60 minute Leviton pushbutton timer and tell them to try running it for 60 minutes after the shower. This should help dry the inside of the duct. If they find they can get away with less, OK.

Putting a lot more insulation on the duct is also a good move. Get some R-30 or 38 batts and wrap the duct completely, then wrap that in foil scrim and tape it with foil tape. R-8 is nothing.

Answered by David Meiland
Posted Jan 16, 2013 2:58 PM ET


David, thanks for your help.

We installed the fan with an AirCycler SmartExhaust. We are currently running a 10 minute delay after the fan switches off, and will run an automatic cycle based on the final airtightness level (air sealing and insulation complete, but windows still to be installed).

If I'm understanding them correctly, the drip is starting WHILE THE FAN IS STILL RUNNING.

If I ran the duct laterally for 8 feet, and then brought it up, do you think that would do it? My biggest concern is reducing flow.

Answered by John Kidda
Posted Jan 16, 2013 3:12 PM ET


If you run it laterally and then up, you will have a pond somewhere in the lateral portion. Best case is to rise slightly at the fan outlet--enough to get above the joists--then slope slightly downward all the way out to a sidewall exit. Sometimes you have to run more pipe than you want to, and/or tell the owner that they are going to have a mechanical wart on the front of their house. Since these folks undoubtedly see the physics of it now, maybe they can be persuaded.

I would still try added insulation first--an hour of grunt work and <$50 worth of stuff and you might solve it.

I'm surprised at your flow measurements. I have checked out an 80 CFM WhisperGreen on the bench with no duct, and got 77 CFM. My test setup is (ahem) not an Alnor, but I think it is pretty close. I'm not sure how far you would have to run to get a sidewall exit, but 50 CFM is usually considered enough for a small/medium bath.

Answered by David Meiland
Posted Jan 16, 2013 4:12 PM ET
Edited Jan 16, 2013 4:13 PM ET.


I'm trying to imagine why the duct is so cold. Is it possible that the entire house is depressurized -- perhaps due to an unbalanced forced-air system? If the whole house is under negative pressure, cold outdoor air might be entering the house continuously through the bath exhaust duct, cooling the duct.

Answered by Martin Holladay
Posted Jan 16, 2013 4:25 PM ET

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