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!
Posted Wed, 01/16/2013 - 13:40
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