Bathroom exhaust

[KSMike]

I’m not sure whether this violates the rules so I’ll take a chance and understand if it gets deleted.

Is there a tool, table, or equation that can be used to determine how long a bathroom exhaust duct can be before steam inside the duct condenses back into liquid?

I’m in the planning stages for a basement bathroom addition but the nearest exit point for an exhaust fan is a good 35’ away with 3 90* turns. Plumbing was stubbed in by the builder so the bathroom location is already fixed.

My thought was to use an in-line fan such as a Fantech FR-150 mounted near the exit (in a pull configuration) using 4” or 6” plastic pipe but now I’m wondering whether *anything* would prevent condensation over such a long run. The only floor drain is in the complete opposite direction.

Thank you, again this is just planning.

[BaldLoonie]

Instead of uninsulated metal pipe or uninsulated flexible tubing, get a good R8 flex duct. I don't think you'll have condensation. Of course a cheapo fan may not be able to overcome 35' so an inline fan like you mentioned may be needed just to get good venting.

A control with a timer that you set for about 10 minutes longer than your shower might be smart to dry out the run.

[Poodle Head Mikey]

"steam" is essentially 100% RH - so some condensation would seem inevitable.

If I were faced with this challenge I would use PVC for the air duct (which is somewhat insulative itself) and then insulate the outside of the PVC pipe with 1/2" wall Rubatex.

Then I would arrange for the power to the exhaust fan through a dehumidification wall stat - set to about 60%. And I would include a 30 minute off-delay in the control of the exhaust fan.

So find a contractor who thinks like me and you should be all set! <g>

PHM
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I’m not sure whether this violates the rules so I’ll take a chance and understand if it gets deleted.

Is there a tool, table, or equation that can be used to determine how long a bathroom exhaust duct can be before steam inside the duct condenses back into liquid?

I’m in the planning stages for a basement bathroom addition but the nearest exit point for an exhaust fan is a good 35’ away with 3 90* turns. Plumbing was stubbed in by the builder so the bathroom location is already fixed.

My thought was to use an in-line fan such as a Fantech FR-150 mounted near the exit (in a pull configuration) using 4” or 6” plastic pipe but now I’m wondering whether *anything* would prevent condensation over such a long run. The only floor drain is in the complete opposite direction.

Thank you, again this is just planning.

[Juan Madera]

SDR35 pipe with long radius ells.. Glued fittings. Do the calcs for round metal ductwork and size the diameter and external static of the fan to suit.

As long as you are moving/providing enough air to dilute the air stream, it should be fine. Just like a diverter hood does on old gas furnaces..

[kdean1]

Steam? You have water in the vent at 212°? There's your problem!

[pecmsg]

I’m not sure whether this violates the rules so I’ll take a chance and understand if it gets deleted.

Is there a tool, table, or equation that can be used to determine how long a bathroom exhaust duct can be before steam inside the duct condenses back into liquid?

I’m in the planning stages for a basement bathroom addition but the nearest exit point for an exhaust fan is a good 35’ away with 3 90* turns. Plumbing was stubbed in by the builder so the bathroom location is already fixed.

My thought was to use an in-line fan such as a Fantech FR-150 mounted near the exit (in a pull configuration) using 4” or 6” plastic pipe but now I’m wondering whether *anything* would prevent condensation over such a long run. The only floor drain is in the complete opposite direction.

Thank you, again this is just planning.

If you don’t run the fan long enough YES you will have condensation forming.

[KSMike]

Steam? You have water in the vent at 212°? There's your problem!

LOL well you haven’t felt how my wife adjusts the water...

These ideas are great, including running the fan after shower use! But still, you guys don’t think I’ll have a problem with water sitting in the duct, getting moldy, etc.? Thank you so much.

[kdean1]

LOL well you haven’t felt how my wife adjusts the water...

These ideas are great, including running the fan after shower use! But still, you guys don’t think I’ll have a problem with water sitting in the duct, getting moldy, etc.? Thank you so much.

I have monitored the humidity of our bathroom. It takes 15-20 minutes to reduce RH from 75% back to 50%.
I installed a timer for the fan. My wife doesn't like the fan running while she's wet so we can set the timer after drying off. Then we don't have to remember to turn it off later.

[micdundee]

I always ran the insulated vent (pipe or flex) uphill and tight, so that any condensation that may occur would run back to the fan and not collect in a pool waiting to rust out and create water damage. Seemed logical to me.

[KSMike]

I have monitored the humidity of our bathroom. It takes 15-20 minutes to reduce RH from 75% back to 50%.

Thats a great help. I’d planned on using a timer but it helps to have a starting point on the time.

I’ve given thought to uphill vs downhill but the fact is I’m going to have to elbow under & around floor joists so I’m not sure it’ll make much difference.