Venting Multiple Fans

[Dmwesq]

I have 2 Panasonic fans with 4 inch ducts, which re both venting through flexible venting into a non-flexible 3 inch duct pipe out of the side of the house. In addition, one of the two fans is somehow missing the plastic damper, but to replace that I would need the entire damper assembly, because according to Panasonic the damper is not sold alone. I know I need to make some changes to this arrangement, and have read some of the various threads on this site and others.

I have been told that if I want to continue to vent both fans out the same vent I need a 6" vent pipe. If I run the vents separate, I would need two 4" vent pipes.

If I go with either of these arrangements, how critical is it to also replace the damper assembly on the one fan missing the damper, or can I take care of that by installing a damper on the vent pipe itself?

I also read that the flexible venting is no good, but I have no kinks in the attic where the flex vent runs, and I don't see how else to bring the vent around the joists where the fans are installed. I would not run the flex to the outside, it is only run until it meets the solid vent pipe that runs to the outside.

And finally, since I have to make some corrections anyway, am I better off running to the roof? The run to the side of the house is fairly short, and I think with the right size venting the side of the house should continue to be OK.

 

[Jadnashua]

The corrugations in the flexible ducting create a lot of turbulence, and slow the air flow down. That could be enough to cause some water vapor to condense, and standing water can lead to mold growth...IOW, it should be avoided (same is true on a clothes dryer...it also slows down the rate at which the clothes dry, and ends up costing more in energy use!). When the duct is too small, it also creates backpressure, makes the fan work harder, costs more to run, and slows down the air flow, which means it may need to run longer to do the same job and also cost more. Most manufacturers do not recommend combining multiple fans into one duct. The dampers are by no means perfect, and the air will try to go in the path of least resistance (and with corrugated stuff, that might very well be into one of the other fans!).

The shorter the run and the fewer changes of direction will affect the efficiency of the fan, and the size duct required. A penetration through the roof can work, but may be problematic in the winter if there is a snow cover unless you elevate the outlet.

Fantec makes a remote fan motor designed to be able to duct multiple rooms, and may be a better solution to your problem.

 

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[Dmwesq]

Fantec makes a remote fan motor designed to be able to duct multiple rooms, and may be a better solution to your problem.

Could you explain this a little? Are you suggesting replacing the 2 existing fans? They are new top of the line Panasonic fans and I would hate to discard them. Or is this something that would work in conjunction with my existing fans?

 

[Jadnashua]

The dampers in a typical fan are not perfect. Running one fan can mean some of the exhaust ends up in the other room along with all of its smells and humidity. Fluids (air works like a fluid) take the path of least resistance. Using one fan with multiple inputs especially when designed specifically for that purpose, can be more efficient. I like the Panasonic fans, I would not try to vent them together unless they were designed for it.