2 bath fans using 1 4-inch duct?

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sw_ross

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I have never done this and would not choose to do this on my own.
The GC on a basement remodel is asking me to use the existing duct for one existing fan for an additional bath fan. They are adding an additional bathroom.

The reason he wants to do this is the challenging logistics of getting the new bath vented to the outside.
Has anyone done this before? If so, what method do you use to keep the new bath fan to feed into the existing bathroom? Some sort of damper setup?
 
This is friom Quora. I have no idea how authoritative it is .....

You can use two bathroom fans feeding into one common exhaust line so long as all of the following are true:
  1. Each fan housing must have its own damper.
  2. The common exhaust vent line must be at least 1 size in diameter larger than the two individual exhaust lines feeding it. That is.. if your individual exhaust lines from each fan are 3″ in diameter, the common line where they get tied together with a wye needs to be 4″ at minimum to allow for both fans running at the same time. 6″ would be better.
  3. The fans need to be of similar capacity. That is you shouldn't pair a 200 CFM fan on a line with an 80 CFM fan.
  4. The common exhaust duct needs to have no “kinks" where the turn radius is less than 2x the diameter of the duct. So a 4” duct minimum turn radius is 8.”
  5. The number of turns in the common exhaust duct, even at the minimum radius, should be limited to 4. Zero turns is the ideal.
There is another way to attack this issue which we use on commercial buildings all the time, and that is to mount the fan after the point where the two bathroom exhausts tie together so that the fan pulls from both bathrooms when it's on.
 
oldsparky52 said:
It would still be important to not mix fan capacities.
Click to expand...
what difference would it make? The key is that you don't create back pressure from one fan that the other fan cannot blow into.

I think as a rule of thumb the quora citation might work but I think kicking it up from 3" to 4" is sketchy. 3" is about 7 square inches of ID. 4" is about 12 square inches. Thus if the calculations showed you needed 3" duct for one fan, running two of them into a 4" line might be undersizing the duct.
 
IRC M1507.4 states you must provide 50 CFM of ventilation intermittently or 20 CFM continuously. If you can achieve this by whatever methods you employ you are within code.
 
sw_ross said:
I have never done this and would not choose to do this on my own.
The GC on a basement remodel is asking me to use the existing duct for one existing fan for an additional bath fan. They are adding an additional bathroom.

The reason he wants to do this is the challenging logistics of getting the new bath vented to the outside.
Has anyone done this before? If so, what method do you use to keep the new bath fan to feed into the existing bathroom? Some sort of damper setup?
Click to expand...
Do you think you could use a centralized remote fan (FanTech) to take care of the two rooms?
 
petersonra said:
Yeah as long as the same rating also means the same back pressure. Just because a fan is rated at a certain CFM has another fan does not mean that they will be able to run against the same back pressure.
Click to expand...
Most of us construction electricians are not capable of calculating this. We do know it's not a very good idea, and from my fuel distribution experience, I know that crazy things can happen when you use 2 pumps into one distribution pipe.
 
sw_ross said:
I have never done this and would not choose to do this on my own.
The GC on a basement remodel is asking me to use the existing duct for one existing fan for an additional bath fan. They are adding an additional bathroom.

The reason he wants to do this is the challenging logistics of getting the new bath vented to the outside.
Has anyone done this before? If so, what method do you use to keep the new bath fan to feed into the existing bathroom? Some sort of damper setup?
Click to expand...
First off it's not the electricians responsibility to exhaust the fan. Especially when a GC is involved. Have him hire a tin knocker or do it himself.
There's no way I'd approve of this if it were two separate dwellings.
I've seen it done improperly all the time I don't know what the outcome was later on. I guess as long as the dampers are in place you should help prevent "blow back" from the other fan.
I'd try to use a remote online fan if possible.
 
Knuckle Dragger said:
First off it's not the electricians responsibility to exhaust the fan. Especially when a GC is involved. Have him hire a tin knocker or do it himself.
There's no way I'd approve of this if it were two separate dwellings.
I've seen it done improperly all the time I don't know what the outcome was later on. I guess as long as the dampers are in place you should help prevent "blow back" from the other fan.
Click to expand...
I agree with this.
I'm getting ready to start a new rough-in, and the tinners have been there already. They Y'd off of a single 4" to exhaust a Master bath and half bath. Then attached a reducer to 3" at the end of each hose.

So sorry for them. My fans use a 4" duct.
And it was really stupid on them because it was just T1-11 siding, so easy to cut a second one.

Typically, if the duct is there I will attach it to the fan, but this one I'm gonna tell the builder I can't because it's no bueno
 
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sw_ross said:
I have never done this and would not choose to do this on my own.
The GC on a basement remodel is asking me to use the existing duct for one existing fan for an additional bath fan. They are adding an additional bathroom.

The reason he wants to do this is the challenging logistics of getting the new bath vented to the outside.
Has anyone done this before? If so, what method do you use to keep the new bath fan to feed into the existing bathroom? Some sort of damper setup?
Click to expand...
Panasonic i believe, has a pretty exhaustive (no pun intended) chart on calculating duct runs, static pressure, sizing etc
 
Here's a chart I found.
I suppose if you wanted to combine two ducts into one, you would just add the CFMs together and make sure the single pipe is large enough.

However, one thing that gets lost in many a conversation is noise level. The manufacturer recommends or dictates a minimum duct size, and it does not only correlate to CFMs.

Any time you restrict the airflow of any type of fan or blower, it increases motor noise AND air turbulence (more noise).

There's an old adage that bigger is not necessarily better. But in ductwork, bigger is almost always better
 

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sw_ross said:
I have never done this and would not choose to do this on my own.
The GC on a basement remodel is asking me to use the existing duct for one existing fan for an additional bath fan. They are adding an additional bathroom.

The reason he wants to do this is the challenging logistics of getting the new bath vented to the outside.
Has anyone done this before? If so, what method do you use to keep the new bath fan to feed into the existing bathroom? Some sort of damper setup?
Click to expand...
Tell your contractor that your not an HVAC Tech and for him to have a professional perform the vent work. I personally would not even attempt that as it sounds like a problem "you" will be married too.
 
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