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Questions about one inline exhaust fan serving two bathrooms

whitenack | Posted in General Questions on

Hi all,

My floorplan has two bathrooms very close to each other, and I am planning to use a single inline fan to cover both exhaust jobs. Here are the details…

One bathroom is 90 sq ft plus a walk in shower that measures 3.5 x 7.5. This bath is long and narrow,15 x 6′ (not counting the shower) with the entrance to the shower on one end of the long side and the toilet on the opposite end. The other bathroom is small, 40 sq ft. Total square footage is 156, including shower, and with 9′ ceilings means the total cubic feet is 1406. If my math is right, I need something in the neighborhood of 187 cfms to get the recommended air changes per minute. Both baths are near a gabled end of the house, so duct runs will be short. I will be doing the ductwork myself to ensure the ducts are as efficient as possible with as little friction as possible.

Here are the questions…

I was considering a WhisperLine fan, and it seems that the CFM options are 120 or 240. I assume the recommendation would be to step up to the larger fan to allow for a little pressure drop from the ducts, or would a 120cfm fan be close enough? It looks like it is a difference between 10 and 5 ACH.

I assume it is possible to have multiple on/off switches for the same fan? What about the timer delay? Can that be triggered in multiple locations, and the delay would reset if the timer is triggered from one location and then triggered again from the other location?

For the bath that is long and narrow, do I need two exhaust ports, one near the shower and one over the toilet? Adding another port means more ductwork and fittings, which adds to the total friction, and a more complicated design, but it might help balance the exhaust to more evenly match the square footage of the two bathrooms.

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  1. Expert Member

    I'll let someone with more experience with ductwork answer those questions.

    As to the controls: Yes you can have multiple switches for the fan. They will be wired just as you would three way switches used for lighting. And yes you can run two timers with delays that will work together. They will also be powered by the same hot from a common circuit as the switches.

  2. GBA Editor
    Martin Holladay | | #2

    There are different philosophies when it comes to bathroom exhaust fans. Some people insist on a powerful fan that removes moisture quickly; other people are satisfied with a smaller fan that takes a little more time but uses less energy.

    Before you can design your system, you need to know whether this fan will be part of a whole-house exhaust-only ventilation system, or whether the fans will only be used for spot removal of odors and humidity. Do you plan to include any other ventilation fans in your house?

    In most cases, I recommend separate fans for each bathroom. It seems inefficient to me to be pulling air from a bathroom when no one is using the bathroom.

    I believe that fans should never be oversized, so I'm inclined to choose a fan with a smaller cfm rating rather than a bigger cfm rating. Operating an exhaust fan always incurs an energy penalty.

    With ductwork, the rules are: it's better to oversize the duct diameter than undersize the duct diameter; use as few elbows as possible; and keep duct runs straight and short.

    For more information, see:

    Bathroom Exhaust Fans

    Does a Home with an HRV Also Need Bath Fans?

  3. charlie_sullivan | | #3

    I agree with Martin's overall advice. On the specific sizing question, one of the nice features of some of panasonic's whispergreen series of fans is that you can set the speed after installing it, so you can dial in the performance you want.

  4. nvman | | #4

    You may also find that a 240 com fan may be noisy even though it is advertise as quiet

  5. whitenack | | #5

    Thanks for the replies, it gives me things to think about.

    I was thinking I could save money and have a neater setup by having one fan and one hole in the exterior. But I understand that that comes at running a 120 cfm fan when only 50cfms are needed. Another advantage to having multiple fans is I could have a fan/light combo and not have to have another hole in the ceiling.

    Also, the smaller bathroom has no outside walls, and the heat/cool load is almost nothing, and probably won't need much, if any, supply vents. A single fan running all the time on low would help bring a little heat and cool into that room and keep the air from getting stale.

    In terms of whole house ventilation, I am still trying to figure that out. A lot of it depends on the blower door test. We are doing a lot of air sealing, but have a lot of double hung windows that I'm not sure how airtight they will be. We live in zone 4A, with very humid summers, so HRVs wouldn't be a good choice for ventilation, and ERVs wouldn't be good for bath humidity removal. Regardless, balanced ventilation may not be in the budget, unless I could go with one of those spot ERVs from Panasonic. I had been leaning toward an exhaust-only strategy, but I worry about drawing in fumes from our attached garage. The garage will be unfinished and no ceiling, with lots of passive venting to the outside, but it is still a concern (maybe unnecessarily?).

    I have another bathroom downstairs that will need a a fan. Maybe a fan downstairs running at 30cfm and a fan in the small bathroom running at 30cfm will get the job done. I can boost and set delays using the built-in controls, and can even turn it all off during the peak load times. The larger bath can have just a regular fan that runs only when called upon.

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