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Old 01-03-2020, 11:01 AM   #1
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Shared duct for 2 bathroom fans ?


Happy New Year to all. I'm new here and looking for some great advice
I 'm updating 2 side-by-side 5'x9' bathrooms and they don't have fans. The tub and shower are either side of the shared 5' wide wall. I don't like ducting fans through the roof - Canada winters can be so cold - so I put in a dropped ceiling each side and made this wall deeper to allow ducting to go down to a laundry room below which also has a dropped ceiling.

Originally I wanted an inline fan there, removing air from both rooms but with dampers & control over which room is being cleared.


I'm taking a second look at this plan (mainly because of cost) and wonder if 2 Panasonic fans will be a better option. It's the "how to exit the air" part that I need help with please.

These fans are 4" duct, 80-100 CFM with LED lights. I'd connect individual 4" ducting in the ceiling and down the wall for each fan - then use a Y and 6" run to exit the house. Each run involves 2 x 90 elbows. Will this arrangement minimize air being pushed into the fan that's not running? They have back-draft dampers. There's also that mysterious factor called static pressure that's lurking around eh?....Looking forward to expert comments.
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Old 01-03-2020, 12:52 PM   #2
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Re: Shared duct for 2 bathroom fans ?


You will be slowing the air in the 6" duct and may end up with ice around the exit flapper.

I would run them both all the way out.
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Old 01-03-2020, 06:47 PM   #3
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Re: Shared duct for 2 bathroom fans ?


Thanks for the suggestion. I was hoping to enlist the help of someone in the HVAC business because I'm sure using dampers in the right places is an important factor.
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Old 01-06-2020, 09:51 AM   #4
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Re: Shared duct for 2 bathroom fans ?


Do you also have the vent in the bathroom? How you planning on getting rid of humidity ?
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Old 01-06-2020, 10:08 AM   #5
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Re: Shared duct for 2 bathroom fans ?


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Originally Posted by EMRYS View Post
Thanks for the suggestion. I was hoping to enlist the help of someone in the HVAC business because I'm sure using dampers in the right places is an important factor.
You could wire then together so they both run at the same time from either switch.
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Old 01-06-2020, 11:06 AM   #6
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Re: Shared duct for 2 bathroom fans ?


Hi Coolair - Sorry if my 1st posting isn't clear to you. I AM adding bathroom venting in both rooms but it doesn't go up through the roof but down 1 level then out. The original plan is using one INLINE fan (installed below the bathrooms) which draws air from grilles in both rooms. Their individual ducts connect to a Y before the inline fan. Dampers would control which room is being vented. The only quote I got was over $2000 !! and I know the fan only costs $440 tops.

My back-up plan is installing 2 regular fans with 4" ducting. These ducts take the air down to a Y fitting. My concern/question is what size duct to use after the Y ? Both fans have back-draft dampers. Does changing to a 6" duct after the Y help bathroom A's fan work efficiently and reduce the pressure on fan B's damper - and vise-versa ?
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Old 01-07-2020, 09:47 AM   #7
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Re: Shared duct for 2 bathroom fans ?


the duct size should be consistent In my opinion. If you change the size of the ducts it will work vice versa as you are expecting.
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Old 01-07-2020, 11:14 AM   #8
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Re: Shared duct for 2 bathroom fans ?


Reducing the number of penetrations in the building envelope has advantages, so venting two bathroom fans out the same vent isn’t a bad idea, but there will be some issues.

(If the bathrooms are on an external wall without a vented soffit above the best solution might be two separate vents out the wall, each with less than one foot of straight ducting.)

You’ll need to review the fan manufacturer’s guidance on maximum duct length to see if your duct routing plan is feasible. Elbows add a lot of equivalent length. Having the fan inlets near the bathroom ceiling with the duct exit one floor down seems like a lot of ducting.

If 4” duct was adequate for one fan for the length, you could run 4” after the ducts are combined. That would be fine if only one fan were running, but will be insufficient if both fans are running.

The flow area of 6” duct is a bit more than twice that of 4” duct, so you could run 6” after the two 4” ducts are combined. That would maintain the same airflow rate though the 4” and 6” when both fans are running, but the flowrate in the 6” would be reduced when only one fan was in operation. If these were fans moving debris, like a dust collection system, that would be a concern because the reduced airflow would allow particles to drop out and accumulate in the duct. That isn’t the case here, so I don’t see a problem with that. The exit damper on the non-operating fan will prevent most airflow from the operating fan from backflowing. They don’t seal perfectly, though. If there is, well, some unpleasant odor being cleared from one bathroom, it might be noticeable in the bathroom with the non-operating fan. If it’s possible to do, Neal’s idea of wiring the two fans to run concurrently would solve that problem. It would require a three-way timer switch, which I’ve never looked for. You’d be venting one bathroom sometimes when it’s not required, but bathroom fans typically don’t run that much, so you wouldn’t be wasting much energy pushing heated air out of the house.

I’d suggest against putting additional dampers in to control airflow. That will be extra work/cost/maintenance without much benefit.

If you want to take this to the next level, do without bathroom fans altogether (check your local building code). Instead use an HRV to pull air from the bathrooms. That stale air is run across a heat exchanger so that cold fresh air being brought in from outside is warmed up. If you’re doing a lot of other work in your house to seal it up and stop air infiltration, adding mechanical ventilation is a good idea. Based on personal experience I can tell you that retrofitting the ducting for an HRV to an existing house can be a challenge.

Chris
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Old 01-07-2020, 12:16 PM   #9
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Re: Shared duct for 2 bathroom fans ?


Chris - Thanks for the detailed reply. I'm with you about using HRV and if we were staying here more than a year or so I'd do it. As it is, the downward-routed ducting could be adjusted to the basement level by a future home owner.
The FanTech kit, using 2 grilles and an inline fan - often used in big bathrooms - was my first call but a local ventilation contractor wants $2000 - way beyond my budget, so I'm adapting that idea with 2 good, independent fans. It was just the final duct sizing that puzzled me !
Thanks again
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Old 01-07-2020, 12:29 PM   #10
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Re: Shared duct for 2 bathroom fans ?


Follow up question... If the last 6-7 ft after the Y is 5" ducting is this a good compromise to the fan efficiency?
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Old 01-07-2020, 12:48 PM   #11
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Re: Shared duct for 2 bathroom fans ?


@Chris616


The switches actually are two single pole switches , so no one can turn off your fan while you are in the shower.
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Old 01-07-2020, 01:59 PM   #12
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Re: Shared duct for 2 bathroom fans ?


Quote:
Originally Posted by EMRYS View Post
Follow up question... If the last 6-7 ft after the Y is 5" ducting is this a good compromise to the fan efficiency?
Diolch
Not sure what you mean by “good compromise to the fan efficiency”. If you tell me the duct lengths and number of fittings I’ll explain how you calculate what the airflow will be.

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Old 01-07-2020, 02:36 PM   #13
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Re: Shared duct for 2 bathroom fans ?


You mentioned 6" ducting has +/- effects. That's why I mentioned 5".


The run is as follows - 1' from each fan housing to the wall'; a 90* to go down about 6.5' to the Y fitting which is fitted into a hole in the floor. Another 90* or even a blend of 2 for a smoother turn to direct the air to the outside, about 5' away. All ducting will be solid metal and I'll use foil tape all over the place.
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Old 01-07-2020, 10:17 PM   #14
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Re: Shared duct for 2 bathroom fans ?


There’s no downside to running 6” duct in this situation, except for the space that it takes up. As I wrote earlier reduced air velocity isn’t really a concern since a bathroom fan is just moving air.

There are much more complicated ways to size ducts, and I’ve made some approximations to make this short, but this will work for a bathroom fan. We’ll see what airflow the fans can push when they’re both running, which will be the worst case scenario for pressure drop.

We’ll start off assuming that 4” duct is a good size for a single fan. When the two ducts combine, pushing twice as much airflow through a 5” duct will make the pressure drop per foot a bit higher than through the 4” duct. Using 6” duct will make it a bit less, so we’ll use that. Since the 6” duct with two fans running has less pressure drop per foot than the 4”, we can conservatively estimate the pressure loss for each fan as if it was running a dedicated 4” duct from fan to house vent. I’m doing this because the fan spec sheet assumes a constant duct size.

To calculate the total length from fan to house vent we add up all the straight lengths, plus “equivalent lengths” for each of the fittings. Depending on how it’s made, a 90 degree elbow can have an equivalent length of from 15’ to 35’, so let’s use 25’. I couldn’t find the equivalent length for the Y, so I’m going to estimate it as 10’. So the total equivalent length from fan to house vent will be the sum of:
- The straight runs you measured (1 + 6.5 + 5) = 12.5’
- Two elbows (2 X 25’) = 50’
- The Y = 10’
For a total of 72.5’. Let’s say 80’ to account for the flapper on the outlet vent (and because that matches one of the lines on the fan spec sheet). You can see what a huge effect elbows have on the pressure drop in a duct and why it’s so important to get as straight a run as possible.

I went to the Panasonic website and found a bathroom fan that has a selectable 50-80-100 CFM flowrate. Model FV-0510VSL1. The diagram below is from the spec sheet. I’ve coloured the lines to make it easier to see. The red line is the pressure drop for 80 feet of 4” duct. The fan has three selectable speeds. To find how much air the fan will push at each speed, look at the intersection of the fan speed line and red duct line:
50 CFM speed line – about 50 CFM
80 CFM speed line – about 78 CFM
100 CFM speed line – about 85 CFM

Chris
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Old 01-08-2020, 01:40 PM   #15
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Re: Shared duct for 2 bathroom fans ?


Great information - and I've taken it onboard. Both fans are still over the tub & shower respectively but now relocated closer to the outside wall. This lets me shorten the connection from each fan to the downward ducting and after they merge it's barely 1' to the outside. Hope to be testing this setup once sparkie has been round to connect them. As a bonus - the fans are closer to the toilets !
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