How to ventilate a room with no windows with one inline fan?

  • #1

Main Question or Discussion Point

Hello! I am building a recording studio which will be air tight and will need fresh air intake and stale air exhaust. I have a general question regarding which is better, using one or two inline fans for the job? I was told that that you could have one fan on the exhaust duct which can pull out stale air and automatically new air would be sucked into the room from the opposite end through the supply duct. Is this true?
 

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  • #2
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I was told that that you could have one fan on the exhaust duct which can pull out stale air and automatically new air would be sucked into the room from the opposite end through the supply duct. Is this true?
Yes and no. In a house, you can use an exhaust and depend on air leaks to let new air in. If your room has few leaks, that won't work. You used the word "airtight". Strictly speaking that implies no leaks and no fresh air.
 
  • #3
russ_watters
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Yes and no. In a house, you can use an exhaust and depend on air leaks to let new air in. If your room has few leaks, that won't work. You used the word "airtight". Strictly speaking that implies no leaks and no fresh air.
...but two ducts!
 
  • #4
Yes and no. In a house, you can use an exhaust and depend on air leaks to let new air in. If your room has few leaks, that won't work. You used the word "airtight". Strictly speaking that implies no leaks and no fresh air.
There would be two holes in this airtight soundproof room. One hole on one side of the room with an exhaust duct and fan attached onto the outside of it and another hole on another side of the room with only duct attached to the outside of it. Would this work?

Here is an aerial view plan of my studio. In the south you can see 4 brown boxes. These are silencer boxes which are kind of like baffles for the sound not enter or escape the studio while air can move through from inside to outside etc. My plan is only to have a fan on the exhaust and I was told that this will suck air from the other side of the room into the room to fill the void.
 

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  • #5
...but two ducts!
yes exactly, there will be a hole on either side of the room with ducts attached to silencers/baffles to not let the sound inside.
 
  • #6
here is a side view of the room where I plan to have the exhaust duct. The other side is identical but just without a fan. I am not sure if I should put a fan on the other side. On an acoustics forum they said this may cause "aerodynamic stalling" if you use two fans and that it is best to just use one fan else you have to callibrate the system so it works properly.
 

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  • #7
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Then yes, with two holes you can make do with one fan.

With exhaust fan only, the room will run slightly negative pressure. With inlet fan only, the room will run slightly positive pressure. Positive pressure will make a very slight improvement in keeping the room dust free. It also makes a difference which way the door should be hinged.

Either way you need a fine screen or a filter on the end where air comes in to keep bugs and dust out.
 
  • #8
Then yes, with two holes you can make do with one fan.

With exhaust fan only, the room will run slightly negative pressure. With inlet fan only, the room will run slightly positive pressure. Positive pressure will make a very slight improvement in keeping the room dust free. It also makes a difference which way the door should be hinged.

Either way you need a fine screen or a filter on the end where air comes in to keep bugs and dust out.
Ok great! And does it matter that the path of air has to squiggle through all the baffle boxes. I am sure they create static pressure but from a physics perspective, can the fresh air intake suffer as a result of the static pressure caused by the silencer/baffle boxes? Here is a picture of one of them.
 

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  • #9
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The more bends, the more friction. The more valuable friction the less flow. In other words a stronger fan.


To choose the right size fan, you need an expert such as @russ_watters
 
  • #10
Baluncore
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Moisture and warm air will rise to the ceiling, so you should remove air from there to prevent condensation and micro-organism growth.

If the room is heated or cooled you might consider a counterflow heat exchanger to reduce running costs.
 
  • #11
The more bends, the more friction. The more valuable friction the less flow. In other words a stronger fan.


To choose the right size fan, you need an expert such as @russ_watters
Moisture and warm air will rise to the ceiling, so you should remove air from there to prevent condensation and micro-organism growth.

If the room is heated or cooled you might consider a counterflow heat exchanger to reduce running costs.
Thanks, I was planning on having the return register at around 75% of the room length as in the diagram. Do you think 1 exhaust vent is sufficient for a 20 X 13ft room?

The other option is to use something like this (see pic). This way I could put two registers in the room at different location and would have the added extra of increasing my cross sectional duct area to make the airflow even more quiet.
 

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  • #12
CWatters
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Have you considered a single room mvhr unit?
 
  • #13
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Plan B would be one wall hole with a co-axial fan + bypass duct. We have such in our wash-room. Incoming air is directed sideways...
 
  • #14
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The rest of the design stuff is mostly in the acoustic part of your design. As Anorlunda pointed out the more bends the more friction. This friction will create noise (a very small amount but observable at some level) however a larger baffle arrangement will allow more external noise. You will need to find the balance that meets your needs. For music recording the Db floor level can be adjusted via a gate for instance but too much gating causes loss of some of the signal.

You would be well advised in a new project to build in some type of box structure on either side in case you need to change the baffles. This way a duct size or path can be changed without reframing the entire wall. Also make sure to not mount the fan to your room or structure. Support it separately and use a minimally sound conductive coupling. It can be amazing how much noise just "comes through the walls."
 
  • #15
The rest of the design stuff is mostly in the acoustic part of your design. As Anorlunda pointed out the more bends the more friction. This friction will create noise (a very small amount but observable at some level) however a larger baffle arrangement will allow more external noise. You will need to find the balance that meets your needs. For music recording the Db floor level can be adjusted via a gate for instance but too much gating causes loss of some of the signal.

You would be well advised in a new project to build in some type of box structure on either side in case you need to change the baffles. This way a duct size or path can be changed without reframing the entire wall. Also make sure to not mount the fan to your room or structure. Support it separately and use a minimally sound conductive coupling. It can be amazing how much noise just "comes through the walls."
On an acoustic forum where I have done most my reading, the advice is to let the duct run straight before reaching the register/grille. So there can be bends in the duct work but not too close to the opening. Although I must say that I am a bit confused by this because I only have two options regarding this (see image). I am not sure which of these will work because as you can see there is a bend in the top one but since it is a curve, will this be acceptable? I would ideally like to do something like the bottom one but again I am not sure if this is even possible or which of these two will be quieter.
 

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  • #16
Have you considered a single room mvhr unit?
Not really, I just need humidification and maybe some cooling in summer but because I am in the basement but it really doesn't get too hot down there. I was going to use a mini split system which will be completely independent to the ventilation. My plan is to have the supply register close to the indoor unit so it can dehumidify the fresh incoming air.
 
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  • #17
Plan B would be one wall hole with a co-axial fan + bypass duct. We have such in our wash-room. Incoming air is directed sideways...
Thanks, it is good to have a plan B :)
Do you think my plan is OK though? Can you see if it has any flaws in it?
 
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  • #18
I might be able to help you a bit as part of my job as a civil engineer is designing sound walls to prevent traffic noises from disturbing residential neighborhoods.

First thing to know is that you get a 5dB reduction in observable sound just by breaking line of site from the generator to the observer . For example if you are outside and you hear a radio playing near by, you can reduce your observed sound level (5dB) by simply stepping behind a wall so that you can no longer see the radio. After you loose line of site to the radio, adding more walls between you and the radio offers no additional reduction in sound.

If you are 50 feet from a radio with 1 block wall between you and the radio (assume no reflections) you will have a 5dB reduction in sound. Adding any number of additional walls between you and that radio 50 feet away will make no difference in the total of 5dB reduction. So in your ventilation system as long as there is one turn between the fan and the microphones then you already have the maximum sound reduction that you will get from baffles (still assuming no reflections).

The biggest reduction you get is from distance. Moving the sound generator further away from the observer is the best way to reduce observed sound. You can calculate how effective this will be using the formula "15*Log([current distance from the sound source]/[planned distance from the sound source]). Moving the fan from 10 feet away from the observer to 20 feet away from the observer will reduce the sound level by 4.5dB.

Part of you problem will be that most ducting is metal and thus reflects sound well. Reflections are tolerable in a straight path but will completely negate your baffles if you don't reduce it. You want the walls of your ducts/baffles to be as soft a material as possible so that they absorb sound instead of reflecting it down the pipe. I would recommend you build your own ducts out of plywood and cover the inside with a soft foam (or carpet if you need to do it cheaply) just remember that you want a good dust filter on the intake to keep your sound absorbing material free of dust.

Also, regardless of the sound from the fan, moving air makes sound too, so you would want to have the air entering and leaving using large ducts and from multiple locations or from a long continuous vent. See the attached image. This will reduce the air pressure on your intake and outlet so that the volume of air moved is the same but the speed and which it is moving is reduced. If a 4in duct would be adequate for a room of this size, use a 1ft ducts and vents instead so that the air can move at a slower pace.

The same rules apply on the room outlet for reducing sound. Large soft walled ducts with one turn to reduce sound and keep the air moving at a slow speed. The longer the duct runs, the better sound will dissipate over distance.
 

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