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I have forced air extraction with a duct running through one classroom then a second. The classroom which is ducted closer to the fan receives much greater air flow through its vents.

duct diagram

To combat this we have to use airflow regulators- basically metal shutters- to restrict the airflow in vents 1 and 2 in order that 3 and 4 receive flow. As these are classrooms a fairly powerful motor is used to renew the air around 15 times per hour so this is rather noisy with a horrible whistling noise.

In this photo you can see vents 3 and 4 with 1 and 2 behind that wall.

vent photo

I have spoken with a ventilation contractor who says having larger vents would alleviate the problem but I imagine I would still need to close large vents in 1 and 2 as much as with the smaller vents and the noise would persist. In all honesty I have had a poor experience in general with ventilation contractors so I don't want to waste more money.

I would appreciate advice and imagine this is a fairly typical scenario. I hope so at least. :)

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As a cheap solution, I would try different registers. You might find that a different style/design, produces less noise. Since the current style has the slats running lengthwise, I might try some where the slats run the other way. – Tester101♦ Dec 13 '12 at 13:39
Maybe a different design is what I was thinking, probably of a larger size. You can't actually see the register, it is above the metal grill in the photo and is adjusted by a screwdriver closing 6 widthways 'gaps'. – mark Dec 13 '12 at 20:56

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One option would be to reconfigure the ducts so that there's an independent run for each classroom, joining into a trunk just before the fan. This way you could use dampers in each duct run to balance the airflow between rooms. If you keep the dampers far away from the registers, you can minimize the effect of the turbulence (and associated noise) from the dampers.

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This might be a very good idea as it would halve the wind speed in the longer running duct. At the end of the my diagram - where the fan is - there is a turn of the duct to 90 degrees to the left. I could look into branching another duct from this for a shorter run in the first classroom. Don't know about dampers though, we have fibre ducts so I'll have to look into it. Thanks. – mark Dec 13 '12 at 20:53
Have added the design digram. I´m thinking of branching from that 500x150 duct and having a short second run at 90 degrees in the first classroom. – mark Dec 14 '12 at 13:30
Going with this answer. Thanks! – mark Dec 15 '12 at 10:29

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