Excessive air flow through a register

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Excessive air flow through a register

Postby ameneses54 » Wed Jul 24, 2013 11:02 am

I just had a new AC system installed in a small condo I recently bought. The problem is that the main and only bedroom is located almost beside the air handler and as such the duct that emerged from the plenum is at most 4 feet long and straight (made of duct board).
In this setup, as the unit is efficient, I have an excessive air flow to the master and as such, the temperature goes sharply down and is unbalanced. Noise is also an issue.
I've tried many registers with louver and opposing damper blades, but if I close them even slightly I get increased noise.
What do you think of the following -
I suspect that the AC company installed a blower which is over supplying in relation to my needs in the bedroom. The problem is that the temperature is fine in the rest of the condo which has high ceilings as it's on the top floor.
What would you think of reducing the overall diameter of the rectangular duct in about 40 - 50% with duct board. The tech already tried this, but he only jammed a wedge about two feet from the register reducing the effective diameter and this caused increase noise probably due to a venturi effect. My idea is to reduce all the ducts diameter to about 6" from the register with duct board or a sound insulation duct liner (very expensive).
I'd really appreciate your opinion on this matter.
Any ideas on what I can do?

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- Excessive air flow through a register

Postby Freon » Fri Jul 26, 2013 1:47 pm

You are on the right track. But first, understand the blower setting is determined by the size of the condenser unit. So I would not go playing with the blower motor speed.

As to your idea, it is a sound one. Restricting the air flow in the duct that feeds the bedroom will reduce the rate of cooling and divert mare air flow to the rest of the condo. The trick is to reduce the duct's diameter in a way that does not create turbulent airflow (noise).

A simple dam in the duct is an example of the worse design creating very turbulent airflow downstream of the dam. It's just like a dam on a river.

Here is what I'd try, Measure the longest inside dimension of the duct. Now cut a piece of flexible material that is also strong, like aluminum flashing. Make the flashing the exact width as the widest part of the duct and the length maybe 12". This length you may have to adjust.

Now place 3 push-pins (with the little plastic tops) equally across the inside of the duct along the longest duct wall that you measured before. The line formed by the push-pins should be parallel to the opening of the duct at the register. Now slide the aluminum flashing back edge (edge furthest from your hands) into the duct until it stops at the line of push-pins. Push the flashing gently so it starts to bow upward in the middle (the edge at the push-pins still remaining in contact with the duct wall). When you have a bow that, at it's highest point in the middle, is about 1/2 the opening of the duct, use 3 more push-pins to hold the front edge (the edge you were holding as you slid the flashing into the duct) in place. You are trying to create a smooth hump in the duct as opposed to a vertical dam. The airflow will gradually rise as it passes over the aluminum hump becoming more restricted and causing the pressure to increase thereby reducing the flow rate. Experimenting with the height of the hump by adjusting the push-pins until you find the correct airflow and least noise. Once you have the hump correctly set, secure it to the duct wall.
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