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Oversized ductwork

Oversized ductwork

Oversized ductwork

I just have a general question I have been wondering about:  What happens to the airflow when your supply air ductwork is oversized?  Say for example, you typically try to maintain a constant 0.1" pressure drop across your ductwork but it's a retrofit job and reusing the existing ductwork gives you a pressure drop of 0.03"?  I know when the duct is undersized you get noise problems due to the velocity of the air but what type of problems do you have when the duct is undersized?  Thanks for your help!

RE: Oversized ductwork

I'm out of my league in this arena, but from just a flow stand point, I would think that oversized ducts will create an uneven flow distribution -- those points farther down the duct work would have insufficient velocity (pressure) to maintain an adequate flow to the areas desired...

RE: Oversized ductwork


Oversizing of the supply ductwork should not be a problem, but make sure that your sheet metal joints are well sealed, since now you have less air to lose!

Oversizing of individual supply air diffusers and their branch lines can be a problem -- if there is no means of throttling air flow, especially on the takeoffs closest to the air handler, you will not be able to balance the system, and remote diffusers will be starved.


RE: Oversized ductwork

So the ratio of pressure drops is 10/3 = 3.33 and velocity is reduced by 1.82 times. (roughly 3.331/2)

Generally main duct velocity is maintained between 1400-1800 fpm and in your case it reduces to 800-1000 fpm. If your branch sizing and grill sizing is proper, there should not be any problem. Grill velocity should be 400-600 fpm, otherwise you may not get proper distribution of air if your room height is too high.

The main problem comes from increased discharge. If you have already selected a blower for 0.1"/100 ft of ducting, the reduced pressure drop will shift the operating point the right. There is increased flow rate in this case and subsequently an increase in power drawn. This may overload your motor.

Note: Increase in flowrate again increases the velocity and pressure drop. Use the performance curve to calculate this.


RE: Oversized ductwork

The only problem you will have is spending too much on sheetmetal costs.  The velocity pressure will simply convert to static pressure.  There will be no problems with downstream diffuser flow rates.  Overall fan power requirements will be lowered resulting in energy savings provided that the system is properly balanced.  

If the fan was selected for 0.1"/100' feet of duct and the design is as you indicated (0.03"/100') then the fan may require a sheave change in order not to provide too much total static pressure to the system requiring balancing dampers to be shut.  

The reasons 0.1 is typically used is that this provides the most economical approach between ductwork and energy usuage.  Some utility companies (CA) offer rebates for low pressure duct design.  You may qualify for one of these.   

RE: Oversized ductwork

Thanks for your help everyone!  I was thinking that if your fan is correctly sized for your ductwork and you stayed below the air velocities recommended by ASHRAE you would be okay.  But then I started thinking about how it is recommended to keep your pressure drop at 0.1"/100 ft.  Now that I know the reasoning behing the 0.1" it's clearer.

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