Solution for existing undersized return air duct

[shobroco]

Any suggestions for solving an existing undersized return? The supply is 12X20 and the return is 12X18, the HVAC unit sits on an exterior pad at grade. The return simply goes up the wall 10' (in an insulated, clad shaft) and into a ceiling plenum. The supply is ducted to each office. Supply volume at the far end of the runs is poor but noticeably increases if the side cover of the unit is removed allowing unlimited outdoor air into the return. There is an economizer on the unit. There is no explanation why the return is smaller than the supply (in my experience it is usually larger). I am a poor structural guy with nodding aquaintance with HVAC asked by a friend to offer an opinion on his poorly performing system. System size seems okay: 5ton for 2300 sq.ft. which is comparable to his older office with 10 ton & 4800 sq.ft. which works fine.

 

[imok2]

Well your ducts are definately undersized I would recommend replaceing them with this: For a friction rate of 0.1 /100 ft your supply should be 14” x 20” and your return should be 14” x 24” for a supply velocity of ~1000ft/min. and a return of~850ft/min. Your A/C is not very efficienr and wasting energy!

 

[shobroco]

Thanks

 

[chicopee]

The return duct may have been sized smaller because of air loss throughout the building. If you allow 15% outside air to be mixed in with return air, then you can see the volume difference between supply and return.

 

[KiwiMace]

Look and see if the economizer is working. Dampers might have jammed. I take it that the measurements above are the actual steel duct, not measuring insulation. Duct/Filters clean? Fan blades in good condition? This could be many things.

 

[shobroco]

I had thought that the smaller return might be because of the economizer. I don't know that it is working, I'll have him check it out.

 

[imok2]

If the econimizer is working then that makes your present ducting even worse because my sizing was based on 1900 cfm supply and 2000 cfm return and if the ducting was based on deminished supply and return then why have an econimizer that is defective? I would think that you would repair the economizer and install the correct duct work or totally remove the economizer

 

[ChasBean1]

For a standard 1 cfm/ft2 (assuming office type space), supply max is 1,380 fpm in the supply duct. Assuming some value such as 20% minimum OA, neither supply nor return velocities are excessive; especially for the 1 cfm/ft2 representing a maximum value. This would not indicate an undersizing issue. There might be more evaluation needed to resolve the problem.

 

[SAK9]

If we can solve all of life's problems with the help of internet,then there would not be many problems left!

 

[willard3]

1 cfm/square foot is a rule of thumb repeated many, many times in arbitration when the system won't cool properly.

 

[ChasBean1]

Sak9, Willard3, just going with what's given, unless I missed a given..

 

[Ziggypump]

1660 cfm supply = 12x20 supply duct (.1"wc/100 ft)
1295 cfm return = 12x18 return duct (.08"wc/100 ft)
or
1455 cfm return = 12x18 return duct (.1"wc/100 ft)

Take your pick. If I was designing the system, then it would have 365 cfm OA min. It may have been sized for 215 cfm OA min. This means that less air is being returned to the unit - loss through walls, exhaust fans... - and that could be the reason for the smaller return duct. Above are two ways that the ducts may have been sized.