how to size a return vent for a room

[GITMO64]

I was called to a job where the customer did some remodeling to
a store, the customer turned it into a dance studio. she added a reception
area that measures 12x2 x 19x8 and the height of the place is 8x9.
When I looked at the vent setup in the reception area had 2 supplies
and no returns. I informed them that they needed a return in order to
circulate the air in the reception area. I put in a return, I used 8' round flex
duct and added a perforated lay in on the ceiling. But the air is still not
circulating. My question is should I have used rect.ductwork instead of
round, is my return to small for the space.? I know how to check cfm
for residential but not sure on commercial units. I brought an
aneomometer but the instructions on it are not clear and I do not
know how the unit works I called the company that makes it and they
told me to go to the place where I purchased it for instructions.

This is my second job were the issue is are flow and it has me
confused.

[secorp]

What is your Static pressure drop across the coil?


Or you can hold a thumb up to the vent if it covers the vent you are OK , if it does not you need to make it bigger!!!!!!!

[GITMO64]

secorp,

I do not understand about the thumb part what do you mean?

[tinner73]

everything in commercial is 14" round.

[GITMO64]

Thanks for the advise Tinner 73

[Savage16]

Also buy and learn how to use a duct calculator, these handy little things can be very useful. Of course its no good if the unit itself was sized improperly for the building or room it is heating/cooling

[VTP99]

everything in commercial is 14" round.

[merlin73]

everything in commercial is 14" round.

that is very funny, Everything is 14", I guess all the work I did with 6"-20" is all incorrect. Well thats the rule of THUMB i guess

[do over]

I don’t want to say to much. That way I will not get beaten up that much. The BTUH load for each room should calculate duct size.

[secorp]

secorp,

I do not understand about the thumb part what do you mean?

Stand back 20 feet a see if your thumb covers the vent, or break down and purchase cardboard cut outs of thumbs on E bay and use them. every five feet you back away equals 5 tons.














































I hope you realize I am kidding......