Do I need return duct?

[Shoester]

I'm putting a 4ton AC system in my shop. The air handler is up-sized to 5 ton. The air handler will be isolated in an 8x16 mechanical room. Do I need to run return duct out to the main shop area, or can I just leave the 'return' hole in the side of the air handler open (filter only) with no ductwork, with the return air simply coming from the mechanical room?

 

[cgrutt]

I'm not a HVAC guy but believe you want the return to be in same conditioned space as the supply vents for operating efficiency and other reasons. I would definitely avoid keeping return in mechanical room if there are any gas operated furnaces or hot water heaters, etc in that same room where the return could pull flu gases back into house and distribute it (and possible CO) into your conditioned space. Also would avoid if mechanical room is damp, etc. Curious what pros have to say about this.

 

[brewchief]

It should be ducted out of the mechanical room, you could put large transfer grills in the walls but you would have to insure they didn't get blocked. 4 tons is 1600 cfm, that's more then the volume of the room every minute, you would have a strong breeze in the mechanical room.
Any gas appliances could have venting issues but I'm guessing no gas if you're using an air handler.

 

[JohnX14]

You want a return duct from the conditioned space. That is the air that the air handler is "conditioning", whether it is being heated or cooled. You don't want to be sucking the air in from the mechanical room to dump into the conditioned space.

 

[PCustoms]

Definitely need some way to return air to the system.

It should be ducted out of the mechanical room, you could put large transfer grills in the walls but you would have to insure they didn't get blocked. 4 tons is 1600 cfm, that's more then the volume of the room every minute, you would have a strong breeze in the mechanical room.
Any gas appliances could have venting issues but I'm guessing no gas if you're using an air handler.

Interesting post. How do you size the return?

I've got a furnace (90k btu IIRC) that was installed with the "return" being an aluminum louver cut into the stairwell door. Furnace would **** the door closed, so obviously undersized.

How do I calc the correct size?

 

[AA/FC]

If the mechanical room is also going to be conditioned, you COULD simply install a "transfer air grill" in the wall between the mechanical room and shop so it can **** air from the shop into the mechanical room, and then into the air handler. It's just a vent that allows air to flow freely between the two rooms when the mechanical room door is closed. It's not the best best way to do it, but it will work.

Edit: I see others have already mentioned this. Carry on....

 

[fitter30]

2" sq inches per 1000 btu for the return. Bigger is better larger filter common size less changing and quieter.

 

[Milton Shaw]

A duct-board insulated duct will cut the noise level from the air handler from noisy to barely hearable. And the return duct should be in the same shop area as the supply ducts vent to. A right angle in the duct will also cut the noise.