misterfixit
Well-known member
I would like to ask for some 2nd opinions on duct sizing.
I have a 30x40 metal building, concrete slab with spray foam insulation. In this 1200' space I have 24x30 that is open floor to ceiling to use as woodworking area, and the remaining 16x30 area which is divided into 3 rooms. 2 rooms are 16x10 and the center room is 12x10. You can see the actual photo below:
looking at the photo, the room on the right which is open the full width is to be grinding/sanding/blasting area for dirty work. The center room will house my lathe and milling machine. and the left room will be desk, soldering and other 'clean' work. The clean room on the left and the center room will have the clear strip doors like a cooler, the right room will be open at all times. Above these rooms is where my central heat/ac unit will be.
My AC unit was purchased through a friend who has an AC business, and he has been serving as a free source of information. He suggested I get a 3 ton system, which I did and have on hand. He has agreed to come out after I get it installed and sweat the lines and commission the unit for me and I will be paying him for that work. I'm not paying him, as per our friendly arrangement and agreement, for telling me what size ducts to use. He did give me a list of suggested duct and registers to use.
My issue is I'm not sure I agree with his suggestions, and since he's doing this part for free I don't want to argue with him and tell him I think he's wrong unless I'm pretty sure he is. But from past experience (installing AC in my Barndomium) and research I think he's off.
Here's my calculations:
3 ton system, should average 1200 CFM
30x40 building =1200 sqft which means 1 cfm/sq ft
16x24 open area should get 720 cfm
middle room is 12x10 should get 120 cfm
the 2 rooms on either side are 16x10, should get 160 CFM each
that all equals 1160 CFM, which when you look at the 4x10 inset outside the middle room makes it all fit.
It looks to me like each of the rooms would work with 8" flex duct, which by my research should deliver 150 CFM.
His suggestion for the rooms is a 7" duct and register in the larger rooms, and a 6" in the middle.
For the larger room/area, and based on the remaining CFM from the unit, it looks to me like I should take off from the plenum with about a 14" flex then possibly wye to a 14x10x10, taking two 10" ducts to registers at the edge of the mezzanine.
His suggestion was for a 10" from the plenum and wye into two 8".
It looks to me like he has undersized the ducting. Before I go back to him I want to make sure I understand what I think I understand.
All help and suggestions are appreciated.
Edited to add: There are no return air ducts. The return is at the unit itself above the rooms, using the 20x20 return on the air handler alone.
Edited again to add: this is a 3 ton Goodman system using an ARUF37C14AD air handler
I have a 30x40 metal building, concrete slab with spray foam insulation. In this 1200' space I have 24x30 that is open floor to ceiling to use as woodworking area, and the remaining 16x30 area which is divided into 3 rooms. 2 rooms are 16x10 and the center room is 12x10. You can see the actual photo below:
looking at the photo, the room on the right which is open the full width is to be grinding/sanding/blasting area for dirty work. The center room will house my lathe and milling machine. and the left room will be desk, soldering and other 'clean' work. The clean room on the left and the center room will have the clear strip doors like a cooler, the right room will be open at all times. Above these rooms is where my central heat/ac unit will be.
My AC unit was purchased through a friend who has an AC business, and he has been serving as a free source of information. He suggested I get a 3 ton system, which I did and have on hand. He has agreed to come out after I get it installed and sweat the lines and commission the unit for me and I will be paying him for that work. I'm not paying him, as per our friendly arrangement and agreement, for telling me what size ducts to use. He did give me a list of suggested duct and registers to use.
My issue is I'm not sure I agree with his suggestions, and since he's doing this part for free I don't want to argue with him and tell him I think he's wrong unless I'm pretty sure he is. But from past experience (installing AC in my Barndomium) and research I think he's off.
Here's my calculations:
3 ton system, should average 1200 CFM
30x40 building =1200 sqft which means 1 cfm/sq ft
16x24 open area should get 720 cfm
middle room is 12x10 should get 120 cfm
the 2 rooms on either side are 16x10, should get 160 CFM each
that all equals 1160 CFM, which when you look at the 4x10 inset outside the middle room makes it all fit.
It looks to me like each of the rooms would work with 8" flex duct, which by my research should deliver 150 CFM.
His suggestion for the rooms is a 7" duct and register in the larger rooms, and a 6" in the middle.
For the larger room/area, and based on the remaining CFM from the unit, it looks to me like I should take off from the plenum with about a 14" flex then possibly wye to a 14x10x10, taking two 10" ducts to registers at the edge of the mezzanine.
His suggestion was for a 10" from the plenum and wye into two 8".
It looks to me like he has undersized the ducting. Before I go back to him I want to make sure I understand what I think I understand.
All help and suggestions are appreciated.
Edited to add: There are no return air ducts. The return is at the unit itself above the rooms, using the 20x20 return on the air handler alone.
Edited again to add: this is a 3 ton Goodman system using an ARUF37C14AD air handler