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Supply Diffusers are preferable on exterior walls. Interior walls are better for Return Diffusers. And most important remember the more the better.
Don't forget to select take-offs with balancing dampers.
Size the Supply Air Trunks at .10 Friction Loss and size the cfm for the diffusers at 100 cfm each.
Size the Return Air Trunks at .08 Friction Loss and size the diffusers at the ratings of the diffuser. Remember all return diffusers have a cfm rating and there is generally a 60% free area of the total size of the diffuser.
If you would like to purchase a duct calculator with instructions on how to use it, we sell the duct calculators with instructions for using for $15.00.
Here are some sizes for supply duct that can be used. Use larger sizing for return air ducting.
- 100 cfm - 6" round 5x6" 4x8"
- 200 cfm - 8' round 6x8" 4x12" ½ ton
- 300 cfm 9" round 10x7" 5x14" ¾ ton
- 400 cfm 10" round 8x10" 6x14" 1 ton
- 600 cfm 12" round 10x12" 8x14'" 1-1/2 tons
- 800 cfm 13" round 10x14" 8x18" 2 tons
- 1000 cfm 14" round 10x16" 12x14" 2-1/2 tons
- 1200 cfm 16" round 10x20" 12x16" 3 tons
- 1400 cfm 16" round 12x18" 10x20" 3-1/2 tons
- 1600 cfm 18" round 14x16" 12x20" 4 tons
- 2000 cfm 18" round 12x25" 16x17" 5 tons
Now if all the above was too involved or too complex or more than you really cared to know about HVAC systems, try a simple method. You'll find this is a fail safe method that works equally as well and produces excellent results.
- Determine the size of the equipment and all other items such as total air required. For example a two ton system takes 400 cfm of air per ton or a total of 800 cfm.
- From the chart above select the 10 x 14 size which is for 2 tons or 800 cfm.
- 100 cfm per take off so select 8 locations for diffusers.
- Placing the air handler or furnace as close to the center of the house and make your main trunk the size above.
- Place a tee in the main trunk and then run two separate trunks with each taking half the air or half the diffusers.
- The 400 cfm trunks are rated at 400 cfm or 8 x 10"
- Layout the drawing and submit it to us for evaluation. Select the main plenum for 800 cfm, make the take off locations, run the take offs and voila instant duct.
No rocket science, just plain simple common sense. Remember that the duct system can only move so much air at a given size duct. You can't force the fan to do something it isn't capable of performing.