I have to add 3 take offs from my main HVAC supply/trunk. Is there a minimum amount of distance I should maintain between each of the take offs?

What should I do if I don't have the minimum distance available? Is it OK if they are right next to each other?


1 Answer 1


There's a "2 foot" rule for separation, but I have seen take-offs on direct opposite sides of the main trunk, I have seen them right above the furnace, and I have seen two side by side for the same 16in joist space.

Nominally throughout the trunk there is air pressure and air velocity. At any branch-off the pressure drops and the velocity increases because air can move more easily: some of it "escapes" down the branch duct. Take offs tap-off the air predominantly based on air pressure, they don't "scoop" the air. Take-offs are slightly angled to take advantage of the velocity, and they are not exclusively pressure based.

At some distance further down the trunk (the rule of thumb is 2 feet) the velocity drops to nominal and the pressure rises to nominal. That's where the next branch-off can be.

If two take-offs are too close to each other, the pressure for both is low and both cannot tap off as effectively.

If you do it the "wrong" way it just means you have lower pressure entering into the two branches, and their velocity or flow is less than if they were properly spaced. A few of these cases won't break things, and usually you can re-balance the flow with proper throttling at the register.

enter image description here

Image: https://hvactoday.com/0719-two-foot-rule/

  • 1
    rses.org/assets/serviceapplicationmanual/630-148.pdf has similar diagrams/recommendations with more detailed discussion behind the HVAC system design in general.
    – TylerH
    Commented May 11, 2021 at 19:06
  • 1
    @TylerH, thanks for the tip. Here's another one that digs deeper into the "static regain method", but says little about the end-cap clearance: pdhonline.com/courses/m246/m246content.pdf
    – P2000
    Commented May 11, 2021 at 19:33
  • @P2000 Thanks, looks like good reading (just skimmed); only thing I can think of is maybe it's intended to provide a 'double dip' of pressure for air entering the room? The pressure would drop to near zero as soon as it reached the end of the duct and entered the room, resulting in a low aspiration ratio... I dunno.
    – TylerH
    Commented May 11, 2021 at 19:47
  • Can I cut a hole for Return at the very end ( perpendicular ) wall in the main trunk? Does 2 foot of rule apply here too for return case ? I don't think so. because there is no static pressure at the end.
    – PowerTech
    Commented May 12, 2021 at 19:21
  • 1
    Return ducts indeed terminate with a intake grill or a short feed to a grill, so that would be ok, just based on experience. Just to be clear: your posted question was for the supply and your comment is about a completely different duct to the air handler. These are two completely separate ductings.
    – P2000
    Commented May 12, 2021 at 19:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.