Is there a minimum for spacing between take-offs from an HVAC trunk line?
There's a &quot;2 foot&quot; 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 &quot;escapes&quot; down the branch duct. Take offs tap-off the air predominantly based on air pressure, they don't &quot;scoop&quot; 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 &quot;wrong&quot; 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.

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