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HVAC Duct Crossing Building Expansion Joints
3

HVAC Duct Crossing Building Expansion Joints

HVAC Duct Crossing Building Expansion Joints

(OP)
We have a 6'x6' square duct with negative pressure crossing from one building to another with 14 inches (+/-) of differntial movement. The duct is not flexible enough to absorb these movemnts without the use of flexible/expansion joints. Any recommendations would be appreciated. Thank you.

RE: HVAC Duct Crossing Building Expansion Joints

A possible 28 inches of total movement?!  Is that at least constrained to movement in a single plane, or is it 28 left-to-right and 28 up-and-down?

That is better than 1/3 of your duct size.  So you will need a long flex joint to prevent a rather sudden change in the duct cross-section and associated variation in system pressure drop.

RE: HVAC Duct Crossing Building Expansion Joints

(OP)
The total movement is 14" (7" each building)and it is in both horizontal directions x & z. No vertical.

RE: HVAC Duct Crossing Building Expansion Joints

You mention that duct is at a negative pressure - how negative? Depending on the pressure you may be able to use relatively standard flex duct to allow for movement.

RE: HVAC Duct Crossing Building Expansion Joints

Is it really possible for the two buildings to move their full possible excursions in opposite directions at the same time?

RE: HVAC Duct Crossing Building Expansion Joints

MintJulep, I would expect that the condition described by sigma1 could occur on only one occasion and that would be an earthquake.
I was thinking along the same lines at Chris.  Unless it is a horribly negative pressure your best bet is probably regular ol' flex duct, oversized just a bit to take into account it's huge pressure drop in comparison to standard metal ductwork.

RE: HVAC Duct Crossing Building Expansion Joints

(OP)
The duct is outddors at the roof level. The joint cannot be too long or it has to be strong enough to support 35 psf snow and/or 21 psf wind in addition to its own weight and negative pressure (P = -9" about -0.35psi). We are thinking of a double joint with a spool between them  (similar to a pipe universal joint)but the weight of the spool can be a problem.

Thank you all.

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