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Frequently Asked Questions
Why does rectangular duct need a band rather than a clip?
Because it is large, light for its size and made of thin sheet.
Support has to spread load across the section.
A clip gripping a corner simply deforms it.
A band, strap or bracket passes under and around instead.
Two hangers per point is the usual arrangement.
They carry a bearer beneath the duct so weight goes into the base.
What sets the support spacing?
The ductwork specification, not the hanger.
Standards publish maximum intervals by duct size and construction.
Unsupported duct sags and opens its joints.
Open joints leak, which matters commercially as well as technically.
A leaky system moves less air for the same fan energy.
Leakage testing is frequently a specified requirement.
How does insulation affect the support?
A band tightened over insulation crushes it.
That creates a thermal bridge.
On cold duct it causes condensation at the support point.
It destroys the performance the insulation was installed for.
Load-bearing inserts carry the weight through the insulation.
Supports outside the insulation are the alternative correct approach.
What weight should be allowed for?
The duct plus insulation, lining and years of accumulated dust.
Empty duct weight understates the load considerably.
Systems accumulate debris over their life.
Lining adds weight throughout the run.
Insulation adds weight and bulk.
Calculating the loaded weight rather than the sheet metal alone is what matters.
Do dampers need their own support?
Yes - a heavy component cannot hang from the duct's own hangers.
Fire dampers are several times the weight of the duct around them.
So are attenuators and in-line units.
Independent support is normally specified.
Fire-rated installations carry additional requirements.
Those differ by market and the local rule governs.
What limits the assembly?
The weakest component, frequently the anchor into the structure.
Cable, gripper, band and anchor each have their own rating.
Anchor performance depends on the base material.
Long spans concentrate load at fewer points.
Safety factors are set by code and by the manufacturer.
Where they differ, the stricter applies.
Is bracing needed?
Where seismic provisions apply, yes.
Wire suspension resists gravity rather than sway.
Large duct presents considerable mass to lateral movement.
Bracing components are specified separately from the suspension.
Requirements differ substantially between regions.
Fire strategy requirements may also apply to ductwork supports in some jurisdictions.