Static Regain Method: Sizing Ducts So Every Outlet Gets Its Air

The static regain method sizes a supply duct so that, after each branch takes off, the drop in air velocity converts some velocity pressure back into static pressure. Sized correctly, that regained pressure offsets the friction lost along the next section, so the static pressure is roughly the same at every outlet and the system needs less balancing.

By Dhruv Agarwal · · 1 min read

Why it matters on a project

The most common HVAC complaint after an office handover is uneven cooling: desks near the air handling unit are cold, desks at the far end are warm. Often the equipment is the right size and the problem is that the ductwork delivers too much air to the near outlets and too little to the far ones.

Duct sizing method is one of the decisions behind that.

The two common methods

Equal frictionStatic regain
Sizing ruleSame pressure loss per metre in every sectionVelocity reduced so static pressure recovers after each branch
Static pressure along the runFalls towards the endRoughly even at each outlet
Balancing effortMore on long runsLess
Design effortLowerHigher, iterative
Duct size at the far endSmallerLarger
SuitsShort, compact systemsLong mains, many branches, VAV systems

Both methods, and the pressure relationships behind them, are set out in the duct design chapter of the ASHRAE Handbook — Fundamentals. Construction of the ductwork itself is commonly to SMACNA standards.

Where it goes wrong on a fit-out

  • Ducts resized on site to clear a beam, without recalculating the run downstream.
  • Extra outlets added late to suit a changed layout, taken from a section sized for fewer.
  • Tight fittings and short radius bends that add losses the design never allowed for.
  • Larger downstream ducts not checked against the ceiling void during coordination.
  • Balancing skipped or rushed at commissioning on the assumption that the design will balance itself.

Standards referenced

Duct design methods in the ASHRAE Handbook — Fundamentals; ductwork construction commonly to SMACNA; air conditioning provisions in NBC 2016, Part 8. Duct sizes and the choice of method for a specific system must be established by the project's HVAC consultant.

Standards referenced

  • ASHRAE Handbook — Fundamentals — Duct design chapter
  • SMACNA — HVAC Duct Construction Standards — Metal and Flexible
  • NBC 2016, Part 8 — Building services — air conditioning (Bureau of Indian Standards)

Frequently asked

Air in a duct has static pressure, which pushes outward and drives air out of diffusers, and velocity pressure, which comes from its movement. When air slows down, part of its velocity pressure turns back into static pressure. Static regain design deliberately slows the air after each branch so that this gain cancels the friction loss along the following section.

Equal friction sizes every section for the same pressure loss per metre, which is simple and works well for short, compact systems. On a long run, though, outlets near the fan see more static pressure than those at the end, so dampers have to throttle the near ones. Static regain aims for even static pressure at every outlet, which suits long supply mains with many branches.

Mainly on large supply systems with long mains and many take-offs, such as a large open-plan floor served by one air handling unit, and in variable air volume systems where each terminal needs a reasonably stable inlet pressure. On a small fit-out served by a few short runs, equal friction is usually adequate.

Generally the downstream sections end up larger than equal friction would give them, because the air is being slowed down. That means more sheet metal and more ceiling void at the far end of the run, which has to be checked against beams and other services during coordination.

No. It reduces the amount of balancing needed and makes it easier to achieve design airflow at every outlet, but site changes, fittings and installation quality all affect the result. Every system still needs airflow measurement and balancing at commissioning.

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