PRESSURE CLASSES & WATER GAUGE (w.g.)

SMACNA pressure classes (Sheet Metal and Air Conditioning Contractors’ National Association)

SMACNA pressure classes 

1/2” w.g.

Low Range

Small residential / light branch duct.

1” w.g.

LOW Range

Small package units, low-velocity duct.

2” w.g.

LOW (standard)

Most commercial low-pressure supply/return and duct downstream of VAV boxes. ‘Default/’ on many jobs.

3” w.g.

MEDIUM Pressure

Main supply trunks on VAV systems.

4” w.g.

MEDIUM Pressure

Medium-pressure supply mains upstream of VAV systems / fan-powered boxes

6” w.g.

MEDIUM / HIGH Large built-up air handlers, high-rise risers

10'“ w.g.

HIGH Pressure

Industrial / process and special systems.

Positive Pressure

The fan is pushing air into the duct - the walls want to bulge OUT. Supply air.

Negative Pressure

The fan is pushing air into the duct - the walls want collapse IN.Return, Exhaust and relief air.

VAV box (Variable air Volume)

A box in the ceiling that throttles airflow to a zone. The main duct feeding the boxes runs at higher (medium) pressure; the duct after the box runs at low pressure..

Seal classes (related)

A) Transverse joints, longitudinal seams AND wall penetrations. (4” w.g. and up - and now required on many jobs regardless of class (energy codes)

B) Transverse joints and longitudinal seams (3” w.g.)

C) Transverse joints only (2” w.g. and below)