Low and High Pressure HVAC Defined
Low Pressure Fittings for Residential and Light Commercial HVAC Applications
Over the years, the terms low pressure and high pressure in duct system design have been given a wide latitude of meanings in the HVAC industry, and they are frequently (and, sometimes simultaneously) applied to “velocity” as well as pressure because these factors are inter-dependent.
Depending on application, the dividing line between high and low pressure has been variously defined between 1500 to 2500 fpm, and nominally as 2000 fpm; and empirical data has shown that duct sections operate satisfactorily over the above range of velocities at 1″ water gage (wg).
Low pressure systems are chosen where duct space allows, where air noise is a consideration, and where particle conveyance such as wood chips or grain is not a requirement.
Space limitations in modern buildings have restricted the size of air conditioning ducts and equipment. Therefore, to convey the necessary volumes of air, higher velocities must be employed. Increased velocities produce higher duct friction losses. In order to maintain flow against the higher duct friction, it is necessary to have greater pressures at the air source. Therefore, the terms “high pressure” and “high velocity” generally go hand in hand. Conversely, this is true of “low pressure” and “low velocity”.
The use of the terms “high velocity” and “high and medium pressure” in this catalog refer to any static pressure class of 3″ wg or greater, and “low pressure” refers to 2″ wg or less.
SMACNA recommendations on pressure and velocity are shown in the table below. The listed classifications pertain to ducts only. Casing and plenum construction designs are provided in the “High Pressure” manual, but their respective designs have been based on historical acceptability.
| Pressure Velocity Classification 1 | ||||
| Former Duct Class | Pressure Rating | Pressure | Seal Class 2 | Seal Class 3 |
| High Pres. | 10″ | Pos. | A | 2000 Up |
| Medium Pres. | 6″ | Pos. | A | 2000 Up |
| Medium Pres. | 4″ | Pos. | A | 2000 Up |
| Medium Pres. | 3″ | Pos. or Neg. | A | 4000 Dn |
| Low Pres. | 2″ | Pos. or Neg. | 2000 Dn | |
| Low Pres. | 1″ | Pos. or Neg. | 2000 Dn | |
| Low Pres. | 1/2″ | Pos. or Neg. | 1500 Dn | |
1Reproduced in part by Permission From SMACNA High Pressure Duct Standards – 3rd Ed.
2Seal Class A: All seams, joints, fastener penetrations and connections sealed.
3General velocity level through this pressure rated section of the system. Certain points may have higher or lower velocities, e.g., fan outlet or restricted passage, yet not require a different pressure class. The designer makes determinations of duct class after analyzing velocities and operating pressures.
