Static Pressure in a HVAC System

The static pressure in most HVAC systems is a result of the height of the building plus a cushion at the top of the building. In general the cushion is around 5 - 10 psi (lb/in²) or 10 - 20 ft (3 - 6 m) water column.

Static Pressure as Head (ft, m)

The static pressure can be expressed as head:

h_s = h_b + h_c                                          (1)

where

h_s = static head (ft, m)

h_b = elevation at the top of the building (ft, m)

h_c = cushion required at the top of the building (ft, m)

Static Pressure as Pressure - (psi)

p_s = h_b + h_c / (144 / γ)                                      (2)

where

p_s = static pressure (psig)

γ = specific weight of water at the operating temperature (lb/ft³)

Example - Static Pressure in a Water Heating System

The static head of a 190 ft tall building and cushion head 10 ft can be expressed as calculated as:

h_s = (200 ft) + (20 ft)

    = 220 ft

With water operating temperature 60 ^oF and specific weight 62.4 lb/ft³ - the static pressure can be expressed as:

p_s = (200 ft) + (20 ft) / (144 / (62.4 lb/ft³))

    = 95.3 psig