Static Pressures in HVAC Systems

Static pressures in HVAC system avoids the tops of the systems drying out

The static pressure of most HVAC systems consists 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), 10 - 20 ft (3 - 6 m) water column.

Static Pressure as Head

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 tallest building (ft, m)

Static Pressure as PSI

It is often common to express static pressure in 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 pressure of a 190 ft tall building and cushion head 10 ft can be expressed as head:

h_s = (200 ft) + (20 ft)

    = 220 ft

If the water operating temperature is 60^oF with specific weight of 62.4 lb/ft³ - the pressure in psig can be expressed as:

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

    = 95.3 psig

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