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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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Related Topics

• Heating - Heating systems - capacity and design of boilers, pipelines, heat exchangers, expansion systems and more

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