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# Static Pressure in a HVAC System

## A minimum static pressure is required in a HVAC system to keep the water in the highest levels of the 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:

hs = hb + hc                                                     (1)

where

hs = static head (ft, m)

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

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

### Static Pressure as Pressure - (psi)

ps = hb + hc / (144 / γ)                                                 (2)

where

ps = 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:

hs = (200 ft) + (20 ft)

= 220 ft

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

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

= 95.3 psig

## Related Topics

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

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