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