Heads in a piping system
pressure headvelocity headstatic fill pressuretotal dynamic headTDHfeet of head2.31 rule
Pressure head, velocity head, static fill pressure and total dynamic head — the feet of water a pump and a gauge are each talking about.
Pressure Head (h = P/ρg)
Converts a pressure into the equivalent height of a fluid column, with g = 9.80665 m/s².
Velocity Head (h = v²/2g)
The kinetic energy of a flow expressed as an equivalent column height, with g = 9.80665 m/s².
Gauge and Absolute Pressure
Absolute pressure is the gauge reading plus the surrounding atmospheric pressure.
Hydronic Static Fill Pressure
Cold fill pressure a closed loop needs to lift water to its highest point plus a safety margin, the SI form of the 2.31 ft per psi rule.
Total Dynamic Head
The head a pump must develop: static lift plus friction losses plus velocity head, all expressed in feet or metres of the pumped liquid.
Net Positive Suction Head Available (NPSHa)
Absolute head available at the pump suction above the liquid's vapour pressure — the margin that keeps a pump from cavitating.
How they fit together
Head is pressure expressed as a height of the fluid being pumped, and that last clause is the whole subject. The familiar 2.31 ft per psi is 144 in²/ft² divided by 62.4 lb/ft³ — cold water only. For a fluid of specific gravity 1.2 the same psi buys you only 1.93 ft, and for 40% glycol it is about 2.2. A centrifugal pump develops head, not pressure: put brine in it and it lifts the same number of feet while the gauge reads higher.
Match the head to the question. Static fill pressure sizes the fill valve and expansion tank — height to the highest point, in feet, divided by 2.31, plus 4 to 5 psi so the top of the system is not at zero. Total dynamic head sizes the pump: static lift plus friction plus any pressure difference between the end vessels. NPSH available protects the suction. The classic errors are two. Adding static fill height into TDH on a closed loop, where it cancels — water coming down the return does the same work the pump would have to do going up, so a closed hydronic loop's TDH is friction only, no matter how tall the building. And confusing gauge with absolute pressure in an NPSH calculation, which is worth 34 ft and turns a marginal suction into an apparently comfortable one on paper while the pump cavitates in the field.