10.1 Hydronic Pressure, Head, Open and Closed Systems, and Resistance

Hydronic Pressure, Head, and System Resistance

Pressure and head are related, not identical

Pressure is force per unit area, usually recorded in pounds per square inch (psi). Head is energy per unit weight expressed as the height of a column of the measured liquid. The conversion depends on specific gravity (SG):

Head, ft of liquid = psi × 2.31 / SG

psi = head × 0.433 × SG

For water with SG = 1.00, 6.5 psi is 6.5 × 2.31 = 15.0 ft of head. For a different fluid, use the approved fluid properties at a representative temperature. A generic glycol percentage or a remembered table is not enough: type, concentration, temperature, and condition can change specific gravity and viscosity.

Head is convenient for comparing a pump with a system because a centrifugal pump develops approximately the same feet of head for fluids of different density at the same speed and impeller geometry. The pressure difference corresponding to that head changes with SG, and the required power can also change.

Closed and open systems

In a completely filled closed loop, the hydrostatic height of the rising column is balanced by the descending column. The circulating pump normally overcomes dynamic resistance from pipe, fittings, coils, valves, strainers, and other equipment; it does not continuously lift the whole water column from the basement to the roof. Static fill pressure is still essential to keep the high point adequately pressurized, limit air entry, and satisfy equipment requirements, but it is not added as circulating head merely because the loop is tall.

In an open system, the pump may have to overcome an unbalanced elevation difference between open liquid surfaces or between a source surface and a free discharge. A cooling-tower arrangement, for example, must be traced from the basin operating level through the distribution point. The exact static term follows the real hydraulic boundary, not the building height alone.

Before classifying a system, trace the fluid path. A nominally “closed” distribution loop can include an open tank or another interface that changes the calculation. Record the operating water levels and valve lineup used during the test.

Friction, equipment loss, and the system curve

Dynamic head loss increases as flow increases. Over the usual turbulent operating range, a fixed system is often approximated by:

H = KQ²

If flow rises by 10 percent without a piping change, the dynamic loss is expected to rise by about 1.10², or 1.21. The constant K is not a universal fitting value; it represents the tested system configuration. A control valve movement, dirty strainer, bypass position, or different equipment path changes the system curve.

The operating point is where the pump curve intersects the system curve. A gauge reading by itself does not prove flow. Use an approved flow-measurement method or a valid manufacturer performance relationship, and consider its uncertainty.

Pump differential head from field readings

The energy equation includes pressure head, velocity head, and elevation head. Across an operating pump, determine the head added between defined suction and discharge stations:

Pump head = change in pressure head + change in velocity head + change in elevation head

With local pressure gauges, convert each pressure to feet of the actual liquid, include the vertical difference between pressure-tap elevations, and include velocity-head change when pipe sizes or velocities differ materially. State the sign convention. A common same-size arrangement with taps at nearly the same elevation reduces to discharge pressure head minus suction pressure head, but that simplification must be demonstrated rather than assumed.

Gauge elevation treatment depends on the measurement setup. For separate gauges connected locally at their taps, the pressure readings are associated with the tap elevations, so the energy calculation needs the station-elevation term. A differential instrument connected through liquid-filled hoses can also respond to the hydrostatic columns in those hoses. Follow the instrument procedure, locate and purge both hoses as required, and do not add an elevation correction a second time if the displayed differential already incorporates it.

Use gauge pressure consistently. A negative suction gauge reading is not automatically an error, and subtracting a negative value increases the measured differential. Verify zero, range, units, pulsation control, hose condition, air removal, stable operating state, and the actual tap locations.

Worked measurement

Suppose a coil differential measurement is 6.5 psi and the fluid has SG = 1.00. The coil loss is:

6.5 × 2.31 / 1.00 = 15.0 ft of head

That result describes the measured condition. To compare it with a coil curve or schedule, also record flow, valve position, fluid identity and temperature, and whether the coil and strainer path match the referenced data. Do not infer design flow from a single pressure drop unless the selected curve and configuration authorize that conversion.

For a pump test, record suction and discharge pressures at the same stable condition, pump speed, motor data, fluid properties, pipe sizes, tap elevations, and flow. If the pump head and measured flow do not agree with the applicable curve, investigate rotation, speed, impeller identification, valve lineup, air, fouling, gauge error, fluid properties, and whether the taps actually represent the intended stations.

Useful diagnostic patterns

A rising coil or strainer pressure drop at comparable flow can indicate added resistance, but comparisons require the same valves, fluid, and test boundaries. A low pump differential paired with low flow can reflect low speed, wrong rotation, air binding, internal wear, or a curve/configuration mismatch. A high differential paired with low flow can indicate a throttled or blocked path. These are diagnostic leads, not automatic conclusions.

Close the test by returning temporary valve or control changes to their authorized state. The record should make the result reproducible: identify the instrument, calibration status, fluid basis, tap locations and elevations, equations, raw pressures, conversions, flow, speed, operating mode, and unresolved limitations.