PlumbingIntermediateDomestic Hot Water Recirculation

Hot-Water Recirculation-Pump Head Calculator

Calculate signed pump head from suction/discharge pressures, gauge elevation difference, and fluid specific gravity.

Why it matters: A visible equation keeps the calculation boundary and user-entered criteria available for technical review.

User inputs

Enter field measurements

5 inputs
Select I-P or SI display units; convert the complete input set before calculating.
psi or kPaSynchronized suction-gauge pressure.
psi or kPaSynchronized discharge-gauge pressure.
ft or mSigned elevation difference between gauge taps.
ratioSpecific gravity at test condition.

Calculated result

Engineering evaluation

Calculation complete—verify project criteria
Recirculation-pump differential head (H_p)25.1 ft
Pressure differential (ΔP)
10 psi

Engineering interpretation

The entered gauge condition yields 25.1 ft signed pump head.

Important limitations

  • Screening calculation only. Adopted codes, project criteria, manufacturer data, measurement uncertainty, and professional engineering judgment remain controlling.
  • Gauge locations, velocity head, check valves, balancing valves, pump curve, minimum flow, and instrument uncertainty require review.

Motor-driven equipment

Estimate electrical demand and energy use

Use nameplate information and an operating schedule to screen motor input power, current, annual energy, and operating cost. Results update automatically.

Optional companion calculation
Choose the motor nameplate output-power convention.
Rated shaft output, not electrical input.
Used to estimate current; it does not change the hp-based energy result.
Use efficiency at the evaluated load point when available.
Use measured or defensible operating load rather than motor nameplate size alone.
Used only for the estimated current calculation.
Assumes all selected motors operate concurrently at the entered load.
Replace the profile with the actual occupied, process, or control schedule.
$/kWh
Use the applicable marginal utility energy charge; demand charges are not included.

Estimated result

Total motor input power (Pin,total)6.21 kW
Current per motor (I)
9.18 A
Total coincident current (Itotal)
9.18 A
Daily energy (Ed)
50 kWh/day
Annual operating hours (Hy)
2,080 h/year
Annual energy (Ey)
12,925 kWh/year
Annual energy cost (Cy)
$1,551/year

Equations and assumptions

Nameplate-based screening method

Pin=PratedLηmI=1000Pin3VPFEy=NPinhddyCy=Eyce

Horsepower is converted using 1 hp = 0.7457 kW. This estimate assumes steady average load and the entered operating-point efficiency. Voltage and power factor affect estimated current, but hp, load, efficiency, quantity, and schedule establish the energy estimate. VFD speed profiles, cycling, standby power, demand charges, and process variation require separate evaluation.

Technical basis: U.S. Department of Energy — Determining Electric Motor Load and Efficiency

Engineering knowledge center

Understand the calculation—not just the answer

2026-08-07 · Free Library
Governing relationships
H_p=2.31(P_d−P_s)/SG+Δz (I-P)
H_p=(P_d−P_s)/(9.80665SG)+Δz (SI)
displayed symbols: ΔP
Current substitution

H_p=25.1 ft from P_d=40, P_s=30, SG=1, Δz=2.

Calculation sequence

Step-by-step method

  1. Confirm the input boundary, formula symbols, and unit system.
  2. Apply the displayed equation to the user-entered values.
  3. Compare the screening result with project criteria, manufacturer data, and applicable standards.

Equation legend

Inputs and calculated quantities

P_s
Pump suction pressure — psi or kPa
P_d
Pump discharge pressure — psi or kPa
Δz
Discharge-minus-suction gauge elevation — ft or m
SG
Fluid specific gravity — ratio
H_p
Recirculation-pump differential head — ft
ΔP
Pressure differential — psi

Engineering calculation disclaimer

These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.

Read the full use and limitation statement