Hot Water Recirc Loop Sizing (ASPE)

Per ASPE Data Book Vol. 4 Ch. 6: derives heat-loss rate, required GPM from set-point delta.

Run the calculator

Example

You enter

Loop length (ft) 200
Nominal copper size (in) 0.75
Insulation thickness (in; 0 / 0.5 / 1 / 1.5) 1
Hot supply temperature (°F) 120
Ambient temperature surrounding pipe (°F) 65
Set-point delta (°F) 10

You get

Heat-loss rate per ft (Btu/hr/ft) 9.35
Total loop heat loss (Btu/hr) 1870
Required recirc flow (GPM) 0.374

Details, formula, and sources

Friction head via Hazen-Williams, and recommended pump size from loop length / pipe size / insulation / temperatures. ASHRAE 90.1-2022 §7.4.4 governs recirc control.

q_per_ft = U * (T_hot - T_ambient). Q_total = q_per_ft * L. GPM = Q_total / (500 * set-point-delta). Friction head via Hazen-Williams (C=140 for copper). Pump-size ladder: 1/40 / 1/25 / 1/20 / 1/12 / 1/6 / 1/4 HP, with 25% wet-rotor wire-to-water efficiency factor.

ASPE Data Book Vol. 4 (Plumbing Engineering Design Handbook) Chapter 6 simplified per-foot heat-loss method. ASHRAE 90.1-2022 §7.4.4 for recirculation control.

aspe.org for TOC.

Estimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.

Field names used by the API: loop_length_ft, nominal_size_in, insulation_in, hot_supply_F, ambient_F, set_point_delta_F, q_per_ft_btu_hr, Q_total_btu_hr, gpm_required

Pipe material Type L copperASPE typical residential recirc loop
Loss-coefficient table U (Btu/hr/ft/°F-delta) by nominal size (0.5 / 0.75 / 1 / 1.25 / 1.5 in) and insulation thickness (0 / 0.5 / 1 / 1.5 in)ASPE Data Book Vol. 4 Ch. 6 simplified per-foot losses
Insulation interpolation linear in insulation thickness between bundled break pointsengineering practice
Pump efficiency 25% wire-to-water for small wet-rotor circulatorsmanufacturer typical (Taco / Grundfos / Bell & Gossett small circulators)
Pump-size ladder 1/40, 1/25, 1/20, 1/12, 1/6, 1/4 HP next-standard roundingcommon North American residential / light-commercial circulator sizes
Length floor loops below 50 ft warned: 'may not need recirc; consider point-of-use heater'spec-v9 §B.4
Insulation floor 0-in insulation flagged as non-compliant for most ASHRAE 90.1 jurisdictionsASHRAE 90.1-2022 §7.4.4