Hot Water Recirc Loop Sizing (ASPE)
Per ASPE Data Book Vol. 4 Ch. 6: derives heat-loss rate, required GPM from set-point delta, 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.
Formula and source
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.
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