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Hot Water Secondary Return Sizing
Size a DHW secondary return circuit (CIBSE Guide G): array-first circuit heat loss, return flow to hold the design ΔT, pipe size and bronze pump duty, plus a dead-leg advisory.
The Ensign ToolboxHot Water Secondary Return SizingSize a DHW secondary return circuit (CIBSE Guide G): array-first circuit heat loss, return flow to hold the design ΔT, pipe size and bronze pump duty, plus a dead-leg advisory.
Circuit segments
No segments added yet — add each straight run of the secondary return circuit
Design temperatures
°C
HSE: store DHW ≥60°C
K
Return flow ṁ = Q ÷ (cp × ΔT); default 5 K — editable, see engine notes
°C
≥50°C general, 55°C healthcare (HSE / CIBSE Journal Module 87)
Insulation & ambient (shared across segments)
°C
Return pipe & limits
m/s
CIPHE Design Guide Table 17: circulation areas ≤1.5 m/s (noise)
Pa/m
CIBSE Guide B1 life-cycle criterion (≤200 Pa/m)
kg/m³
Default at 60°C (thermexcel.com); adjust if flow temp differs materially
Pa·s
Bronze pump duty
m
m head
%
Typical 5–15% on total head
%
Small bronze wet-rotor circulators typically 20–40%; larger end-suction 50–65%
m/s²
Dead-leg advisory
m
mm
Default = 15 mm copper internal bore
Add at least one circuit segment (length, OD, insulation) to size the return