Sizes a DHW secondary return circuit: circuit heat loss, the return flow needed to hold your design ΔT, the smallest pipe that meets velocity/pressure-drop limits, indicative bronze pump duty, and a dead-leg volume check.
Tender-stage only. Verify final design to project spec and CIBSE Guide G.
For design guidance only. Always verify with a qualified engineer.
About this hot water secondary return calculator
This free hot water secondary return calculator sizes a domestic hot water (DHW) secondary return circuit, the circulating loop that keeps stored hot water at temperature at every outlet, so occupants don’t wait and dead legs stay short. It is aimed at building services engineers, plumbing designers and M&E contractors sizing the return leg, pump and insulation at tender stage. Enter your circuit segments (length, pipe outside diameter and insulation thickness) plus the flow temperature and design ΔT, and it returns the circuit heat loss, the required return flow, the smallest pipe meeting the velocity and pressure-drop limits, an indicative bronze pump duty and a dead-leg volume check. Everything runs in your browser. Nothing is uploaded.
How the secondary return is sized
Circuit heat loss uses the cylindrical-resistance method: per segment Q′ = ΔT ÷ (Rcond + Rsurf), where Rcond = ln(r₂÷r₁) ÷ (2πk) is the insulation resistance and Rsurf the surface film. The return flow needed to hold your design ΔT is ṁ = Q(kW) ÷ (cp × ΔT) with cp = 4.186 kJ/kg·K. The return pipe is chosen by Darcy–Weisbach with a Colebrook–White friction factor: the smallest DN tier whose velocity and pressure-drop per metre both sit under your limits. Indicative pump duty is Ph = ρ·g·Q·H, divided by efficiency for input power, and the dead-leg volume V = (π÷4)·d²·L is checked against a 0.5 L advisory. The method follows the CIBSE Guide G lineage; the 0.5 L figure is a CIBSE Journal CPD advisory, not a single statutory HSE limit.
Frequently asked questions
What design ΔT should I use for the secondary return?
The temperature drop the return flow is sized to hold is editable, a small ΔT (say 5 K) keeps every outlet close to the flow temperature but needs more return flow and a larger pump. It is not a single verified CIBSE Guide G clause, so set it to your project’s circulation strategy. A separate minimum-return-temperature check (50°C general, 55°C healthcare) flags whether the returned water still meets anti-Legionella guidance.
Why is 1.5 m/s the default maximum velocity?
In circulation areas CIPHE guidance limits velocity to about 1.5 m/s to avoid erosion-corrosion and noise, and CIBSE Guide B1 caps the pressure-drop rate near 200 Pa/m on life-cycle grounds. The calculator steps through DN15–DN50 and picks the first size meeting both; if none does, it reports the circuit as undersized.
What is the dead-leg check telling me?
It works out the water volume held in a representative branch (the pipe from the circulating loop to an outlet) and compares it to a 0.5 L advisory limit, longer or wider dead legs cool and stagnate, raising Legionella risk. It is an advisory prompt to keep branches short, not a statutory pass/fail; HSE guidance sets no single minimum length or ratio. Always verify the final design against project spec and CIBSE Guide G.