How do I size a DHW return pump?

Calculate heat loss from the return pipe run, convert to required circulation flow to maintain temperature at the index outlet, then add pump head for pipe friction.

What temperature drop is acceptable on the return loop?

Typical practice: 5–10°C drop across the index run. Tighter drops need higher flow and larger pipe — balance against pump energy.

When is a return loop required?

Where the index draw-off is more than ~15 m or 30 s from the source, or where the brief requires instant hot water at remote outlets.

BuildFlo Design Guide · Water · CIBSE / BS 8558

DHW return circulation pump calculator

Size secondary hot water return flow and pump head from pipe heat loss. Free DHW return pump calculator.

What this calculator does

DHW return pump helps UK building services engineers with secondary hot-water return flow and pump head from pipe heat loss. The calculation follows CIBSE / BS 8558 methods used in everyday MEP design — enter your project inputs below for a first-pass result.

Whether you searched for "dhw return pump", "dhw return pump calculator", or "how to dhw return pump", this page gives you a working tool plus the context to use it on a real job.

When to use it

Use this for early-stage sizing and sanity checks on UK building services work. It is not a substitute for issued calculations — verify against the project specification, drawings, and relevant standard before sign-off.

How to use it

1. Fill in the inputs for your scenario (defaults are starting points only). 2. Click Calculate — results include the method and assumptions where applicable. 3. Verify against the project specification and relevant standard before issuing.

Typical output unit: l/s.

DHW return pump calculator

Secondary hot-water return flow and pump head from pipe heat loss

Heat loss basisHot water flow temperature (°C)Hot water return temperature (°C)Surrounding air temperature (°C)Total flow pipe length (m)Total return pipe length (m)Index leg - flow (m)Index leg - return (m)Flow pipe heat emission (W/m·K)

Used when heat loss basis is emission

Return pipe heat emission (W/m·K)Flow pipe heat loss (W/m)

Used when heat loss basis is linear

Return pipe heat loss (W/m)Index flow pressure drop (Pa/m)Index return pressure drop (Pa/m)Balancing valve loss (kPa)Non-return valve loss (kPa)Heat exchanger loss (kPa)Other fittings loss (kPa)Flow margin (%)Head margin (%)Minimum circulation flow (l/s)

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Tips

Defaults are indicative — replace with project-specific values from the architect, structural, or MEP spec.
For issued calculations, save results in your BuildFlo project and attach to the calculation folder.
Need the regulation citation? Open BuildFlo chat in the app and ask about dhw return pump under UK standards.

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Frequently asked questions

How do I size a DHW return pump?: Calculate heat loss from the return pipe run, convert to required circulation flow to maintain temperature at the index outlet, then add pump head for pipe friction.
What temperature drop is acceptable on the return loop?: Typical practice: 5–10°C drop across the index run. Tighter drops need higher flow and larger pipe — balance against pump energy.
When is a return loop required?: Where the index draw-off is more than ~15 m or 30 s from the source, or where the brief requires instant hot water at remote outlets.