Primary pipework sizing

How heat duty, design temperature drop and hydraulic resistance determine heat-pump primary pipework and pump selection.

Primary pipework carries heating water between the heat pump and the point where heat is distributed or hydraulically separated. It has to carry the design heat output at the intended water temperature drop without exceeding the available pump head, creating unacceptable noise or compromising the heat pump’s minimum-flow requirement.

There is no universal pipe diameter for a given heat-pump model size. The answer depends on internal bore, length, fittings, valves, fluid, circuit arrangement and the actual design duty.

Start with heat and temperature difference

The required water flow follows the heat-transfer relationship:

heat transferred = mass flow × specific heat capacity × temperature difference

For a given heat duty, a smaller flow-to-return temperature difference requires more flow. A larger difference reduces flow but changes emitter temperatures and may fall outside the heat pump or system design.

The design temperature difference should come from the selected heat pump, emitter design and manufacturer requirements. A blanket assumption applied to every product can give the wrong flow.

The calculation should cover the highest simultaneous duty expected on that circuit. Space heating and cylinder charging may have different flows, valve positions and hydraulic paths, so both operating modes need checking.

Internal diameter is what matters

Nominal pipe descriptions do not establish the internal waterway. Pipes with the same outside diameter can have different wall thicknesses and bores depending on material and system.

Hydraulic calculations should use:

actual internal diameter
total straight length on the relevant index circuit
equivalent resistance of bends, tees and transitions
valves, strainers, filters and meters
heat exchangers, manifolds and hydraulic separators
antifreeze or other fluid concentration and design temperature

A simple external-size table cannot account for those differences.

Pressure loss and the pump curve

Water loses pressure as it travels through pipe and components. Resistance rises quickly with flow, so a marginal circuit can become much harder to pump when the required flow increases.

The designer adds the pressure losses around the hydraulically most demanding path and checks them against the pump’s available head at the design flow. The useful figure is residual head available to the external system, not the pump’s maximum head at zero flow.

Integrated heat-pump pumps still need this check. Their available head may already be reduced by internal heat exchangers, valves or hydraulic modules. Where an external pump is used, its selected operating point should sit on the relevant pump curve with suitable control authority.

Velocity is a check, not the sizing method

Excessive water velocity can increase noise, erosion risk and pressure loss. Very low velocity can make air removal and heat distribution harder in some arrangements. Acceptable limits depend on the pipe material, location and system guidance.

Selecting a pipe from a generic velocity rule alone is incomplete. The final choice needs to satisfy heat duty, pressure loss, pump head, acoustic requirements and manufacturer limits together.

Existing and microbore systems

Existing small-bore pipework is not automatically unsuitable for a heat pump. Its suitability depends on the flow required through each section and the resistance of the complete route.

A short branch serving one modest emitter has a different duty from a long main carrying the full building load. Problems often occur where several radiators share a restrictive main, old valves have small waterways or a one-pipe system is treated as a modern two-pipe circuit.

The survey should record accessible material and size, trace circuit layout where possible and identify concealed sections. If the route cannot be confirmed, the design should state the assumption and the commissioning plan for verifying flow.

Minimum flow and active water volume

The heat pump may specify a minimum flow rate and a minimum active system water volume. Both must be available in every relevant operating state, including when thermostatic valves or underfloor actuators close.

These are separate requirements:

pipe size helps determine whether the required flow can be circulated
active volume determines how much water remains thermally available to the heat pump

A bypass can preserve flow but may send warm water directly back to the unit and promote cycling. A volumiser adds series water volume. A buffer or low-loss header can separate flows. None should be added without identifying the hydraulic problem it is meant to solve.

Direct and separated circuits

In a direct system, the heat-pump flow and emitter flow are substantially the same. This can minimise mixing and pump energy when the circuit can maintain the required flow.

A four-pipe buffer, low-loss header or plate heat exchanger creates primary and secondary circuits with separate flows and pumps. If those flows do not match, mixing can raise the return temperature or lower the emitter supply temperature. The design must therefore calculate both sides and the controls must coordinate them.

Hydraulic separation can be justified by multiple circuits, incompatible fluids, complex zoning or pump-head constraints. It is not a substitute for sizing the pipework on either side.

Antifreeze and external pipework

Where a monobloc circuit contains antifreeze, its concentration changes fluid density, heat capacity and viscosity. That affects both required flow and pressure loss. The hydraulic calculation and pump selection should use the fluid properties at the design concentration and temperature.

External water pipework also needs the specified insulation, weather protection and freeze strategy. Increasing antifreeze concentration beyond the design value can add pumping resistance and reduce heat transfer, so it should not be treated as a harmless extra margin.

Installation details that affect the calculation

The installed route should match the design. Extra flexible hoses, undersized isolation valves, strainers, non-return valves and unrecorded fittings all add resistance. Flexible connections should have a suitable bore and bend radius rather than becoming the narrowest part of the primary circuit.

Air vents, dirt separation, flushing points, drains and service access need positions that allow the system to be commissioned and maintained. Pipe support, expansion and vibration isolation also matter, especially near the unit.

As at 22 July 2026, MIS 3005-I:2025 is the MCS installation and commissioning standard used for this entry. It requires commissioning against a documented procedure and the manufacturer’s requirements. The exact hydraulic design remains product and system specific.

Commissioning evidence

Commissioning should establish that the calculated duty can be circulated. Useful records include:

design and measured flow in each operating mode
flow and return temperatures under a meaningful load
pump model, setting and available-head basis
valve and balancing settings
fluid type and antifreeze concentration
filter and strainer condition
primary and secondary measurements where circuits are separated

A measured temperature difference alone does not prove flow unless heat output and operating conditions are also understood. Equally, a displayed flow value should be checked against sensor accuracy and the location in which it is measured.