Commissioning flow rates

How a heat-pump water circuit is purged, balanced and proved against its design flow, temperature difference and manufacturer limits.

Commissioning proves that water can move through a heat-pump system at the rate its design and equipment require. It also checks that the flow is distributed to the rooms, that air and debris have been removed and that the controls are asking the system to operate as intended.

There is no single correct litres-per-minute figure for every heat pump. The target belongs to the selected unit, its operating output, the design temperature difference and the hydraulic arrangement.

Start with the design values

Commissioning uses the final system design, not just the heat-pump model number. The relevant values include:

room-by-room and whole-building design heat loss
outdoor design temperature
design flow and return temperatures
required heat-pump circuit flow rate
the manufacturer’s minimum and permitted flow range
pressure losses through pipework, valves, strainers, emitters and heat exchangers
pump selection and control mode
minimum system volume and any hydraulic separation

The current Heat Pump Association air-to-water commissioning checklist records the property heat loss, outdoor design temperature, design flow temperature, heating temperature difference and heat-pump circuit flow rate. Recording those together makes it possible to compare the installed system with the design.

How heat output, flow and temperature difference relate

For water, the heat carried by the circuit is approximately:

heat output (kW) = flow rate (litres/second) × 4.18 × temperature difference (K)

Rearranged into litres per minute, the approximate required flow is:

flow rate (litres/minute) = 14.3 × heat output (kW) ÷ temperature difference (K)

This is a useful sense check, not a substitute for the manufacturer’s data. The fluid may contain glycol, the heat pump may be modulating, and a buffer or low-loss header can create different flows on the primary and emitter sides.

Temperature difference, often written as delta T or ΔT, means flow temperature minus return temperature. A low measured difference can indicate high flow, low heat transfer or an operating point below design output. A high difference can indicate insufficient flow, restrictions or emitters removing more heat than expected. The reading only means something when the compressor and circuit are in a reasonably steady operating state.

Prepare the circuit before measuring it

A flow reading on a dirty or air-bound system proves little. Preparation for performance checks includes:

confirm that the pipework matches the final schematic
flush and clean the system using the specified procedure
fill with the correct treated water or heat-transfer fluid
purge high points, emitters, the heat pump and hydraulic components
clean strainers and magnetic separators after initial circulation
set the system pressure and expansion arrangement
open every valve needed for the test condition
confirm that non-return, bypass and zone valves are fitted and oriented correctly
check that sensors are attached in the right positions and read plausibly

Air can make an electronic flow sensor erratic and can stop parts of an emitter circuit heating. Debris can produce an apparently acceptable flow at first, then collect in a strainer and reduce it after handover. Rechecking after the initial run is therefore part of useful commissioning.

Measure the right circuit

The displayed heat-pump flow is helpful where the manufacturer declares how it is measured and its accuracy. A calibrated commissioning meter can provide an independent check. Pump speed or percentage is not itself a flow measurement.

In a direct circuit, the heat-pump flow and emitter flow are substantially the same. With a buffer, volumiser, low-loss header or plate heat exchanger, the design may contain two pumps and two distinct flow rates. Both sides need checking because a strong primary flow can conceal poor circulation through the house.

The test record identifies:

which meter or sensor produced the reading
the pump mode and setting
which zones and emitters were open
compressor state and approximate output
flow and return temperatures at the relevant points
any bypass or hydraulic-separation flow

Measurements taken during start-up, hot-water priority or defrost should not be mistaken for steady space-heating performance.

Balance the emitters

Balancing distributes flow so each room can receive its design heat. It is not achieved by opening every lockshield fully or forcing a fixed temperature drop at every radiator regardless of its load.

The installer starts from the design flow for each circuit or emitter, sets available balancing devices, then checks heat-up and temperature behaviour. The HPA checklist asks whether all emitters heat evenly with a similar temperature difference across flow and return. “Similar” is a commissioning check; the design still decides the appropriate individual flow.

Thermostatic valves, zone actuators and underfloor loops should be tested through their full operating sequence. The system must also retain the heat pump’s minimum flow when zones close, using the hydraulic solution in the design rather than an improvised open radiator.

Pump setting and pressure loss

The circulation pump has to deliver the target flow against the resistance of the complete circuit. Raising its setting may overcome a restriction, but it can also create valve noise, excessive electricity use and unwanted bypass flow.

If the design flow cannot be reached, investigate the cause before accepting a higher pump speed. Possibilities include:

undersized or unexpectedly long pipework
closed or incorrectly set valves
a blocked strainer
air in the circuit
a restrictive heat exchanger or buffer connection
an incorrectly selected pump mode
too many small zones closing at once
a commissioning sensor or flow meter error

Microbore pipe does not automatically fail a heat-pump design. Its length, circuit arrangement and required room flow determine whether pressure loss and velocity are acceptable.

Weather compensation and final settings

Flow rate is commissioned alongside controls. Weather compensation normally varies flow temperature with outdoor conditions, allowing the heat pump to run cooler in mild weather. The maximum design flow temperature is therefore a cold-weather endpoint, not necessarily the everyday setting.

The handover record should include the weather-compensation curve, maximum flow temperature, pump mode, zone logic and hot-water priority settings. A temporary high fixed flow temperature used for testing should not be left as the normal operating mode.

The engineer should run space heating and domestic hot water separately, then prove recovery from defrost where practicable. Backup or immersion heaters must be checked without leaving them enabled as an unnoticed substitute for inadequate heat-pump output.

Commissioning record

The commissioning pack records:

design and measured heat-pump circuit flow
design and observed flow and return temperatures
pump make, model, mode and setting
system pressure and heat-transfer fluid details
flushing, water-treatment and strainer checks
balancing settings or schedule
weather-compensation and control settings
any buffer, bypass or hydraulic-separation arrangement
faults, departures from design and corrective action

A tick saying “system balanced” is weak evidence without the values and conditions behind it.

Signs that flow needs investigation

Frequent flow alarms, noisy valves, rapid cycling, a large unexplained temperature difference, rooms that never warm, or reliance on backup heat all justify a commissioning review. The right response is to compare design, hydraulic measurements and control behaviour, not to change random settings until the alarm stops.