A water underfloor heating system can look complete long before it is ready to heat a room. Pipe may be clipped down, the manifold fitted and controls wired, but trapped air, incorrect flow rates or an unsuitable mixing temperature can still leave cold areas and call-backs. This water system commissioning guide explains the practical checks needed to bring a wet UFH installation into service safely and efficiently.

For homeowners, commissioning provides assurance that the system has been installed as designed before finishes make access difficult. For installers, it is the point at which the pipework, manifold, pump, controls and heat source must operate as one system. Do not treat it as a quick final switch-on.

Start commissioning before water enters the system

A thorough visual inspection should happen before filling the system. Check that every circuit is clearly labelled at the manifold and matches the room plan. Confirm that pipework is properly supported, bends are not kinked, and no pipe has been damaged during later building work.

At the manifold, check that flow meters, isolation valves, drain points and automatic air vents are fitted correctly and accessible. Flow meters should be clean and able to move freely. Ensure each loop has its correct actuator or manual cap, and that the supply and return connections have not been crossed.

The system specification matters here. A low-profile retrofit system may have shorter loops and different resistance characteristics from a large new-build installation in screed. Likewise, a heat pump-led design generally needs lower water temperatures and carefully considered flow rates. Commission against the design, not simply against what appears to work on the day.

Before filling, have the following information available:

  • the circuit schedule, including design flow rates and room names
  • the manifold, pump and mixing valve instructions
  • the heat source requirements, particularly minimum flow and temperature limits
  • the floor build-up and screed manufacturer’s heating protocol
  • the selected water treatment requirements and pressure-test record

Filling and flushing each underfloor heating loop

Air is one of the main reasons a new water system performs poorly. It can cause noise, unstable flow readings and rooms that heat slowly or not at all. The most reliable approach is to flush one circuit at a time rather than attempting to fill the entire manifold together.

Close all loops at the manifold, then open one circuit fully. Connect the filling and drain hoses to the appropriate manifold valves, directing discharge safely to a suitable drain. Flush the open loop until the water runs clear and free from bubbles. Then close that loop, open the next and repeat until every circuit has been flushed.

This takes longer than a general fill, but it is usually time well spent. Long pipe runs, changes in level and tightly spaced circuits can retain air pockets that a quick fill will not remove. Once each loop is clear, reopen the circuits and fill the system to the pressure specified for the installation.

For sealed heating systems, the normal cold operating pressure is often around 1 to 1.5 bar, although the final figure depends on property height, equipment and manufacturer instructions. Do not confuse normal operating pressure with the higher pressure used for pipe testing. If the UFH system connects to an existing heating circuit, the rest of that system may also need flushing and treatment before final handover.

Add inhibitor or other approved treatment at the correct concentration once flushing is complete. Where glycol is required for frost protection or a specific heat pump arrangement, use only a product approved for the system components and allow for its effect on pump performance and heat transfer. More glycol is not automatically better.

Pressure testing the pipework

Pressure testing confirms that the pipework and manifold connections are sound before the floor is covered or brought into regular operation. Test pressure, duration and procedure should always follow the pipe manufacturer’s instructions and the project specification. Pipe systems are not all tested to the same criteria.

In many installations, the circuits are pressure tested before screeding and kept under pressure while the screed is laid. This helps identify accidental damage immediately. Record the starting pressure, the time, ambient conditions and the final reading. A minor pressure change can result from water temperature changing, so investigate alongside temperature rather than assuming every movement indicates a leak.

Never use compressed air as a casual substitute for a proper water test unless the pipe manufacturer explicitly permits it and the installer is working to an approved method. Pressurised air stores significantly more energy and can create a serious safety risk if a component fails.

After the floor build-up is complete, inspect the manifold again for weeping joints, damaged gauges and loose connections. A system that passed a pre-screed test still needs checking after other trades have finished work nearby.

Set the manifold flows to the design

Filling a system proves that water can move through it. Balancing proves that the right amount reaches each room. Every loop has a different length and heat requirement, so opening every flow meter to the same position is rarely correct.

Start with all relevant zone valves and actuators open. Set each manifold flow meter to the rate stated on the design schedule, commonly expressed in litres per minute. Longer loops may need a higher setting, while a small en-suite or well-insulated room may need less. Adjust in small increments and allow the readings to settle before moving on.

If a circuit will not achieve its target flow, do not simply force every other loop down. Check first for an air lock, a closed valve, a stuck flow meter, a pinched pipe or insufficient pump head. On larger systems, the pump setting and differential pressure bypass arrangement may need attention as well.

Balancing should also reflect how the property will be used. An open-plan area with large glazing may have a different heat-loss profile from an internal bedroom, even when the floor areas appear similar. The design output, pipe spacing, floor covering and heat source capacity all affect the final settings.

Commission the pump, blending controls and heat source

Water UFH normally operates at a lower flow temperature than radiators. A manifold pump and blending arrangement may therefore be needed where the primary system runs hotter. Set the mixing valve in line with the design temperature, then confirm the actual flow temperature at the manifold once the system is running.

For a boiler system, avoid setting UFH temperatures unnecessarily high in the hope of faster warm-up. Floor heating has a large thermal mass, particularly in screed. Excessive flow temperatures can make control less stable, reduce efficiency and risk damaging sensitive floor finishes.

For heat pumps, low-temperature operation is a core part of efficient performance. The system must still meet room heat losses, but the target is usually the lowest practical flow temperature with adequate circulation. Check the heat pump’s required minimum system volume, flow rate and control arrangement. A poorly configured pump group or incorrectly closed actuators can lead to flow faults even where the pipework itself is sound.

Verify that the manifold pump runs in the intended mode and that its speed is suitable for the calculated resistance. A high setting may create noise and waste electricity; too low a setting can leave distant circuits under-supplied. There is no universal setting that suits every manifold.

Test room controls and zone operation

Each thermostat should call for heat, open its corresponding actuator where fitted, and communicate correctly with the wiring centre and heat source. Check zone by zone rather than assuming that a powered wiring centre is correctly configured.

Set a thermostat above room temperature and allow enough time for the actuator to open. Confirm that the relevant manifold circuit shows flow, then verify that the pump and boiler or heat pump receive a demand where the control strategy requires it. Repeat the process for every zone, including towel rail or secondary heating controls if they are linked to the same system.

Pay particular attention to floor temperature limitation in bathrooms, timber floors and rooms with manufacturer-imposed floor surface limits. The thermostat sensor type and location matter. A misplaced floor probe can create inaccurate readings, while no probe where one is required can put a finished floor at risk.

Bring the floor up to temperature gradually

Do not run a newly screeded floor at full temperature to see whether it works. Cementitious and anhydrite screeds require curing time and a controlled heat-up programme. Follow the screed manufacturer’s instructions, as the timing and permitted temperature increases vary by product.

A typical programme raises the water temperature gradually in small daily steps, holds it as specified, then reduces it gradually before floor finishes are fitted. Record the programme, particularly on projects where flooring warranties depend on correct commissioning. Timber, vinyl and other temperature-sensitive finishes may have stricter surface limits than tile or stone.

Once the system is operating, walk the rooms and check for consistent warming after adequate run time. Underfloor heating is designed for even, steady comfort rather than the rapid response of a small radiator. A cold strip near a manifold, persistent air noise or a room that lags well behind its neighbours deserves investigation before handover.

Record the settings and hand over with confidence

A good commissioning record should include circuit flow settings, test pressures, water treatment used, flow temperatures, pump settings, thermostat locations and the heat-up programme. Give the customer clear instructions on normal operation, including why frequent large thermostat setbacks may not suit a high-mass floor.

For complex projects, this record is also valuable months later when flooring changes, a heat source is upgraded or a service engineer needs to diagnose performance. The Underfloor Heating Company can help with the system components and technical guidance needed to match a manifold, controls and floor build-up to the project.

Commissioning is where careful installation becomes dependable comfort. Taking the time to flush, balance and document the system protects the floor, supports efficient running costs and gives everyone confidence that the heating is ready for the years ahead.