Short answer: To commission water underfloor heating, fill and flush each loop on its own, vent the air, set each loop's flow rate, set the pump and blending valve, prove every thermostat opens the right actuators, then run the first heat-up. The functional heat-up to BS EN 1264-4 starts 21 days after a cement screed or 7 days after an anhydrite screed, at 20 to 25°C for 3 days, then the maximum design flow temperature for 4 more days. It proves the system works, but it doesn't prove the screed is dry: test the moisture before the floor finish goes down.
Commissioning is the stage between a pressure-tested, screeded floor and a working heating system. Done well, every room heats evenly and the screed dries without cracking. Skipped, it is the most common reason a new floor feels lukewarm. This guide gives the order of work, the flow rate sums, the two kinds of screed heat-up and a commissioning record you can copy. For the install itself, see how to install water underfloor heating.
What does commissioning underfloor heating involve?
Commissioning means testing and adjusting the system until it works as designed and uses no more energy than it needs. In England, Part L requires fixed heating systems in dwellings to be commissioned this way.
- Check the pressure test sheet and the screed age.
- Fill, flush and vent each loop.
- Set the flow rate for each loop.
- Set the pump and the flow temperature.
- Prove each zone: thermostat, actuators, pump and heat source.
- Run the first heat-up schedule for your screed.
- Test the screed moisture before the floor finish.
- Fill in the commissioning record and hand it over.
How do I fill, flush and vent underfloor heating loops?
Fill and flush one loop at a time, with the other loops closed, so the full flow pushes air and debris out of that loop. Flushing all loops together lets water take the easiest path and leaves air in the longest loops.
- Close the manifold's flow and return ball valves.
- Set all flow meters or flow setting valves fully open.
- Close every loop except the first, using the actuator valve caps or flow meter shut-offs.
- Connect a fill hose to the flow bar fill valve and a drain hose to the return bar drain valve.
- Run water through until it comes out clear and free of air, then close that loop and open the next.
- When every loop is done, open all loops and let the manifold air vent clear the bars.
- Bring the system up to normal working pressure for your heat source, then open the ball valves.
If you need antifreeze, use a nitrite-free glycol dosed to the maker's instructions. If air keeps coming back after filling, check the air vent and repeat the loop-by-loop purge.
How do I work out the flow rate for each loop?
Each loop needs enough flow to carry its heat output at the design temperature drop between flow and return. Flow in litres per minute is the loop's output in watts divided by 70 times the temperature drop in °C.
The 70 comes from water's heat capacity: 4.19kJ per kg per °C, converted to litres per minute. A smaller temperature drop needs more flow.
| Loop output | Flow at 5°C drop | Flow at 7°C drop |
|---|---|---|
| 500W | 1.4 l/min | 1.0 l/min |
| 750W | 2.1 l/min | 1.5 l/min |
| 1000W | 2.9 l/min | 2.0 l/min |
| 1250W | 3.6 l/min | 2.6 l/min |
Worked example. A 12m² kitchen loop with a design output of 60W/m² gives 720W. At a 5°C drop, 720 ÷ (70 x 5) = 2.1 l/min. A 7m² utility loop at the same output gives 420W, or 1.2 l/min. Set each flow meter to its figure, starting with the loop that needs most flow.
Our auto-balancing manifold lets you preset each loop's flow with its setting key, and holds it as other zones open and close. Its flow meters read 0 to 4 l/min. For hand balancing with flow meters, see balancing underfloor heating loops.
How do I set the pump and the flow temperature?
Set the pump so the loop with the highest demand reaches its flow rate, and set the flow temperature to suit the stage you are at: low for the first heat-up, then the design temperature.
- Grundfos UPM3 Auto: AUTOADAPT sets the pump to the system on its own. It also has proportional pressure, constant pressure and constant curve settings.
- UPM3 Flex on a non-mixing set: the heat pump can control pump speed by PWM. Without a PWM signal, use one of its pump settings.
- Blending valve: our thermostatic valve mixes to between 20 and 55°C and needs no wiring. Set it to 20 to 25°C for the start of the first heat-up.
- Heat pump on a non-mixing set: there is no blending valve, so you set the flow temperature on the heat pump controller.
Never let the water at the pipes exceed 55°C in a cement or anhydrite screed, or the screed maker's lower limit if it has one.
What is the difference between functional heating and drying heating?
Functional heating proves the system and the screed can take heat. Drying heating is a longer, separate schedule that uses the floor to drive moisture out of the screed so it is ready for the floor finish.
Functional heating follows BS EN 1264-4: 20 to 25°C for 3 days, then the maximum design flow temperature for 4 days, starting no earlier than 21 days after a cement screed or 7 days after an anhydrite screed. Our install guide has the full table.
Drying heating is set by the screed maker. As an example, one calcium sulphate screed maker gives this schedule from 7 days after laying:
| Step | Flow temperature | Time |
|---|---|---|
| Start | About 20°C | 24 hours |
| Raise | +5°C a day | Until 45°C |
| Hold | 45°C | 5 days |
| Lower | -5°C a day | Back to the start |
| Off | Cool | 48 hours, then test |
Use the schedule from your own screed's data sheet. Ventilate while heating, so the moisture you drive out can leave the building.
How do I know the screed is dry enough for the floor finish?
Test it. Neither heat-up proves the screed is dry, and the floor layer is responsible for checking it is ready before laying tiles, wood, vinyl or carpet.
- Sand and cement screed: commonly tested with a hygrometer. Most coverings, including tiles and resilient floors, need the surface to read below 75% relative humidity. Wood often needs drier.
- Calcium sulphate (anhydrite) screed: the makers prefer a CM test or an oven-dried sample, giving % moisture content. One maker gives 0.5% for tiles in cement-based adhesive and 0.4% for bonded engineered wood.
Turn the heating off and let the floor cool before testing, as your screed maker specifies. Drying rates and depths are covered in our guide to screed depths and drying times.
How do I test the thermostats and zones?
Turn each thermostat up on its own and check that only its actuators open, the pump runs and the heat source fires. Then turn it down and check they close and stop.
- Set every thermostat to frost or off.
- Call for heat on one zone. Its wiring centre zone light should come on.
- Check the right actuators open. Thermal actuators take a few minutes to open fully.
- Check the pump starts and the boiler or heat pump receives the call.
- Feel the loop's flow and return pipes warm at the manifold.
- Turn the zone off and repeat for the next.
- Set the floor limit on any floor-sensing thermostat to suit the floor finish.
What goes in a UFH commissioning record?
A commissioning record lists what was set and measured, so the owner and any future engineer can see how the system should run. Some screed makers require a signed record before the floor finish goes down.
| Item | Record |
|---|---|
| Pressure test | 6 bar, 60 minutes, date, signed |
| Screed | Type, laid date, heat-up dates |
| System pressure | Cold fill pressure |
| Flow temperature | Valve or heat pump setting |
| Each loop | Room, length, flow in l/min |
| Pump | Mode or speed setting |
| Zones | Thermostat to actuator map |
Leave the record, the loop plan and the product instructions next to the manifold or with the homeowner.
Products for this job
- Auto-balancing stainless steel manifold: from £62 for 2 ports
- Blending valve pump pack with Grundfos UPM3 Auto: £266
- Non-mixing pump set with UPM3 Flex: £207
- Manifold flow meter: £10
- Pressure and temperature gauge for mixing units: £10
- Manifold ball valve set 1" with temperature gauges: £26 a pair
- Heatmiser UH8 wiring centre: £77
Prices inc VAT, October 2026. Browse pumps, actuators and valves.
Start with the right design: MaxHeatUFH ProLoop is free. Add your rooms and floor type, and get a loop design with loop lengths and a full kit list with prices, ready for commissioning. Start your design
Frequently asked questions
When can I turn on underfloor heating after screeding?
For the functional heat-up to BS EN 1264-4, wait at least 21 days for sand and cement screed or 7 days for anhydrite screed, or longer if your screed maker says so. Start at 20 to 25°C.
Can I use underfloor heating to dry the screed?
Only by following a drying schedule from your screed maker, after the earliest start date. Don't turn the heating straight to full to speed things up, as that can crack the screed.
What flow rate should an underfloor heating loop have?
It depends on the loop's heat output and the design temperature drop. A 720W loop at a 5°C drop needs about 2.1 l/min. Divide the watts by 70 times the drop.
What temperature should I set the blending valve to when commissioning?
Start at 20 to 25°C for the first 3 days of the heat-up, then raise it to the design flow temperature. Keep the water at the pipes at or below 55°C in screed.
Does the heat-up mean I can lay tiles?
No. The heat-up shows the system works. Test the screed moisture before laying any floor finish.
Who should commission underfloor heating?
A heating engineer usually commissions the heat source connection, and an electrician tests the wiring. Installers can complete the loop settings and record.
How long does commissioning take?
Filling, flushing and setting flows takes a few hours for a typical manifold. The heat-up schedule takes at least 7 days, and a drying schedule two weeks or more.
Sources: Approved Document L (gov.uk), IMI Hydronic UFH commissioning instructions (EN 1264-4), Gyvlon post-installation guidance, Ronacrete: drying of screeds (BS 8204)