Switching on underfloor heating for the first time: what to expect

The first switch-on is the one time in the life of an underfloor heating system when going slowly genuinely matters. A new screed still holds construction moisture, and driving heat into it too early or too hard can crack it. The pipework, fittings and controls also get their first real workout, so the first week is when small commissioning issues show themselves.

This guide covers when you can switch on, the first heat-up routine for screeded systems, what a normal first warm-up feels like, what noises and readings to expect, and the equivalent checks for electric systems. It assumes the system has already been filled, flushed and pressure tested: if not, start with commissioning underfloor heating.

How soon after screeding can you turn underfloor heating on?

For a cement (sand and cement) screed, heating can begin 21 days after laying at the earliest; for a gypsum-based screed, 7 days after laying is complete. Those waiting periods come from the EN 1264-4 heat-up protocol used across the industry, and they exist because a screed that is forced dry too early can curl and crack over the pipes.

Waiting 21 days is not the same as the screed being dry. Full drying for floor coverings takes much longer and depends on thickness: our guide to screed depth and drying time has the sums. The first heat-up is actually part of that drying process: it drives moisture out in a controlled way. Always check your screed supplier's own instructions too, because some flowing screeds specify their own earliest heating date, and a liquid screed installer's warranty can depend on it.

What is the first heat-up routine for a screeded floor?

EN 1264-4 sets a two-stage routine, and it is worth following to the letter and keeping a record of:

1. Wait the curing period. 21 days for cement screed, 7 days for gypsum-based screed, or longer if the screed maker says so.
2. Stage one: 25°C flow for 3 days. Set the mixing valve or heat source so the water entering the loops is around 25°C and leave every loop open and running for 3 days.
3. Stage two: maximum design temperature for 4 days. Raise the flow to the system's maximum design temperature and hold it for 4 days. For context, the general ceilings in screed systems are 55°C for cement and calcium sulphate screeds and 50°C for gypsum, though a well-designed UFH system runs below its ceiling.
4. Record it. Note the dates, temperatures and anything unusual. Screed suppliers and flooring installers ask for this record, and a moisture test is still required before the final floor covering goes down.

Only after this functional heat-up, and a passed moisture test, should the floor covering be fitted. Turning the system straight up to full temperature on day one is the classic new-build mistake, and it is irreversible if the screed cracks.

How long will the floor take to warm up the first time?

Longer than it ever will again. A thick screed over a cold subfloor has to absorb hours of heat before the surface temperature moves much, so on the first run it is normal for the floor to feel barely warm for several hours and for the room to take a day or more to reach its set temperature. The system is also heating the structure around it: walls, subfloor and furniture all start cold.

Once the building has heat in it, response gets dramatically quicker, and a system running steadily only ever tops up. If the floor still feels slow weeks later, that is a different problem: compare your system against typical UFH warm-up times. Timber and overlay systems carry far less mass, so their first warm-up feels much closer to normal running and the EN 1264-4 screed routine does not apply to them.

Resist the urge to turn the thermostat up to force the pace: the thermostat sets where the heating stops, not how fast it gets there. Set the final room temperatures you actually want: see UFH temperature settings, and let the system get there in its own time.

What noises and readings are normal at first switch-on?

Some, and they mostly fade:

• Clicks and creaks as pipes, panels and boards expand with first heat. Persistent loud ticking from one spot is worth investigating: see underfloor heating noises.
• Gurgling means air still in the loops. Vent it rather than living with it: bleeding air from UFH shows how.
• A small pressure drop as the system heats and air comes out of the water is common; repeated topping up is not. A manifold pressure gauge end unit makes the trend easy to watch.
• Flow meters settling at slightly different readings than when cold: re-balance after the first heat-up if rooms heat unevenly.
• A warm, slightly damp smell from a new screed as moisture leaves it in the first days of heating is normal; it should clear as the screed dries.

Check the manifold visually each day during the first week: no weeps at the loop connections, pump quiet, actuators opening when their thermostats call.

What about switching on electric underfloor heating for the first time?

The waiting rule moves from the screed to the adhesive. An electric mat or cable under tile must not be switched on until the tile adhesive and any levelling compound have cured, and cure times are the adhesive maker's to state, so check the bag or data sheet: forcing the cure with heat weakens the bond. The mat's insulation resistance and conductor resistance should have been tested before, during and after tiling; if those records exist, the first switch-on is low-drama. If they do not, test before energising: see testing electric UFH during installation.

The final connection must be made by a qualified electrician (Part P / BS 7671), and first heat is a good moment to set the thermostat's floor limit and check the floor probe is reading sensibly. Run the first session at a modest floor temperature and step up over a few days, particularly over a fresh levelling compound.

What should you check before pressing the button?

A five-minute pre-flight list for a wet system:

1. System pressure at the manifold is at its commissioning level, with no visible weeps. The install should already have passed a proper test with a hydraulic pressure test pump.
2. Loops open: flow meters or lockshields open per the design, or evenly if no design figures exist yet.
3. Mixing valve set low: 25°C for a new screed's first heat-up.
4. Thermostats calling: each zone's thermostat turned up enough to open its actuators for the heat-up, then programmed properly afterwards.
5. Heat source settings: boiler or heat pump flow temperature set to suit the mixing valve, not fighting it.
6. Paperwork ready: dates and temperatures logged as you go.

If the first switch-on is also the system's commissioning, do the full routine: filling, purging loop by loop, balancing, rather than just this list.

Frequently asked questions

How long after screeding can I turn on underfloor heating?

At the earliest, 21 days for a cement screed and 7 days for a gypsum-based screed, per the EN 1264-4 heat-up protocol, and later if the screed manufacturer says so.

What temperature should the first heat-up start at?

Around 25°C flow temperature, held for 3 days, then the maximum design temperature for a further 4 days. The two-stage routine comes from EN 1264-4.

Why does new underfloor heating feel barely warm?

A comfortable UFH floor runs only a few degrees above room temperature, and on the first run the screed, subfloor and walls all absorb heat before the surface warms. Give it a day or two before judging performance.

Is it normal for the pressure to drop at first switch-on?

A small drop is common as dissolved air leaves the water and is vented. A continuing fall that needs regular topping up suggests a leak or an open vent and should be investigated.

Can I lay flooring straight after the first heat-up?

Not automatically. The heat-up is a functional test and drying aid; a moisture test on the screed is still required before the floor covering is laid.

When can electric underfloor heating be switched on after tiling?

Only after the adhesive and leveller have fully cured: the adhesive manufacturer's stated cure time governs it. Energising early to speed the cure weakens the bond.

Do timber and overlay systems need the EN 1264-4 heat-up?

No. The staged heat-up protects wet screed. Low-mass timber and overlay systems can be brought up to temperature normally, following the board maker's instructions.