Commissioning Underfloor Heating Before the Floor Goes Down
When the screed can first be heated, the functional heating programme, the 21-day commissioning cycle, and why a heated screed is not yet a dry one.
A floor laid over underfloor heating that has never been run is a floor laid before the building is finished. The heating has to be taken through its working temperatures with nothing on top of it, the screed it sits in has to be dry, and those are not the same thing. This guide sets out the order of events, with the published figures for each step.
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Step one: let the screed cure
A heated screed is not heated straight away. IMI, a maker of underfloor heating components, gives the earliest start for functional heating in its commissioning instructions, which follow EN 1264-4: 21 days after laying for a cement screed, 7 days for an anhydrite one. The screed manufacturer's own information comes first where it differs.
Step two: functional heating
The same instructions set the programme: begin at a flow temperature of 20 to 25 °C and hold it for 3 days, then set the maximum design temperature and hold it for 4 days. Functional heating proves the system and takes the screed through the temperatures it will see in use, while there is still nothing on top of it to suffer for it.
The Contract Flooring Association describes the same principle from the floor layer's side: in all instances the heating must be run through a full drying and commissioning cycle of heating up and cooling down before any subfloor preparation or floor covering, and that period is usually around 21 days. The heating engineer and the floor layer should agree in advance whose programme is being run and when it ends.
Step three: measure, do not assume
Heating a screed is not the same as drying it. IMI says so directly: functional heating does not ensure that the screed has reached the moisture content needed before it can be covered, and that readiness is checked by the floor layer. The check in UK practice is a hygrometer sealed to the screed in an insulated box, with the heating switched off for at least 4 days before, and left to reach equilibrium: at least 4 hours on a 50 mm screed with the membrane beneath it, 72 hours where the membrane is below the base slab. Readings not exceeding 75 % relative humidity count as dry for resilient floor coverings such as vinyl, and 65 % for solid timber.
How long the screed takes to get there is a separate figure. BS 8204-1 allows one day per millimetre for the first 50 mm of a levelling screed and longer per millimetre above that, so a 50 mm screed in good conditions takes about two months. A date on the calendar says when to start testing, not that the test will pass.
Step four: the floor arrives before it is laid
The Contract Flooring Association asks for a steady room temperature of 18 to 27 °C, reached 48 hours before installation, so that the flooring can be kept in the room and reach room temperature before it is laid. For multilayer floors such as rigid vinyl, the MMFA asks for at least 48 hours in the same conditions as the installation area, at roughly 15 to 25 °C and 35 to 65 % relative humidity. What the heating itself should be doing on the day of fitting is set by the instructions for the floor, and they are worth reading before the fitter arrives.
Step five: the floor's limits take over
Once the floor is down, the heating works to the floor's limits rather than the screed's. BEAMA notes that most vinyl and linoleum makers state a maximum surface temperature of 27 °C and that most vinyls should not be heated above 26 °C; for real wood, Junckers and the Contract Flooring Association give 27 °C. For parquet it advises a floor temperature sensor always fitted, and for any floor a sensor wired to the thermostat is what keeps the surface where the instructions say.
Common questions
How long after screeding can underfloor heating be switched on?
IMI's commissioning instructions, following EN 1264-4, give the earliest start for functional heating as 21 days after laying for a cement screed and 7 days for an anhydrite screed. The screed manufacturer's information comes first where it differs.
How long does commissioning take?
IMI's functional heating programme is 3 days at a flow temperature of 20 to 25 °C followed by 4 days at the maximum design temperature. The Contract Flooring Association puts the full heating-up and cooling-down cycle before any floor covering at usually around 21 days.
Does running the heating dry the screed enough to lay the floor?
No. IMI states that functional heating does not ensure the screed has reached the moisture content needed for covering. A sealed hygrometer reading not exceeding 75 % relative humidity, or 65 % for solid timber, taken with the heating off for at least 4 days, is the test.
Who decides that the screed is ready for the floor?
The floor layer, on a measurement. IMI's instructions say the readiness for covering is checked by the floor layer, and in UK practice that check is a hygrometer sealed to the screed.
What temperature should the heating run at once the floor is down?
The figure in the floor's instructions. BEAMA notes that most vinyl and linoleum makers state 27 °C at the surface and that most vinyls should not go above 26 °C; for real wood, Junckers gives 27 °C.
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What the published standards say
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