Underfloor heating (UFH) systems have been around since Roman times, with the earliest references dating back to 350 BC. The “Hypocaust” system used heated hot air, generated by burning fires.
Fast forward to more recent years, UFH systems have evolved just like floor covering systems. They are now a common heating option for commercial and domestic environments, whether an electric or a boiler-heated system.
When visiting sites, I see issues with UFH systems not being commissioned, which can cause bigger problems. Luckily, there is enough assistance and resources for all stakeholders out there.
The changes set out by Part L of Building Regulations, concerned with reducing energy consumption, are expected to drive significant changes. From the end of 2025, new buildings will be required to utilise low-temperature and efficient heat sources. This could mean electric systems using photoelectric panels as the energy source and, more commonly known option – heat pumps.
Over the past five years, the use of underfloor heating has grown significantly and has been accompanied by a notable increase in the sales of liquid-applied flowing screeds. In this period, the use of flowing screeds has grown from about 15-16% of the total m2 screeded per year in the UK to about 40%. Because of this, it is important to understand the procedures involved when a UFH system is part of the screeded subfloor.
Some manufacturers of floor preparation and adhesive products conduct site surveys to provide product and process recommendations for preparing subfloors and installing floor coverings on subfloors that incorporate UFH systems.
Prior to physically starting the preparation and installation process, a lot of investigation can be conducted. Confirming what products and how they were installed is key. The most obvious benefit of a site inspection is a review of the physical attributes of the project itself, which can often reveal a conflict, i.e., an uncommissioned UFH system.
It is essential that the screed is preheated before any floor covering is installed, as prescribed in BS EN ISO 11855-5, BS EN1264-4, BS 8204-1 and BS 8204-7.
BS 8204-1 Screeds, bases and in-situ flooring, states:
“… the usual operating surface temperature of a heated levelling screed is about 27°C; however, some locations operate at higher temperatures, e.g., 35°C”.
BS 8203 Code of practice for the installation of resilient floor coverings, states:
“When used with many flooring materials UFH can cause problems if the temperature at the interface between the subfloor and flooring exceeds 27°C or is subject to rapid fluctuations in temperature. In the majority of installations, this temperature will not need to be exceeded if the building insulation meets the requirements of Part L of the Building Regulations.”
Unfortunately, in my experience, the UFH is often not commissioned, creating a ‘monkey on the back’ scenario. Is it assumed that you have accepted the subfloor’s status quo if you continue to apply a floor covering? All parties involved need to be on the same page, and a compliant subfloor needs to be handed over to the flooring installer. This is an ideal-world scenario but one that is important to check for.
In all instances, UFH systems must be run through a full drying/commissioning cycle of heating up and cooling down before any subfloor preparation or installation of decorative floor coverings. This period of commissioning is usually around 21 days.
On the first day, set the water temperature of the underfloor heating at approximately 25°C by setting the thermostat (TRV) on the manifold or by setting the boiler thermostat to a lower temperature. Then, each day, raise the water temperature by 5°C up to a maximum of 50°C. Only then will the water contained in the screed/concrete be able to evaporate. When starting the system for the first time, it may take a relatively long time (up to one week), before the water in the underfloor heating pipes and the floor surface gets up to temperature.
After a full cycle has been carried out, moisture tests can be performed. If necessary, the process needs to be repeated until a definitive moisture reading of ≤65-75% RH has been achieved if moisture control systems are not being applied (depending on the subfloor base and surface finish to be applied).
As mentioned before, many manufacturers offer site inspections that provide assessments within industry-approved codes of practice and established standards. As such, all guidance provided by the British Standards Institute, the Contract Flooring Association and other leading industry sources advise that applications undertaken to heated bases with uncommissioned UFH systems must not take place.
After a full commissioning cycle, the system must be turned off for a minimum of 48 hours. After this point, a moisture test must be carried out to determine the moisture content of the substrate. Dry readings would allow the application to commence, whereas wet readings would require further running of the system to ‘dry out’ the substrate to acceptable levels. Alternatively, a recommendation for utilising a moisture control system must be given.
It is critical to point out that if products are applied to heated bases without completing the full commissioning cycle, and if the heating system is not turned off for at least 48 hours before application, most manufacturers will not support the installation. In such cases, product performance guarantees and warranties will be void. Any failures that result from not following the recommended process for applying products on heated bases will not be covered, and liability will rest with others.
It is strongly advised that the approved codes of practice be fully adhered to, as failure to do so may lead to significant issues with the applied preparation products and final floor coverings.
The consequences:
In brief, if the complete flooring system is installed and the UFH system is only turned on afterwards, it is highly likely that issues such as the following may arise:
- Substrate curling
- Cracks within the base
- Delamination of applied products
- Thermal shock (expansion and/or contraction) leading to floor failures.
- Softening of base material
- Blistering
- Mould formation
Thanks to the CFA working group members and wider manufacturers, the CFA Guidance Document “Beyond Installation: Guidance on Underfloor Heating” is an excellent resource for referencing and obtaining the exact information required by all stakeholders.
