This Is How It Works: Laying Underfloor Heating on Milled OSB Boards

The components of the underfloor heating sets with milled OSB boards depend on the selected area. Detailed information can be found in the comprehensive PDF installation instructions and on the corresponding product page. You can also find additional accessories and suitable tools in our shop.

Note

Despite the simplified installation, the following also applies to FLEXIRO OSB underfloor heating: Work on heating systems requires sufficient technical knowledge and skills. If in doubt, have the installation carried out by a specialist.

First, the arrangement of the heating circuits is determined. The installation location of the RTL box is selected so that the heating pipes of all heating circuits can be routed directly to the control box without the need for extensions.

• 2.9 m² = 1 heating circuit

• 5.8 m² = 2 heating circuits

• 8.7 m² = 3 heating circuits

• 11.6 m² = 4 heating circuits

The pipes of the underfloor heating system can, in principle, be shortened. However, care must be taken to ensure that the differences in length between the individual connections do not exceed 15%.

Caution: Underfloor heating on OSB boards should not be operated continuously at system temperatures exceeding 45°C. Higher temperatures can damage the wood fibre boards.

The subfloor must be level, clean, dry and stable. The milled OSB boards are not suitable for use as a static floor construction. If the existing floor does not have sufficient load-bearing capacity, the installation is carried out as multi-layer system on additional standard OSB boards.The thickness of the additional boards and the number of layers depends on the expected load.

The first step is to draw up a layout plan for the arrangement of the milled OSB boards and the positioning of the RTL box. It should be possible to route the heating pipes from the RTL box to the heating surface and back without any crossings.

To ensure accurate and efficient temperature control, position the RTL box so that the thermostat head can detect the room air temperature without obstruction. The RTL valve must remain accessible after installation. There should be a distance of at least 20 cm to the floor so that the pipe ends can still be inserted into the individual connections after the distributors have been installed.

Please refer to the separately enclosed installation and operating instructions for the RTL box.

The next step is to lay an edge insulation strip, which should be significantly wider than the installation height, around the floor area to be heated. Columns or similar structures should also be enclosed with the edge insulation strip so that the entire floor structure can expand.

The edge insulation strip must later be grouted to ensure permanent elasticity. Once the work is complete, it is trimmed and covered with skirting boards.

Before installation, transfer the planned heating surface to the floor and cut the milled OSB boards to size as required. For cutting, use a hand-held circular saw, jigsaw or hand saw with a suitable blade.

Please note: The milled wood fibre boards are only required for the designated heating surface. Standard OSB boards are used in the unheated edge areas and to completely fill any additional layers that may be required. Additional boards can be ordered from the shop on request. 

The process of laying the boards begins with the supply board. The milled OSB boards are then laid with the grooves aligned, ready for the heating pipe to be laid in them later. Cross joints are not a problem for the top layer. The boards are fixed to the lower OSB layers using four screws in the centre of the milled plates located at the corners of the boards, without the need for additional pre-drilling. Once all the boards have been fixed in place, they are screwed down using additional screws spaced approximately 30 cm apart. This means that approximately 18 screws are used per board.

When laying the milled OSB boards directly on a straight, load-bearing floor, it is recommended that a suitable adhesive is used instead of screws.

To keep the connection length of the heating pipe as short as possible, it is best to start laying it near the distribution fingers. The distance between the grooves is 10 cm. This corresponds to the minimum laying distance and the pipe's smallest bending radius. The distance between the underfloor heating and the room walls should be at least 5 cm.

If the heating pipe is damaged

Kinked or damaged sections must be cut out. The pipe must then be reconnected using a permanently sealing coupling (see the accessories section for plug connectors). Protect the coupling from direct contact with the casting compound or top layer by wrapping it in PE film. Mark the position of the coupling. If necessary, the pipes can also be extended using the same procedure.

After installation, the heating pipe should lie firmly in the grooves. If the pipe pops out in places, it can be secured with wide staples or similar.

Meandering and spiral laying patterns are commonly used. We use the spiral or helical pattern because it ensures even heat distribution and facilitates twist-free laying thanks to the larger radii. Care must be taken not to damage the pipes when walking on the OSB boards to lay them.

The distribution fingers are connected to the RTL box using a Eurocone screw connection. The Eurocone has a self-sealing effect, while the metal screw connection performs a purely securing function. Over-tightening must be avoided at all times so as not to compromise the tightness of the heating system.

The heating pipes are connected to the distribution fingers using a simple plug-in assembly. The heating pipes, which have been cut straight and possibly shortened, are inserted directly into the plug-in connections of the distribution fingers until they reach the stop.

As a rule, thin-layer underfloor heating is integrated into the existing heating system in parallel with other radiators or as a replacement for an old radiator. To do this, a T-piece (not included in the scope of delivery) is inserted into the flow and return pipes of the heating system.

Please ensure that the flow and return are correctly assigned in the RTL box. Swapping the flow and return in the control box will disable the thermostatic valve and prevent the underfloor heating from functioning.

Before commissioning the underfloor heating system, it is essential to ensure that the system and all associated components are correctly installed and tight. For this, a pressure test pump is available as an accessory in the shop.

See video for exact procedure and connections for pressure test

We strongly recommend testing the underfloor heating system before covering the pipe system with levelling compound. This is your last chance to correct any installation errors or damage!

First, apply a thin layer of adhesive primer to the OSB boards using a suitable tool. Depending on the consistency, a paintbrush, brush or roller may be used. This ensures that the levelling compound adheres well to the OSB boards. The drying time is 1-2 hours. Follow the manufacturer's instructions.

The levelling compound is then poured on thinly and smoothed out. The minimum coverage of the heating pipes is 5 mm. All grooves, even those not occupied by pipes, must be completely filled with levelling compound. Once the levelling compound has dried briefly, cut a decoupling mat to size for the area and lay it on top. Spread the levelling compound over the decoupling mat.

Floor covering

Once the levelling compound has dried thoroughly, the selected floor covering can be laid. It is possible to lay ceramic or porcelain stoneware tiles in a 330 x 330 mm format. However, please check with the tile manufacturer to ensure that larger formats are suitable for the floor construction. Please note that laws, regulations, guidelines and standards must be observed when planning and constructing heating systems and carrying out drywall work. Floor coverings must be labelled 'suitable for underfloor heating'.

Once the floor covering has been laid and has dried naturally, the heating process can begin. If tiles are being laid, the underfloor heating must not be switched on until the tile adhesive has completely hardened. Where possible, commissioning should be carried out in accordance with the specifications of a heating protocol.