Frequently Asked Questions with Answers about our Radiant Heating Kits

Of course, you can also install several FLEXIRO sets in a large room and control them via RTL boxes.
Installation plan for a low profile underfloor heating system in a 32 sqm room

In the bathroom, there are many surfaces on the floor that can remain inactive because they do not transfer heat efficiently to the room anyway. This is the case, for example, under the bathtub. If you do not thermally activate this area, then a 7.5m² kit is already sufficient.
Instructions for installing an underfloor heating in bathrooms

Particularly when modernising existing buildings, there is often insufficient room height to install underfloor heating. Conventional underfloor heating has a construction height of 100 mm and more. The old floor would have to be completely ripped out and disposed of. This results in additional costs and considerable work. Milling grooves into existing concrete floors is also labour-intensive and requires special milling machines. Moreover, this is only possible if the floor slab’s structural capacity allows grooves to be milled. This effort is generally not justifiable, especially for small heating surfaces.

With a construction height of just 17mm (without the surface covering), the FLEXIRO low profile underfloor heating kit therefore provides an excellent option when modernising small heating surfaces and, in particular, individual rooms.

The height of the knobbed mat and clip rail with the heating pipe is 14 mm. In addition there is the self-levelling compound, which must be laid according to the manufacturer's specifications. The self-levelling compound should have a depth of at least 3 mm for ceramic surface coverings (tiles etc.). This results in an average construction height of 17 mm without the surface covering.

The surface covering has a big influence on the energy efficiency and the response time of the FLEXIRO underfloor heating. If possible, select a covering that has good thermal conductivity. Ceramic tiles are particularly suitable. Materials such as cork or timber floorboards and parquet take a long time to warm through and require a higher supply temperature.

We at FLEXIRO have put together kits for you that are specially designed for modernising smaller areas. This makes it easy to plan your new radiant heating. However, please feel free to get in touch with us if your project requires an individual solution: Contact form.

The Bostik company says yes! However, floating timber structures are not permitted as they are highly susceptible to deformation. Installation is therefore only possible on fixed timber structures, whereby these must be uncoupled. Please contact us for further information on laying on fixed timber structures!

The existing substrate must always be level, clean, dry and load-bearing.

FLEXIRO underfloor heating can be installed on OSB boards. Bostik's instructions clearly state that the OSB boards must be bonded to the substrate. Floating OSB boards are not suitable. The OSB boards must be primed before laying the FLEXIRO underfloor heating. In this case, the pipes should be covered with at least 5 mm of screed.

If there is an unheated room above the ceiling activated with FLEXIRO and the old ceiling has no thermal insulation or the heat transfer coefficient of the ceiling is unknown, we recommend adding an additional insulation layer. In any case, thermal insulation above the ceiling heating will shorten the reaction time of the heating.

The substructure for the ceiling heating should comply with the dry construction guidelines for suspended ceilings. When using CD profiles and direct hangers, a suspension height of approx. 5 cm can already be sufficient. A small distance to the raw ceiling is also required for the installation of the FLEXIRO connection pipes and plug connections. The dry construction guidelines also permit the use of wooden beams as a substructure for the plasterboard. However, their stability and torsional stiffness is usually less good compared to CD profiles made of metal.

Yes, it is possible to use the ceiling heating kit for cooling in summer. Sufficient cool water supply is required though. Further details can be found at: How to use a ceiling heating as cooling ceiling. 

Yes, the FLEXIRO wall heating can be mounted on an exterior wall. However, the exterior wall should have a maximum U-value of 0.30 W/m2K. If insulation is installed behind the wall heating to minimise heat loss to the outside, a building physics check of the construction may be required to prevent condensation in the wall when the wall heating is not in operation. During operating, the wall heating will dry-heat the affected wall rather than causing moisture damage to the construction.

Such a combination is possible as long as the max. number of 4 heating circuits is not exceeded. In addition, it must be ensured that the differences in length between the individual connections do not exceed 15%. Otherwise there could be problems with the hydraulics. For combinations of wall and underfloor heating systems within the scope of these guidelines, we will be happy to submit an individual offer.

Yes. If assembly according to the assembly instructions is not possible, the plasterboard panels with integrated heating pipe can also be glued directly to the wall in accordance with the dry construction guidelines. According to the manufacturer, the required connectors for the heating pipe are suitable for grouting and bonding compounds. We strongly recommend carrying out a pressure test before the bonding process.

Yes. Before the panels are fitted, the arrangement of the heating pipes can be seen quite clearly on the back. A normal cavity box for holding a light switch has a diameter of 68 mm. The heating pipes have a distance to each other of approx. 90 mm. The opening for the switch should be pre-drilled from behind before mounting the drywall elements.

After installing the drywall panels, you can visualise the arrangement of the heating pipes in heating mode with a thermal imaging camera. The pipes have a distance of 100 mm. In this way, pictures can also be hung later. Alternatively, if the walls are not tiled, you can spray them slightly damp when the heating is on. Where a heating pipe runs, the moisture will dry off more quickly.

For the FLEXIRO wall heating dry construction, a standard gypsum plasterboard 12 mm thick is used. For damp rooms, dry construction recommends an impregnated gypsum plasterboard GKBI. Please ask for a customised offer.

The control of the heating temperature via the RTL principle (=return temperature limiter) is an established control technique. It is particularly suitable if the new radiant heating is to be integrated into an existing heating system with higher flow temperatures. The RTL valve permanently measures the heating water temperature in the heating circuit. If this is too high for a radiant heating system (the limit is about 45°C), the valve interrupts the flow through the heating circuit and no further hot water flows into the heating circuit. Thus, the water in the heating circuit then cools down again. If the temperature falls below the maximum temperature of the radiant heating, the RTL valve opens again.

The thermostatic head in the Multibox 4K-RTL can be replaced by a Smarthome servo drive. The retail market offers various brands for this purpose, depending on the Smarthome system. The product to be selected must have a connection thread of M30x1.5 and have the corresponding adapters for the IMI thermostatic head K installed as standard in the Multibox.

The Kompabox RTL is used exclusively in conjunction with conventional heating systems with high flow temperature. The Multibox can also handle radiant heating systems with lower flow temperatures. Check our head-to-head comparison of temperature control boxes.

• Installation dimensions Multibox 4K-RTL in mm: 190×240×60 (H×W×D), threaded connection 3/4"
• Installation dimensions Kompabox RTL in mm: 150×230×60 (H×W×D), threaded connection 3/4"

An RTL-Box is required for heating systems that bring the water into the circuit with a flow temperature of up to 65°C. The RTL stands for return temperature limiter. The Multibox 4K-RTL limits the return temperature and has a flow shut-off including a vent valve and a regulating spindle with which hydraulic balancing can be carried out.

The control takes place without additional auxiliary energy and can be individually adjusted via the thermostatic head supplied. This functions as a continuous proportional controller. The change in the room air temperature (controlled variable) is proportional to the change in the valve stroke (control variable). If the room air temperature rises, the temperature in the temperature sensor expands and acts on the corrugated tube. This throttles or increases the water supply in the underfloor heating circuit via the valve spindle.

Even if you replace the heat generator with a low-temperature heat generator at a later date, the Multibox 4K-RTL can still be used. Because the flow temperature is then moderately below 45°C, the Multibox will not need to regulate and you can still use the actuator.

The sets compiled by FLEXIRO offer the advantage that all components contained in the set are technically well coordinated and thus ensure perfect functioning. With the appropriate expertise, the components can also be installed differently from the set structure. Thus, the FLEXIRO radiant heating can also be connected to a standard heating circuit manifold. Upon request, the FLEXIRO sales department will be pleased to prepare a quotation for individual sets or for sets without RTL control box. However: In the case of individual solutions, FLEXIRO cannot take any responsibility or guarantee for the proper functioning of the heating system.

The components of the FLEXIRO sets are carefully dimensioned to match each other in terms of technology. It is very unlikely that the heating pipes will not allow sufficient heating water to pass through if they are installed correctly. If the heating surface does not get warm, please first check whether the Multibox or Kompabox is correctly connected to the flow and return of the heating system. Perhaps only one heating pipe has been kinked. If several heating circuits were installed, one heating pipe may have been disproportionately shortened in length. As a result, the heating circuit with the longest connection pipe may not heat up properly. The pipe lengths of the individual heating circuits should be approximately the same.

The heating capacity in general depends on the installation distance, insulation and floor covering, etc. The information in our product flyers provides data on the average heating capacities of the kits.

Due to the installation of the low profile underfloor heating directly below the surface, additional insulation is not required in most cases. This keeps the installation height low. Exceptions are floors above unheated basements and the ground. The requirements of the Energy Saving Ordinance must be observed. Further information on thermal insulation for underfloor heating

The existing substrate must be level, clean, dry and load-bearing.

FLEXIRO underfloor heating cannot be connected to a single-pipe heating system.

The T-pieces are not included in the underfloor heating system because we don’t know which pipe dimension your existing heating has. However, you can easily obtain the T-pieces from specialist dealers.

No, because we don’t know which pipe dimension was used on site. However, the connecting piece can be ordered individually in the Web shop under Underfloor heating accessories.

Yes. It is a multi-layer (5 layers) pipe made of PE-RT with an EVOH layer (oxygen-tight according to DIN 4726).

All parts that form part of a FLEXIRO underfloor heating kit are also available as spare parts.

The low profile underfloor heating is designed for mounting directly on the existing surface. The modernisation of an existing and already insulated floor usually does not require additional insulation of the underfloor heating. Therefore we only sell special insulation boards for bathrooms and other wet areas. Additional floor insulation can be bought at any specialist store. Once the insulation has been applied, you need to ensure a stable surface before installing the underfloor heating.

Most of the kits can be delivered to EU-Countries and Switzerland. Detailed information is provided here: Countries and shipping costs. If you do not find your country on the list, please use the contact form to make an enquiry. We will tell you the corresponding shipping costs in accordance with the country of destination.

The invoice and delivery addresses can be specified separately.

No, the plasterboards of the drywall wall and ceiling heating system are delivered by a forwarding agent. You will be informed of the delivery time window in good time by telephone. Therefore, please leave a telephone number.

When we begin shipping the FLEXIRO underfloor heating you will receive an email with the tracking number so that you can track the shipment.

Once the payment has been received, we can usually deliver in approximately 2-4 working days.

There are two possibilities: By immediate bank transfer or prepayment. The advantage of the immediate bank transfer is that the payment is made immediately and therefore the goods can be shipped immediately. With prepayment by bank transfer, 1-2 working days elapse until the payment is booked. The goods can only be shipped once the payment has been received.

Yes. This is of course possible by email or telephone. In case of cancellation, please always quote the order number.