What Should Be Considered when Planning the Heating Surface of an Underfloor Heating?

The room size is usually not identical with the heating surface of an underfloor heating system. The heating power of an underfloor heating system is lost below permanently installed equipment and installations such as cupboard walls, kitchen units or shower trays. These areas should be left out when planning the heating surface. In addition, a minimum distance of 10 cm from the walls is deducted from the room size.

As a rule of thumb, an average occupancy rate of 75% of the room area is assumed. For a 15 m2 room, this means a heating surface of 11.25 m2. Depending on the room conditions and the demand for heating power, the occupancy rate can be between 60 and 85%.

To determine the required heating surface before purchasing underfloor heating, the unheated areas should be recorded in a planning sketch. The required heating area results from the distribution of the heating circuits over the remaining area and the determination of the installation location of the control box. Especially in the bathroom, if the sanitary facilities are cleverly divided and the installation planning is good, the 5 m2 set can already be sufficient for heating 10 m2.

Special requirements of a low profile underfloor heating

The FLEXIRO system is designed for retrofitting underfloor heating to areas up to 10 m2. Why is this so?

The PE-RT heating pipe with a diameter of only 10 mm not only allows flexible laying but also low installation heights of at least 17 mm. This means that underfloor heating can also be retrofitted to existing floor areas. Additional thermal insulation is unnecessary in most cases.

However, the thin FLEXIRO heating pipe has a higher frictional pressure on the inside of the pipe compared to conventional underfloor heating systems with heating screed and a construction height of 50 mm and more. The pressure resistance increases with the length of the pipes and reduces the water pressure of the heating system. Since the FLEXIRO sets should work on as many heating systems as possible, we have limited the sets to a maximum of 4 heating circuits á 30 m heating pipe. This results in a maximum set size of 10 m2.

Large Rooms

The required heating surface is larger than 10 m2. Can I still use the FLEXIRO underfloor heating?

In addition to savings in the occupancy level, the underfloor heating sets can be combined for larger rooms. However, each set must be connected to its own control box. As a rule, larger rooms were heated via several radiators before the conversion. The resulting connection options can also be used for the underfloor heating sets. The installation for a 32 m2 room could look as follows, for example, when using two 10 m2 sets.

The Multibox 4K-RTL with the option of Smarthome expansion offers a particularly convenient way to control several sets uniformly. However, the control of underfloor heating is rarely changed in everyday life once the optimum room temperature has been found, and because they are installed flush-mounted, the Kompaboxes are hardly noticeable.

Individual Solutions

In addition, by combining underfloor heating with wall or ceiling heating, the desired heating output can be achieved even with smaller sets. It is also possible to connect several systems to one heating circuit distributor. If you have a large-scale project, consult a specialist company for advice on the design of the heating surfaces and the choice of technology.

We are committed to supporting do-it-yourselfers in the realisation of their underfloor heating to the best of our ability. For some initial conditions, thin-bed systems are simply not suitable. In all other cases, we are happy to discuss possible solutions and make individual offers. Ask our technical advisors.

Retrofitting Underfloor Heating in Bathrooms » What you need to know

  • How do I calculate the heating surface?
  • What heat output is possible?
  • How do I protect the floor structure from splashing water?


We provide answers to the most important questions when retrofitting underfloor heating in bathrooms.