What Needs to Be Considered when Planning the Heating Area of an Underfloor Heating System?

The size of a room does not generally correspond to the heating surface of an underfloor heating system. The heat output of an underfloor heating system is wasted under permanently installed fixtures such as cupboard walls, kitchen units or shower trays. These areas should be avoided when planning the heating area. 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% is assumed. For a 15 m² room, this means a heating surface of 11.25 m². Depending on the room conditions and heating requirements, the occupancy rate can be between 60 and 85%.

In order to determine the required heating surface before purchasing underfloor heating, the unheated areas should be recorded in a planning sketch. The required heating area is calculated by distributing the heating circuits over the remaining area and determining the location of the control box. Particularly in the bathroom, a 5 m² set can be sufficient to heat 10 m² if the sanitary facilities are cleverly arranged and the installation is well planned.

Special features of thin layer underfloor heating

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

The PE-RT heating pipe, with a diameter of just 10 mm, not only allows flexible installation, but also low installation heights of at least 17 mm. This means that underfloor heating can be retrofitted to existing floor surfaces. In many cases, additional insulation is not required.

However, the thin FLEXIRO heating pipe has a higher frictional pressure on the inside of the pipe compared to conventional underfloor heating systems with heated screed and an installation height of 50 mm and more. The pressure resistance increases with the length of the pipe and reduces the water pressure of the heating system. As the FLEXIRO kits are designed to work on as many heating systems as possible, we have limited the kits to a maximum of 4 heating circuits, each with 30 m of heating pipe. This results in a maximum kit size of approx.10 m².

Large rooms

The required heating area is greater than 10 m². Can I still use FLEXIRO underfloor heating?

In addition to the savings in occupancy, the underfloor heating sets can be combined for larger rooms. However, each set must be connected to its own control box. Before the conversion, larger rooms were usually heated by several radiators. The resulting connection options can also be used for the underfloor heating sets. For example, the installation for a 32 m² room could look like this, using two 10 m² sets.

Sketch of underfloor heating area with 2 sets

The Multibox 4K-RTL with the option of smarthome extension is a particularly convenient way of controlling several kits and boxes in a standardised way. However, once the optimum room temperature has been found, the control of underfloor heating systems is rarely changed on a day-to-day basis, and the flush-mounted installation means that the kompaboxes are barely noticeable.

Individual approaches

In addition, smaller sets can be used to achieve the desired heating output by combining underfloor heating with wall or ceiling heating. It is also possible to connect several systems to a heating circuit manifold. For larger projects, consult a specialist company for advice on the design of the heating surfaces and the choice of technology.

We are committed to helping DIYers achieve their underfloor heating goals. There are some situations where thin layer systems are simply not suitable. In all other cases, we are happy to discuss possible solutions and provide customised quotations for sets without kompabox. Ask our technical advisors.

Retrofit underfloor heating in the bathroom trending_flat That's what counts

  • 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 low profile underfloor heating in the bathroom.

Retrofit underfloor heating in the bathroom