FAQ

The maximum permissible length of the heating circuit in water-based surface heating systems is largely determined by the fluid dynamics of the pipework. As the length of the pipework increases, the pressure drop caused by frictional resistance rises continuously. This pressure loss depends on the internal pipe diameter, pipe material, surface roughness, flow rate, and the temperature and viscosity of the heating medium.

Particularly in the case of small-diameter heating pipes, such as PE-RT pipes with an external diameter of 10 mm, comparatively high specific pressure losses occur due to the reduced hydraulic cross-section. If, in addition, a higher flow rate is required due to high heating loads, the pressure loss increases disproportionately. At a flow rate of 86 l/h, the specific pressure loss of a 10 mm PE-RT heating pipe is typically around 7–8 mbar/m (R).

For a heating circuit with a pipe length of 50 m, this results in:

Δp≈7.5 mbar⋅50 m≈375 mbar

In addition to the pipe length laid in the heating surface area, all supply and return pipes must be taken into account hydraulically, as these also contribute to increasing the total resistance.

In the case of heating circuits operating in parallel within a heating circuit manifold, the hydraulically most unfavourable or longest heating circuit always determines the required head of the circulation pump. For this reason, the individual heating circuits should be designed to have pipe lengths that are as uniform as possible. To ensure uniform heat distribution and defined flow rates, hydraulic balancing in accordance with accepted engineering practice is also required.

To ensure reliable operation within a wide variety of system configurations whilst meeting the requirements for energy efficiency, hydraulic stability and standard-compliant system operation, the maximum recommended heating circuit length for FLEXIRO systems is limited to 50 m. This design takes into account typical operating conditions of modern low-temperature surface heating systems and enables operation with standard high-efficiency circulation pumps without impermissibly high pressure losses or flow velocities.

Under favourable hydraulic conditions — for example, with powerful high-efficiency pumps, low system temperature differentials or reduced flow rates — the technically feasible heating circuit length may be slightly exceeded in individual cases. However, the maximum permissible pressure losses, the pump design, and the requirements for hydraulic balancing and uniform heat distribution must always be taken into account.