ᐅ Underfloor heating pipe used as a protective conduit / empty duct in the ground?

Hello,
During renovation work in an old building, a floor heating system is to be installed in the basement. The old floor has been completely excavated to a depth of 30 cm (12 inches) below the future finished floor level.
In the soil beneath the screed, within the capillary-breaking layer made of gravel, various cables (NYY-J) are to be laid. Are no longer used floor heating pipes, for example 20 x 2 mm (0.8 x 0.08 inches), made of PE-RT or PE-Xa or PE-Xc suitable as empty conduits or protective tubes?

Best regards

Opinion from a non-expert: I don’t think so. If the above-mentioned conduit experiences too tight bending radii, it kinks, creating sharp indentations on the inside! These can cause high localized pressure on the cable. The NYY-J cable also doesn’t have much bending room inside a 20/2 underfloor heating pipe. A 5x 2.5mm² cable has about a 15mm (0.6 inch) outer diameter, so pulling it through an internal diameter of 16mm (0.6 inch) won’t be easy. Even a 5x 1.5mm² cable won’t slide through smoothly. Additionally, the thermal insulation properties of the composite pipe could be counterproductive in this case (heat increases the cable’s internal resistance). Good quality cable conduit is relatively inexpensive, so I wouldn’t take any risks...

gunterh schrieb:

In the ground, beneath the screed, cables (NYY-J) are to be installed in the capillary-breaking layer of gravel.

If you are going to use underground cables anyway, just lay them directly on or in the soil. That is what they are designed for.