Thermostat calls for heat (indicator shows heating) but underfloor heating remains cold

Safety first: Turn off power at the breaker before touching any wiring. Underfloor heating circuits are mains‑voltage in many regions.

Steps to diagnose no‑heat output:

• 1. Confirm thermostat is calling for heat:
  • Set the thermostat to a high temperature and verify the heating indicator or relay click is present.
• 2. Check load wiring at thermostat:
  • With power off, remove the thermostat.
  • Inspect the load (heating) terminals for loose or disconnected wires.
  • Tighten terminal screws as needed.
• 3. Measure output voltage:
  • Restore power and, if you are qualified, measure voltage at the load terminals when there is a heat call.
  • No voltage indicates a failed relay/triac or internal fault.
• 4. Inspect junction boxes and heating cable:
  • Trace the cable from the thermostat to any junction boxes.
  • Check for loose connections or damaged cables.
• 5. Test heating element resistance:
  • With power off and cables disconnected, measure resistance of the heating mat or cable.
  • Infinite resistance indicates a break in the element.

When to call a professional: Breaks in underfloor heating elements usually require a specialist with cable fault‑finding equipment. A failed thermostat output stage should be replaced by an electrician.