Short answer
The return sensor measures water coming back to the boiler. The control uses flow and return together to calculate ΔT and protect the heat exchanger. A faulty return sensor can cause false ΔT lockouts or allow the boiler to run with unsafe conditions.
Common causes
Open or short circuit.
Drifted resistance.
Poor contact with the pipe.
Wiring damage.
What the engineer checks
Measure resistance against the table and compare with flow sensor and contact thermometer readings.
Repair and prevention
Replace the faulty sensor and confirm ΔT readings are correct at full fire.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
Need this done on site?
Manufacturer support
We diagnose this fault on all major commercial boiler brands:
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