Short answer
A high ΔT means the water is leaving the boiler much hotter than it entered — the sign of too little water flow for the heat being put in. Many commercial boilers modulate down or lock out above a set ΔT, commonly around 20–30°C depending on the model, to protect the heat exchanger.
Common causes
Pump failing or set too low.
Blocked strainers, valves or heat exchanger.
Air in the system.
Incorrect hydraulic design or header arrangement.
Faulty flow or return sensor.
What the engineer checks
Measure actual flow and return temperatures.
Check pump speed, head and operation.
Verify sensor resistance against the manufacturer's table.
Repair and prevention
Restore the design flow rate, clean strainers, vent air, correct pump settings and replace faulty sensors.
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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