Short answer
A low ΔT means the return water is almost as hot as the flow. The boiler reaches temperature quickly, cycles off and on, and a condensing boiler stops condensing because return temperatures are too high. It points to too much flow, short-circuiting, or a system that is not drawing heat.
Common causes
Pump speed too high.
Bypass valves stuck open or wrongly set.
Low-loss header causing boiler water to recirculate.
Weather compensation or controls set too high.
Little heat demand from the building.
What the engineer checks
Measure ΔT across boiler and system.
Check bypass and header arrangement.
Review control strategy and pump settings.
Repair and prevention
Rebalance pump speed and bypasses, set weather compensation correctly and target return temperatures below about 55°C for condensing.
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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