Furnace frequently shuts down and restarts, with large temperature swings

Safety: Adjust only user‑accessible settings; do not change service parameters without guidance.

• Review the boiler temperature setpoint and hysteresis in the Defro controller; increase hysteresis slightly to reduce cycling.
• If using a room thermostat, ensure it is placed away from direct sunlight, drafts, or heat sources.
• Consider enabling or optimizing buffer tank control if installed.
• Check that radiator valves are not all nearly closed, which can cause rapid temperature rise.

For persistent short cycling, consult a heating engineer to evaluate boiler sizing and system design.