Short answer
Short cycling is a boiler firing, reaching temperature quickly and stopping, many times an hour. It wastes energy, wears ignition components and fans, and on cascades reduces efficiency. It usually comes from oversized plant, low flow-return ΔT, or poor control settings.
Common causes
Boiler output much larger than the load, especially in summer.
Low ΔT or short-circuiting through headers and bypasses.
Control differential set too tight.
Sensors placed where they see temperature change quickly.
Cascade sequencing bringing on too many boilers.
What the engineer checks
Count starts per hour from the controller.
Review cascade and BMS settings.
Measure ΔT and check hydraulic layout.
Repair and prevention
Adjust anti-cycling times and modulation, correct cascade sequencing, improve hydraulic separation, and consider buffer volume where loads are small.
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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