Short answer
High system pressure is usually caused by expansion not being accommodated or water being added. If it reaches the safety valve setting, the valve discharges, which then causes low pressure later.
Common causes
Expansion vessel undersized, flat or failed.
Filling loop or pressurisation unit letting water in.
Cold fill pressure set too high.
Plate heat exchanger leaking from mains or DHW side into heating.
What the engineer checks
Measure cold and hot pressure.
Check vessel pre-charge and sizing.
Isolate filling points to see if pressure still rises.
Repair and prevention
Re-charge or replace the vessel, repair passing fill valves and set the cold fill pressure correctly. Replace a safety valve that has discharged repeatedly.
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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