Short answer
Low system pressure on a sealed commercial heating system means water is being lost or the pressurisation equipment is not maintaining pressure. Most boilers lock out below a minimum pressure to protect the heat exchanger and pump.
Common causes
Leaks at valves, radiators, fan coils or pipework.
Safety valve discharging due to over-pressure.
Expansion vessel with lost charge or failed membrane.
Pressurisation unit fault or empty break tank.
Recent draining without proper refill.
What the engineer checks
Check the expansion vessel pre-charge with the system depressurised.
Inspect the pressure relief valve discharge.
Survey for leaks and check pressurisation unit logs.
Repair and prevention
Repair leaks, re-charge or replace the vessel, service the pressurisation unit and refill with inhibitor. Persistent top-ups introduce oxygen and scale — fix the leak, do not just keep filling.
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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