Quick answer
A pressurisation unit alarm usually means system pressure is too low or too high, the unit's pump has failed, or its break tank is low. Note the alarm message, the system pressure reading and whether boilers have locked out, check for visible leaks, and call a commercial engineer. Do not change unit settings, open valves or try to refill a sealed system yourself.
What the unit does
A pressurisation unit keeps a sealed heating or chilled water system at its working pressure by topping up water from a break tank. Expansion vessels absorb expansion as water heats.
Common alarms
| Alarm | Possible meaning |
|---|---|
| Low pressure | Leak, empty break tank, failed unit pump |
| High pressure | Expansion vessel failure, overfilling, control fault |
| Pump fault | Failed pump or motor, electrical fault |
| Low tank level | Break tank supply or float valve problem |
| Excessive top-ups or run time | Significant leak or passing valve |
Safe checks
- read the alarm message and the system pressure gauge
- check whether boilers or chillers have locked out
- look for visible water on the plant room floor or from safety valve discharge pipes
- note whether the unit is running constantly
Do not adjust settings, open drain points or valves, or try to refill the system. Sealed systems hold hot water under pressure.
What to report
- The alarm text and time.
- The pressure reading and normal operating pressure if marked.
- Which plant has stopped.
- Any leaks seen and where.
- Access arrangements.
What happens next
An engineer checks the unit, break tank, vessels and safety valves, then looks for the leak. See pressurisation unit repairs and pipework repairs.
Frequently asked questions
Is a pressurisation unit alarm an emergency?
It is urgent if boilers have locked out on low pressure, pressure keeps falling, or safety valves are discharging. A single alarm that has cleared, with pressure normal, can usually be reported for a scheduled visit.
Why does the unit keep topping up?
Frequent top-ups mean water is leaving the system, through a leak, a passing safety valve or air vent, or because a failed expansion vessel is causing pressure swings.
UK guidance and references
Primary and industry sources relevant to this subject. Always check the current edition and how it applies to your building.
- Pressure systems Health and Safety Executive
This guide is general information for commercial building operators. It does not replace an assessment by a competent engineer. If you smell gas or a CO alarm sounds, leave the area and call 0800 111 999; if there is a risk to life, call 999.