In short
A pressurisation unit keeps a sealed heating or chilled water system at the right pressure. When it fails or alarms, system pressure can fall and boilers lock out, or pressure can rise and safety valves discharge. Record the pressure reading and alarm, do not adjust settings or valves, and call a commercial engineer to find the cause, which is often a leak or vessel fault.
Is it urgent?
Call now if
- Boilers have locked out on low system pressure
- The pressurisation unit is in alarm and pressure keeps falling
- Safety valves are discharging or pressure is abnormally high
- The unit is running constantly, suggesting a significant leak
Usually suitable for scheduled work
- The unit tops up occasionally and pressure holds
- A minor alarm that has cleared with pressure normal
- Annual servicing of the unit and expansion vessels
What pressurisation units do
Sealed commercial heating and chilled water systems need a minimum pressure to keep water from boiling, protect pumps and allow boilers to operate. Pressurisation units top up water from a break tank to hold that pressure, while expansion vessels absorb thermal expansion.
Common faults
| Symptom | Possible causes |
|---|---|
| Low-pressure alarm, boilers locked out | Leak, failed unit pump, empty break tank, failed controls |
| Unit tops up very often | Leak, passing safety valve or air vent, failed expansion vessel |
| High pressure, safety valve discharging | Expansion vessel failure, overfilling, control fault |
| Unit in fault, will not run | Pump failure, electrical fault, low break tank level |
Commercial heating system pressure problems
Pressure loss can lock out every boiler in a plant room at once. See commercial boiler repairs and plant room repairs.
Safe inspection and isolation
Sealed systems contain hot water under pressure. Do not open valves, drain points or vessel connections, and do not change unit settings. Scalding and flooding are real risks.
Professional repair
The engineer checks the unit, break tank, pumps, controls, expansion vessels and safety valves, then looks for where water is being lost. Finding and fixing the leak matters as much as fixing the unit. See commercial pipework repairs.
Water quality
Frequent topping up dilutes water treatment and brings in fresh oxygen, which increases corrosion. After repairs, ask for system water treatment to be checked.
Pressurisation Unit Repairs: frequently asked questions
Pressurisation unit alarm. What should Facilities Managers check and report?
Note the alarm message, the system pressure gauge reading and the unit's set pressure if shown, whether boilers have locked out, whether the unit is running or topping up frequently, and any signs of leaks in the plant room or building. Do not change settings or open valves.
Why does system pressure keep dropping?
Usually because water is leaving the system through a leak, a passing safety valve or an automatic air vent. A failed expansion vessel can also cause pressure to swing. The pressurisation unit is often reporting a problem elsewhere rather than being the fault itself.
What does an expansion vessel do?
It absorbs the increase in water volume as the system heats up. If its air charge is lost or the membrane fails, pressure rises when hot and falls when cool, causing safety valves to discharge and the unit to top up repeatedly.
Can low pressure damage boilers and pumps?
Boilers are protected by low-pressure lockouts. Pumps can cavitate or run dry if pressure is too low, causing damage. That is one reason to treat persistent low pressure promptly.
Do pressurisation units need servicing?
Yes. Pumps, controls, break tanks, expansion vessel charge and safety devices should be checked as part of planned maintenance. Some pressure equipment falls under the Pressure Systems Safety Regulations, so check with your competent person.
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
- Pressure Systems Safety Regulations 2000 legislation.gov.uk