Expansion vessels and pressurisation units: what site teams should watch for
Commercial engineers covering Suffolk, Essex and Cambridgeshire
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On a sealed commercial heating system the expansion vessel and the pressurisation unit work together. The vessel absorbs the extra water volume as the system heats up. The pressurisation unit keeps the cold-fill pressure where the engineer set it and tops up small losses. When either part is neglected, pressure swings, relief valves discharge, boilers lock out and plant rooms get wet. This guide explains what site teams should watch for, how often manufacturers typically want the vessel pre-charge checked, and what must be left to a competent engineer.
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What each part does
Hamworthy's instructions for its Weymouth pressurisation units (a common commercial range) describe the arrangement clearly. With the system cold and at fill pressure, the expansion vessel is empty of system fluid. As the water heats and expands, the vessel takes up that volume by compressing the gas charge behind a diaphragm. When the system cools, pressure falls. If some water has been lost, the pressurisation unit's pump restores the set cold-fill pressure from a small header tank.
The same manual sets the vessel's gas charge at 0.1 bar below the cold-fill pressure. That relationship is what keeps the vessel ready to accept expansion without the system pressure climbing to the safety valve. It is also why a lost charge shows up as pressure swings and relief-valve discharge.
How often should the expansion vessel pre-charge be checked?
Always follow the manufacturer's instructions for the equipment on your site. As one published commercial example, Hamworthy's Weymouth servicing schedule recommends:
| Interval | What the engineer checks (Weymouth example) |
|---|---|
| Every 6 months | Expansion vessel charge pressure (vessel isolated and emptied on the water side, then measured at the Schrader valve). A significant drop may mean a faulty diaphragm. |
| Every 12 months | External corrosion on the vessel, float-valve integrity on the pressurisation unit, and (on the Advanced unit) the high/low pressure safety circuits. |
| Every 4 years | Remove and inspect the diaphragm for wear or ageing, and inspect the vessel interior for corrosion. Significant corrosion can mean replacing the vessel. |
Those intervals are for that product range. Your plant may use a different make. Ask your engineer to record the correct schedule for your vessels and pressurisation unit in the site log.
Signs of a lost charge or pressurisation fault
HSE's leaflet INDG261 says to look for tell-tale signs of pressure-system problems. One it names is a safety valve that repeatedly discharges, which "could be an indication that either the system is over-pressurising or the safety valve is not working correctly." On sealed heating plant a common cause of over-pressurising is an expansion vessel that has lost its gas charge, so the expanding water has nowhere to go. Other signs trained site staff often spot first include:
- System pressure that climbs sharply when the boilers fire and drops when they cool, instead of staying near the engineer-set range.
- The pressurisation unit pump running often, or never stopping, which can point to a leak, a vessel problem or a wrong set point.
- Alarm lights or BMS messages for high or low system pressure.
- Water at a tundish or discharge pipe from a relief valve.
- Rust, weeping or physical damage on the vessel shell.
None of those observations is a diagnosis. Log what you see and call a competent commercial engineer. Do not try to "cure" a discharging relief valve by capping or adjusting it.
Weekly visual and gauge checks for trained site staff
A short weekly walk-round is good practice on many sites. It is not a substitute for the manufacturer's service intervals, for PSSR examination where that applies, or for your own risk assessment. Site staff stick to visual and gauge checks, log the readings and report anything abnormal. They do not open, empty, recharge or adjust pressurised equipment.
- Pressure gauge: read the system gauge on the pressurisation unit or boiler header and compare it with the cold-fill / normal range your engineer has recorded. Log date, reading and initials. A trend matters more than one odd reading.
- Pressurisation unit: note any fault lights, alarms or a pump that runs constantly. Check that the header tank has water and that the area around the unit is dry.
- Expansion vessel: look for external rust, dents, weeping at connections, and whether the vessel feels unusually hot or cold compared with normal (report changes; don't dismantle anything).
- Relief valves and tundishes: look for discharge, scale or corrosion. Never lift, adjust, cap or plug a relief valve.
- Housekeeping: keep access clear, keep ventilation open, and keep the plant room free of stored combustibles.
Our commercial plant room safety checklist covers the rest of the room, and our guide to pressure relief valves on industrial water heaters covers T&P and expansion relief on hot-water plant.
Pressurisation unit or expansion vessel: which part is causing the problem?
Because the two parts share one job, a fault in either shows up as "pressure problems". The pattern of the readings is usually the best clue to where an engineer should start. Treat this as a guide to what to report, not a diagnosis.
| What the site sees | More likely to point at | Why |
|---|---|---|
| Pressure rises sharply on heat-up, falls back when the plant cools, relief valve weeps when hot | Expansion vessel (lost gas charge, failed diaphragm or vessel too small) | The expanding water has nowhere to go, so pressure climbs until the relief valve opens |
| Pressurisation pump starts and stops many times an hour, pressure fairly steady | Expansion vessel with little or no charge, or a set point close to the pump cut-in | Small volume changes are not being absorbed by the vessel, so the pump keeps correcting them |
| Pump runs for long periods or the break tank keeps refilling, with no visible high pressure | A leak on the system, a passing relief valve, or a vent letting water out | The unit is doing its job and replacing water that is being lost somewhere else |
| Low pressure alarm and boilers locked out, pump not running | Pressurisation unit (pump, switch, controls, empty break tank or tripped supply) | The unit is not topping up, so the system falls below the boilers' minimum pressure |
| High pressure alarm with the plant cold | Pressurisation unit over-filling (set point or pressure switch), or a mains filling loop left open | The cold system should sit near the cold-fill pressure; extra water has been added |
| Problems started after radiators, heaters or a new circuit were added | Expansion vessel now undersized for the bigger system | More water means more expansion; the original vessel may no longer be large enough |
If you can, note the gauge reading cold first thing, again when the boilers have been running for an hour, and how often the pump runs. Those three facts save time on the visit.
What a pressurisation unit service covers
Pressurisation units are often left off the plant service because they sit quietly in a corner. A competent engineer's visit should normally include, following the manufacturer's instructions:
- Recording the settings: cold-fill pressure, pump cut-in and cut-out points, high and low pressure alarm settings, and the vessel pre-charge, written in the plant log so the next engineer can compare.
- Pump and controls: pump start and stop at the right pressures, no unusual noise or seal weeping, and any pump-run or fill-limit alarm working.
- Safety switches and interlocks: the low pressure switch stopping the boilers and the high pressure alarm signalling as designed, including to the BMS where fitted.
- Break tank and water supply: float valve working, tank clean, and the air gap that keeps system water out of the mains supply intact.
- Expansion vessel: the gas charge checked with the vessel isolated and emptied on the water side, then corrected and recorded; shell checked for corrosion.
- Water treatment link: a unit that tops up often brings in fresh oxygenated water and dilutes inhibitor, so frequent top-ups are a reason to test the system water and look for losses.
Most sites have this done at the same visit as the annual boiler service. Ask for it by name, because "boiler service" on its own does not always include it. Our guide to what a commercial boiler service includes explains the rest of the visit.
When the system has changed
An expansion vessel is sized for the water content, flow temperature and height of the system it was fitted to. Adding a new wing, more radiators, unit heaters or an extra calorifier increases the water volume, and a vessel that was right ten years ago can now be too small. The same applies when flow temperatures are raised. If pressure problems started after building work, tell the engineer what was added and when, so the vessel and the cold-fill pressure can be checked against the system as it is now.
What only a competent engineer should do
Checking or restoring an expansion vessel pre-charge means isolating the vessel from the system, emptying the water side, measuring the gas charge at the Schrader valve, and adjusting it with a suitable gauge and an oil-free air or nitrogen source. That is engineer work. The same applies to setting cold-fill and high/low pressure switches, replacing diaphragms, and testing or replacing relief valves.
HSE's guidance on using pressure systems says the user must fit suitable protective devices, keep them working, carry out suitable maintenance, train the people who operate and maintain the plant, and have equipment examined where the Pressure Systems Safety Regulations 2000 (PSSR) require it. INDG261 adds that once protective devices are set, they "cannot be altered except by an authorised person", and that a written-scheme examination is "not a substitute for regular and routine maintenance".
Does PSSR apply to your sealed heating system?
PSSR applies to systems containing a "relevant fluid". HSE's guidance includes steam at any pressure, compressed gases above 0.5 bar and pressurised hot water above 110°C. Many low-temperature hot water heating systems fall outside the written-scheme requirement, but the general duty to keep plant safe still applies, and manufacturer maintenance still matters. Where PSSR does apply, the written scheme must cover all protective devices, every pressure vessel, and pipework where a defect may give rise to danger, and a competent person must examine the system to that scheme. If you are unsure, ask a competent person or your insurer's engineering inspector.
For pressure loss, leaks, expansion vessel or pressurisation unit faults, see commercial boiler pressure and leak repair. For wider plant room help see our commercial boiler services, or go to emergency commercial boiler help if plant is failing now.
Safety note
Under the Gas Safety (Installation and Use) Regulations 1998, gas work must be carried out by a Gas Safe registered engineer, and we apply that rule on every site. Don't attempt DIY work on gas appliances, burners, flues or safety devices. On pressure systems, trained site staff should stick to visual and gauge checks, log the readings and report anything abnormal. Never adjust, cap, plug or test-lift a relief valve yourself.
Talk to a Gas Safe commercial engineer
Fast Commercial Boiler Help is a Gas Safe registered (985448) commercial heating business based at 4 Market Hill, Clare, Sudbury, Suffolk CO10 8NN. Our team of over 10 local engineers covers East Anglia and East London, working on commercial boilers, plant rooms, water heaters, and warm air, radiant and unit heaters. We work on commercial premises only.
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Frequently asked questions
How often should we check a commercial expansion vessel pre-charge?
Follow the manufacturer's instructions for your vessel and pressurisation unit. As one published example, Hamworthy's Weymouth schedule recommends checking the expansion vessel charge pressure every 6 months, checking the vessel for external corrosion every 12 months, and inspecting the diaphragm every 4 years. Your engineer should confirm the interval for the equipment on your site. Trained site staff do weekly visual and gauge checks only; they do not measure or adjust the pre-charge.
What are the signs that an expansion vessel has lost its charge?
Common signs include system pressure that rises sharply when the boilers fire and falls when they cool, a relief valve that discharges on heat-up, and a pressurisation unit that runs more than usual. HSE's INDG261 notes that a safety valve which repeatedly discharges may mean the system is over-pressurising or the valve is not working correctly. Log what you see and call a competent engineer rather than adjusting anything.
Can our caretaker recharge the expansion vessel?
No. Restoring the pre-charge means isolating and emptying the vessel on the water side, measuring the gas charge and adjusting it with suitable equipment. That is work for a competent engineer. Where PSSR applies, vessels and protective devices are also examined by a competent person under the duty holder's written scheme of examination. Site staff should read and log the gauges and report anything outside the normal range.
Is a pressurisation unit the same as an expansion vessel?
No. The expansion vessel absorbs the volume change as the water heats and cools. The pressurisation unit maintains the set cold-fill pressure and tops up small losses. They work together on a sealed commercial system, and both need the checks set out in the manufacturer's instructions.
Do we need a written scheme of examination for our expansion vessels?
Only if the system falls under the Pressure Systems Safety Regulations 2000. HSE lists relevant fluids as including steam at any pressure, compressed gases above 0.5 bar and pressurised hot water above 110°C. Where PSSR applies, the written scheme must cover all protective devices and every pressure vessel. Many low-temperature hot water systems are outside that requirement, but they still need proper maintenance. A competent person can confirm.
What does a pressurisation unit service include?
Typically: recording the cold-fill pressure, pump cut-in and cut-out and alarm settings, checking the pump, pressure switches and boiler interlocks, checking the break tank, float valve and air gap, and checking the expansion vessel gas charge with the water side isolated. Follow the manufacturer's schedule for your unit and ask for it to be included in the annual plant service.
Why does our pressurisation unit keep topping up?
A unit that tops up often is usually replacing water that is being lost, for example through a small leak, a passing relief valve or an automatic air vent. A vessel with no gas charge can also make the pump cycle. Log how often it runs and call a competent engineer; frequent top-ups also dilute the inhibitor in the system water.
Can the expansion vessel and pressurisation unit be replaced separately?
Yes. They are separate components, and often only one has failed. An engineer should confirm the vessel size still suits the system and set the new part to match the existing cold-fill pressure before anything is replaced.
Sources and further reading
- Hamworthy Weymouth pressurisation units: installation, commissioning and servicing instructions
- HSE INDG261: Pressure systems, a brief guide to safety
- HSE INDG178: Written schemes of examination
- HSE: Pressure Systems Safety Regulations 2000 (PSSR)
- HSE: Using pressure systems
- PSSR 2000, regulation 8: written scheme of examination (legislation.gov.uk)
- HSE L122: Safety of pressure systems ACOP
- HSE: Gas safety for employers
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