Pressure relief valves on industrial water heaters: weekly checks and why neglect is dangerous

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Industrial and commercial water heaters in factories, kitchens, hotels, care homes and leisure sites store a lot of hot water under pressure. The small valves on top of them, the pressure relief valve and the temperature and pressure (T&P) relief valve, are the last line of defence if the controls fail. This guide explains why neglecting them is dangerous, and sets out simple weekly checks to run alongside the annual service.

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Commercial heating tips and advice · 25 September 2026 · Fast Commercial Boiler Help, Gas Safe 985448

Why stored hot water under pressure is dangerous

Water held above 100°C under pressure does not boil, but if the vessel fails and the pressure drops suddenly, it can flash to steam. HSE's Approved Code of Practice for pressure systems (L122) treats hot water held above its boiling point at atmospheric pressure as a "relevant fluid" for that reason. HSE's leaflet INDG261 is blunt: "If pressure equipment fails in use, it can seriously injure or kill people nearby and cause serious damage to property."

The safety devices on an unvented water heater

Approved Document G (section G3) says an unvented hot water storage system should have at least two independent safety devices, in addition to the thermostat that controls the water temperature. An acceptable approach is:

a non-self-resetting energy cut-out that disconnects the heat supply if the water overheats; and
a temperature relief valve, or combined temperature and pressure relief valve, that safely discharges water in the event of serious overheating.

The guidance says these relief valves should be sized for the total power input of the system and fitted directly on the storage vessel so that the stored water does not exceed 100°C. Most systems also have an expansion relief valve and an expansion vessel, with discharge routed through a visible tundish to a safe place.

How neglect creeps in

Scale: the Andrews ClassicFlo manual tells the service technician to check the T&P valve at least once a year to make sure it has not become encrusted with lime and that it seats properly and operates freely.
Blocked or capped discharge: the same manual says not to plug the T&P relief valve outlet. A valve that dribbles is sometimes "fixed" by capping it. That removes the protection completely.
Repeated discharge ignored: HSE's INDG261 notes that a safety valve that repeatedly discharges may mean the system is over-pressurising or the valve is not working correctly. On sealed systems a common cause is an expansion vessel that has lost its charge, but it needs diagnosing.
Corrosion: Andrews advises inspecting the sacrificial anodes annually and replacing them as needed. Depleted anodes leave the vessel unprotected.

Weekly visual and gauge checks for trained site staff

A short weekly walk-round by trained site staff is a sensible routine for many sites. They make visual and gauge checks, log the readings and report anything abnormal. It is not a specific legal requirement, and it doesn't replace the manufacturer's service intervals, the written scheme of examination or your own risk assessment. It is simply how small changes get spotted early. Nothing is opened, lifted or adjusted.

System pressure: read the boiler or pressurisation unit gauge and compare it with the normal range your engineer has recorded. Log it. A steady rise or fall matters more than one reading.
Hot water temperature: note the storage temperature gauge where fitted. HSE's legionella guidance says hot water should be stored at 60°C or above and reach 50°C at outlets within a minute (55°C in healthcare premises).
Tundishes and discharge pipes: look for water running through any tundish. Any discharge from a relief valve is a warning sign, not a nuisance. Check that discharge pipes are intact, not capped, and not blocked or damaged.
Relief valves: look for weeping, scale crust, corrosion or paint over the valve. Don't lift the easing lever. The Andrews manual warns that doing so releases hot water under pressure.
Expansion vessels and pressurisation unit: look for alarms, fault lights, a pump that runs constantly, and external rust or damage on vessels.
Around the heater: leaks, damp floors, rust staining, soot or scorching on gas-fired units, and any smell of gas or fumes.
Record and report: date, initials and what you saw. Report anything abnormal to a competent commercial engineer promptly. Never adjust, cap, plug or test-lift a relief valve yourself.

Our commercial plant room safety checklist covers the rest of the room, and our guide to yearly heater servicing covers the appliances.

What the engineer does at the service

At a planned service, a competent engineer checks relief valve operation and replaces valves where needed, tests the energy cut-out and thermostat, inspects anodes, and on gas-fired heaters checks combustion, flueing and gas safety. Under the Gas Safety (Installation and Use) Regulations 1998, that gas work must be done by a Gas Safe registered engineer. Expansion vessels need their gas charge checked with the vessel isolated and drained. Hamworthy's instructions for its Weymouth pressurisation units, for example, list a charge check every 6 months, an external corrosion check every 12 months and a diaphragm inspection every 4 years. Always follow the schedule for the equipment you actually have. Where PSSR applies, relief valves, expansion vessels and pressure vessels are also examined by a competent person under the duty holder's written scheme of examination.

Does PSSR apply to your water heaters?

The Pressure Systems Safety Regulations 2000 apply to systems containing a relevant fluid. HSE lists steam at any pressure, compressed gases above 0.5 bar and pressurised hot water above 110°C. Where PSSR applies, the user must have a written scheme of examination, drawn up or certified by a competent person, covering "all protective devices", every pressure vessel, and pipework where a defect may give rise to danger (regulation 8). Many low-temperature water heating systems fall outside the written-scheme requirement, but the general duty to keep them safe still applies. If you're unsure, ask a competent person or your insurer's engineering inspector.

For water heater and plant room work in hotels and kitchens, care homes and factories, see our commercial water heater and calorifier repair and planned maintenance pages. For pressure loss, leaks or pressurisation unit faults, see commercial boiler pressure and leak repair. If a valve is discharging now, go to emergency help.

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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Call or WhatsApp 07947 578332 (a photo of the appliance data plate or the fault display helps) · Email [email protected]

Frequently asked questions

How often should a T&P relief valve on a commercial water heater be checked?

Follow the manufacturer's instructions for your model. For example, the Andrews ClassicFlo manual says the combined temperature and pressure relief valve should be checked at least once a year, as part of maintenance by a qualified service technician. Trained site staff can look for weeping, scale or discharge during weekly visual checks and report it, but must never operate, adjust, cap or plug the valve.

Water is dripping from the tundish. What should we do?

Treat it as a warning that a safety device is operating or failing. Do not cap or plug the discharge. Note when it started and what the pressure and temperature gauges show, keep people clear of hot discharge, and call a competent commercial engineer. If there is a smell of gas on a gas-fired heater, call 0800 111 999.

Are weekly pressure checks a legal requirement?

There is no specific legal requirement for weekly checks. The legal duties are to keep equipment safe and, where PSSR applies, to have a written scheme of examination and follow it. A weekly look-and-log routine is good practice that helps you meet those duties and spot problems early.

Can our caretaker top up the pressure or recharge the expansion vessel?

Adjusting or recharging pressurised equipment should be left to a competent engineer. Expansion vessel charge checks, for example, involve isolating and draining the vessel. Where PSSR applies, vessels and protective devices are examined by a competent person under the duty holder's written scheme of examination. Your caretaker's job is to read and log the gauges and report anything outside the normal range.

Does a written scheme of examination cover relief valves?

Where PSSR applies, yes. Regulation 8 requires the written scheme to cover all protective devices, along with every pressure vessel and pipework where a defect may give rise to danger. The scheme must be drawn up or certified by a competent person.

Sources and further reading

Call 0800 0996 007

Call or WhatsApp 07947 578332 · [email protected]

General guidance only. This article is general information for commercial premises. It is not a substitute for advice from a competent engineer who has seen your site and equipment, or for the manufacturer's instructions. Duty holders remain responsible for their own legal compliance.