Reading a commercial heating pressure gauge: normal ranges, rises and drops
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On a sealed commercial heating system the pressure gauge is the first thing many facilities teams look at when something feels wrong. Used properly, it is a simple weekly check: read the gauge, compare it with the engineer-set range, log the reading and report anything abnormal. Used badly, it becomes guesswork and DIY top-ups. This guide sticks to visual and gauge checks only. It explains what the reading is meant to show, what rises and drops often mean, and when to call a competent engineer.
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What the system pressure gauge shows
Hamworthy's instructions for its Weymouth pressurisation units (a common commercial range) place a 0–6 bar system pressure gauge on the front of the cabinet. That gauge shows the water pressure in the sealed system at the unit's connection point. The same manual defines system cold-fill pressure as the water pressure the pressurisation unit is set to maintain when the plant is cold. With the system cold and at fill pressure, the expansion vessel is empty of system fluid. As the water heats and expands, the vessel absorbs that volume and the gauge shows only a small rise. When the system cools, pressure falls again. If some water has been lost, the pressurisation unit's pump restores the set cold-fill pressure.
There is no single "correct" bar reading for every commercial site. Cold-fill and high/low switch settings are calculated for the plant, its height and its safety valve. On the Weymouth range, for example, the published cold-fill band is 1.0–3.4 bar, with an example set-up of about 1.8 bar cold fill, a low-pressure switch around 1.3 bar and a high-pressure switch around 3.65 bar. Your engineer should write the cold-fill / normal operating range for your plant in the site log. That recorded range is what weekly checks compare against.
How to log a gauge reading (visual check only)
A short weekly walk-round by trained site staff is good practice on many sites. It is not a specific legal requirement, and it does not replace manufacturer servicing, PSSR examination where that applies, or your own risk assessment. Stick to reading and logging. Do not open, drain, top up, adjust switches or touch relief valves.
- Find the right gauge: usually on the pressurisation unit front, or on the boiler / primary header. Use the gauge your engineer pointed out in the log.
- Note the plant state: cold / just started / fully warm / after a long idle. A small rise when hot is normal on a sealed system with a working expansion vessel.
- Read and compare: write the date, time, reading (bar), plant state and your initials. Compare with the engineer-set cold-fill / normal range in the log.
- Watch trends: a steady creep up or down over several days matters more than one odd reading after a busy heating cycle.
- Look around the gauge: pressurisation-unit fault lights, a pump that never stops, water at a tundish, damp floors, or BMS high/low pressure alarms. Log those too.
- Report, don't fix: anything outside the recorded range, repeated alarms, or relief-valve discharge needs a competent commercial engineer. Never adjust, cap, plug or test-lift a relief valve.
What rises and drops often mean (not a diagnosis)
None of the patterns below is a DIY diagnosis. They are signals to log and pass to an engineer.
| What you see | What it can point to (engineer to confirm) |
|---|---|
| Pressure climbs sharply when boilers fire, then falls hard when they cool | Expansion vessel not absorbing volume (often a lost gas charge). Expanding water then has nowhere to go. |
| Gauge sits at cold-fill and the pressurisation pump runs often or constantly | Hamworthy's Weymouth fault table links continuous pump running and "system pressure runs consistently at cold fill" to a small system leak or a wrong cold-fill switch setting — both engineer jobs. |
| Pressure falls below the recorded low range when plant is idle | Loss of water (leak, drain left open, vessel/diaphragm issue) or a pressurisation fault. |
| Pressure above the recorded high range, or high-pressure alarm / boiler lockout | Over-pressurising. On Advanced Weymouth units a high-pressure switch can shut the boilers down. HSE's INDG261 says a safety valve that repeatedly discharges may mean the system is over-pressurising or the valve is not working correctly. |
| Water at a tundish or discharge pipe | A protective device operating or failing. Do not cap or plug the discharge. Call an engineer. |
HSE's guidance for users of pressure systems says you must know the operating conditions and the safe operating limits of the equipment, fit suitable protective devices and keep them working, carry out suitable maintenance, and train people who operate or maintain the plant. INDG261 adds that once protective devices are set, they "cannot be altered except by an authorised person".
When to call a competent engineer
Call promptly if the gauge is outside the recorded range and stays there, if high or low pressure alarms repeat, if a relief valve is discharging, if the pressurisation pump never stops, or if boilers lock out on pressure. If you smell gas, call the National Gas Emergency Service on 0800 111 999 and follow your site procedure.
Under the Gas Safety (Installation and Use) Regulations 1998, gas work must be done by a Gas Safe registered engineer. Adjusting cold-fill or high/low pressure switches, recharging expansion vessels, tracing sealed-system leaks and testing or replacing relief valves are engineer tasks. Where the Pressure Systems Safety Regulations 2000 (PSSR) apply, protective devices and pressure vessels are also examined by a competent person under the duty holder's written scheme of examination. INDG261 notes 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. If you are unsure, ask a competent person or your insurer's engineering inspector.
For a deeper look at vessels and pressurisation units, see our guide to expansion vessel and pressurisation unit checks. For pressure loss, leaks or pressurisation faults, see commercial boiler pressure and leak repair. If plant is failing now, go to emergency commercial boiler 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.
Call or WhatsApp 07947 578332 (a photo of the appliance data plate or the fault display helps) · Email [email protected]
Frequently asked questions
What is a normal reading on a commercial heating pressure gauge?
There is no universal figure. The normal reading is the cold-fill / operating range your engineer recorded for your plant. On one published commercial example (Hamworthy Weymouth), cold-fill can be set between 1.0 and 3.4 bar, with an example around 1.8 bar cold fill. Always use the range written in your site log, not a number from another building.
Why does the pressure rise when the boilers heat up?
On a sealed system the water expands as it heats. The expansion vessel is meant to absorb that volume so the gauge only shows a small rise. A sharp climb on heat-up often means the vessel is not absorbing expansion (commonly a lost gas charge). Log it and call a competent engineer rather than adjusting anything.
Our pressurisation pump never stops. What should we do?
Log the gauge reading, whether the plant is hot or cold, and any alarms or discharge at tundishes. Hamworthy's Weymouth instructions list continuous pump running among faults that can point to a leak or an incorrect cold-fill switch setting. Do not alter pressure switches yourself. Call a competent commercial engineer.
Can our caretaker top up the system when the gauge is low?
No. On a commercial sealed system the pressurisation unit is meant to maintain the set cold-fill pressure. Manual top-ups, switch adjustments and vessel recharges are engineer work. Trained site staff should read and log the gauge against the engineer-set range and report anything outside it. Where PSSR applies, protective devices and vessels are examined by a competent person under the written scheme of examination.
Are weekly gauge checks a legal requirement?
There is no specific legal requirement for a weekly gauge check. HSE expects users of pressure systems to know the safe operating limits, keep protective devices working, maintain the system and train the people who use it. Where PSSR applies there must also be a written scheme of examination. A weekly look-and-log routine is good practice that helps you meet those duties and spot problems early.
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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