Short answer: HSE's HSG274 Part 2 (Table 2.1) asks for a monthly check that the calorifier's flow temperature is as close to 60 °C as practicable without going below it and that the return is not below 50 °C, plus a yearly internal inspection and clean. The Pressure Systems Safety Regulations 2000 apply only to parts holding a relevant fluid, such as a steam primary.
What HSG274 asks for
A calorifier is the storage water heater at the centre of most commercial hot water systems. HSE's technical guidance on Legionella, HSG274 Part 2 (2024), sets out the routine checks in its Table 2.1. The site's Legionella risk assessment sets the actual frequency: HSE says the table gives "an indication of the frequency of inspection and monitoring" (para 2.79).
| Check | What HSE says | Frequency |
|---|---|---|
| Flow temperature | Thermostat settings "should modulate as close to 60 °C as practicable without going below 60 °C" | Monthly |
| Return temperature | Not below 50 °C | Monthly |
| Internal inspection and clean | Inspect internally by removing the inspection hatch or using a boroscope, and clean by draining the vessel; adjust the frequency to the conditions recorded | Annually, or as indicated by the rate of fouling |
| Purge where there is no hatch | Purge any debris in the base of the calorifier to a suitable drain | Annually, or more often if the risk assessment or inspection findings say so |
| Initial flush from the base | Collect it and inspect clarity, quantity of debris and temperature | Annually, or more often if the risk assessment or inspection findings say so |
In the same table, the hot water circuits served by the calorifier have their own monthly checks: return legs of principal loops at a minimum of 50 °C (55 °C in healthcare premises), and sentinel outlets on non-circulating systems reaching 50 °C within one minute.
Design points that affect the service visit
- Somewhere to measure. HSE expects thermometer or immersion pockets on the flow and return, and in the base of the calorifier (para 2.37).
- A drain you can reach. The drain valve should be "in an accessible position at the lowest point and as close as possible to the vessel, so that accumulated particulate matter can be safely drained" (para 2.37).
- The cold zone at the base. In indirect calorifiers, cold water enters at the base, below the coil, where the water may support microbial growth. HSE suggests a timer-controlled shunt pump so that at least 60 °C is reached throughout the vessel "for at least one continuous hour a day" (para 2.25).
- Clean before you shunt. A shunt pump should not be run, and no alteration made, before the calorifier is cleaned and descaled, "as operating the pump may disturb sludge or sediment" (para 2.26).
- Parallel vessels. Where there is more than one calorifier, HSE says they should be connected in parallel and each deliver at least 60 °C (para 2.37).
- Standby units. A standby calorifier needs "a suitable procedure to ensure the risk is controlled before they are brought back into service" (para 2.61).
- Sampling under pressure. Samples from the hot water outlet and the base should be taken only where it is safe to do so, because HSE notes that "some systems are under considerable pressure" (para 2.124).
Is a calorifier a pressure system?
The Pressure Systems Safety Regulations 2000 (PSSR) apply to pressure systems used at work (reg 3(2)). A pressure system is one that "contains or is liable to contain a relevant fluid" (reg 2(1)). A relevant fluid is:
- steam;
- a gas, or a liquid with a vapour pressure greater than 0.5 bar above atmospheric "at either the actual temperature of the liquid or 17.5 degrees Celsius", at a pressure greater than 0.5 bar above atmospheric;
- a gas dissolved under pressure in a solvent in a porous substance (acetylene).
HSE's summary of the Regulations puts the hot water case simply: relevant fluids include "pressurised hot water above 110 °C". HSE's Approved Code of Practice and guidance, L122, describes it as "hot water contained above its boiling point at atmospheric pressure" (para 19), and adds that "only those parts of a system which contain a relevant fluid fall within the scope of the Regulations", except protective devices on such a system (para 20).
So the answer depends on the calorifier's primary side:
| Calorifier type | Relevant fluid? | What follows |
|---|---|---|
| Heated by steam | Yes: steam, at any pressure | The steam side and its protective devices are in scope. The 250 bar-litre exception does not apply to steam: L122 says "where the relevant fluid is steam all the regulations apply, irrespective of the vessel pressure and size" (para 213). A written scheme of examination is needed (reg 8). If a control fault could heat the stored water above its boiling point at atmospheric pressure, the secondary side is "liable to contain" a relevant fluid as well; the competent person decides whether the shell and its relief valves go in the scheme. |
| Heated by pressurised hot water above its boiling point (HSE: above 110 °C) | Yes | As for steam, subject to the exceptions in Schedule 1. |
| Heated by low temperature hot water, or an electric immersion, storing water at around 60 °C | Not from the stored water, which is below its boiling point | PSSR does not apply to those parts, provided the water cannot be driven above its boiling point under foreseeable operating conditions. L122 says the user "should be able to show clear evidence that the system does not contain (and is not liable to contain) a relevant fluid" (para 21). |
A gas-charged expansion vessel on an unvented system holds compressed gas. Whether it forms part of a pressure system is a decision for the user with a competent person. Where it does, Schedule 1 Part II para 2 lifts regulations 5(4), 8 to 10 and 14 (marking, the written scheme, examinations and records) if the product of pressure in bar and internal volume in litres is less than 250 bar litres for each vessel. The duty to maintain the system (reg 12) still applies.
L122 says the user or owner "should decide, in consultation with a person competent to advise as appropriate, whether a written scheme of examination is required" (para 113). For more on schemes, examinations and reports, see our guide to pressure systems in plant rooms.
Worked example: an office with two calorifiers
A six-storey office has two 1,000-litre indirect calorifiers in parallel, heated from gas boilers at 80 °C flow, with a shunt pump on a time clock and a circulating hot water system with four principal loops. The risk assessment adopts the Table 2.1 frequencies.
| Visit | Calorifier tasks |
|---|---|
| Monthly | Flow and return temperature of each calorifier; return temperature at each of the four principal loop returns; shunt pump timer and operation |
| Yearly | Isolate, drain and inspect each calorifier in turn through the hatch; clean; record the initial flush from the base (clarity, debris, temperature); check the drain valve can be reached and operated; bring the vessel back into service under the site's procedure |
The primary is hot water at 80 °C, below its boiling point, so PSSR does not apply to the calorifiers on their primary or secondary side. The maintenance contractor records that position, with the boiler flow temperature and the boilers' overheat cut-outs it rests on, in the site file. The gas-charged expansion vessels on the system are a separate question for the user and a competent person, as above. Inspecting one vessel at a time keeps hot water on during the yearly visit.
If the same building were heated from a steam main, the steam side of each calorifier would need a written scheme of examination drawn up or certified by a competent person, and examinations by that competent person at the intervals in the scheme (regs 8 and 9).
Who does what
- The dutyholder (the employer or person in control of the premises) owns the Legionella risk assessment and, for a steam or high-temperature calorifier, is the PSSR "user" who must hold the written scheme.
- The competent person under PSSR, an individual (other than an employee) or a body, as regulation 2 defines it, draws up or certifies the scheme and carries out the examinations.
- The maintenance contractor takes the monthly temperatures, carries out the yearly inspection and clean, and reports results outside the limits with how urgent they are.
- The water hygiene specialist, where one is appointed, reviews the records and advises on sampling and the risk assessment.
What to record
- Calorifier identity, location, capacity and primary heat source
- Monthly: flow and return temperatures for each calorifier, with the date, time and instrument used
- Yearly: condition found at inspection, amount and type of debris, initial flush clarity, debris and temperature, cleaning done
- Shunt pump timer setting and whether it ran
- Readings outside the limits and the action taken
- The PSSR position: whether a written scheme is needed, and if so the date of the last examination and the next one due
Running calorifier visits in Field Suite
- Equipment register: each calorifier at its site, with make, model, serial number, location, service interval and job history; capacity and primary heat source go in its notes.
- Contracts: a 1-month visit line for the temperature checks and a 12-month line for the inspection and clean, each created as a job a set number of days before it is due.
- Your own form: build the checklist with the form builder. Number questions record temperatures with their unit. Number answers cannot fail a form on their own, so add a Pass/Fail question such as "Flow at or above 60 °C" next to each reading.
- Faults: a low return temperature or a heavy deposit becomes a fault marked by urgency, which the office can turn into a quote.
See planned maintenance software, read about booster set servicing for the cold water side, or book a demo.
Field Suite is job management software. The site's Legionella risk assessment, the written scheme where one is needed and the manufacturer's instructions decide how a calorifier is maintained.
Questions
Does every calorifier need a written scheme of examination?
No. Only parts holding a relevant fluid are in scope. A calorifier heated by steam needs one for its steam side; one heated by hot water below its boiling point does not, on that basis.
How often should a calorifier be opened and inspected?
HSG274 Table 2.1 says annually, or as indicated by the rate of fouling, with the frequency adjusted to what each inspection finds.
Is a flow temperature of 58 °C acceptable?
Not under Table 2.1, which says the setting should modulate as close to 60 °C as practicable without going below 60 °C.
Can a new shunt pump be started straight away on an old calorifier?
HSE says not before the calorifier is cleaned and descaled, because the pump may disturb sludge or sediment.
Sources
- HSE, Legionnaires' disease: Technical guidance, HSG274 Part 2 (2024), paras 2.25, 2.26, 2.37, 2.61, 2.79, 2.124 and Table 2.1. https://www.hse.gov.uk/pubns/priced/hsg274part2.pdf
- Pressure Systems Safety Regulations 2000, regulation 2 (interpretation). https://www.legislation.gov.uk/uksi/2000/128/regulation/2
- Pressure Systems Safety Regulations 2000, regulation 3 (application). https://www.legislation.gov.uk/uksi/2000/128/regulation/3
- Pressure Systems Safety Regulations 2000, regulation 8 (written scheme of examination). https://www.legislation.gov.uk/uksi/2000/128/regulation/8
- Pressure Systems Safety Regulations 2000, regulation 9 (examination in accordance with the written scheme). https://www.legislation.gov.uk/uksi/2000/128/regulation/9
- Pressure Systems Safety Regulations 2000, Schedule 1 (exceptions). https://www.legislation.gov.uk/uksi/2000/128/schedule/1
- HSE, Pressure Systems Safety Regulations 2000 (PSSR). https://www.hse.gov.uk/pressure-systems/pssr.htm
- HSE, Safety of pressure systems, Approved Code of Practice L122 (second edition, 2014), paras 19 to 21, 113 and 213. https://www.hse.gov.uk/pubns/priced/l122.pdf
Last checked: 5 November 2026 against the sources above.
Checked in detail: HSE HSG274 Part 2 (2024) Table 2.1 and paras 2.25, 2.26, 2.37, 2.61, 2.79, 2.124; PSSR 2000 regs 2, 3, 8, 9 and Schedule 1 on legislation.gov.uk (no outstanding effects); HSE PSSR page; HSE L122 (2014) paras 19 to 21, 113 and 213.
Field Suite is job management software for UK building services firms, not a consultancy. Your own risk assessments, contracts and advisers decide what applies to your work. Spotted a mistake? Email info@fieldsuiteapp.com and we correct it.