Short answer: Building Regulations requirement G3 governs how an unvented cylinder is installed, and Approved Document G gives the usual way to meet it: two independent safety devices besides the thermostat, and discharge pipework that shows any discharge and takes it somewhere safe. G3 sets no service interval. Manufacturers do: Mixergy and Dutypoint ask for an annual service by a competent person, including relief valve, strainer and expansion vessel checks.
What G3 requires
Requirement G3 is in Part G of Schedule 1 to the Building Regulations 2010. Approved Document G (2015 edition with the 2016 and 2024 amendments) gives the government's guidance for England (it also covers excepted energy buildings in Wales). Elsewhere in the UK, check the local building standards guidance. G3(3) says a hot water system with a storage vessel must incorporate precautions to:
- "prevent the temperature of the water stored in the vessel at any time exceeding 100˚C"; and
- "ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building".
G3(3) does not apply to a system that heats or stores water only for an industrial process. G3(4) limits the hot water delivered to a fixed bath to 48°C, but only in new dwellings and dwellings formed by a material change of use.
| Topic | Guidance | Paragraph |
|---|---|---|
| Safety devices | At least two independent safety devices, in addition to the thermostat. An acceptable approach: a non-self-resetting energy cut-out, plus a temperature relief valve or combined temperature and pressure relief valve | 3.17, 3.18 |
| Up to 500 litres and under 45 kW | Should be a proprietary unit or package meeting a standard such as BS EN 12897:2006 | 3.22 |
| Over 500 litres | Generally bespoke; designed to the safety requirements in 3.17 by an appropriately qualified engineer | 3.26 |
| Markings | Maker, model, capacity, operating pressure and expansion valve pressure, safety device data, and a warning to the user and installer that stays visible after installation | 3.23, 3.24 |
| Relief valves | Temperature relief valves in a unit or package are factory fitted, not disconnected other than for replacement, and not relocated | 3.39 |
| Who installs | Installers with the necessary skills, shown for example by registration with a competent person scheme or a current registered operative skills card for unvented hot water systems | 3.40 |
| Notification | Installing an unvented system is notifiable building work, unless the installer is registered with a competent person scheme for it and self-certifies | 3.41, 3.42 |
The discharge pipework
Approved Document G sets out an arrangement that meets G3(3)(b). It is the part of the system most worth checking on every visit, because changes made by others, such as a new ceiling, a blocked gully or a plastic gutter, can make a safe discharge unsafe.
- D1: a short length of metal pipe from the relief valve to the tundish, not smaller than the valve outlet (3.50, 3.51).
- Tundish: vertical, in the same space as the cylinder, lower than the valve, with no more than 600 mm of pipe between the valve outlet and the tundish. Any discharge should be visible at the tundish (3.54, 3.55).
- D2: at least 300 mm vertical below the tundish before any bend, then a continuous fall of at least 1 in 200 (3.56). Metal, or another material shown to withstand the discharge and marked to say so (3.57). At least one pipe size larger than the valve outlet, and larger again for longer runs (3.58 and Table 3.1).
- Where it ends: somewhere safe, for example a trapped gully below a fixed grating, a low-level discharge with a guard, or a metal hopper and downpipe (3.61, 3.62). The discharge is hot water and steam, which can damage asphalt, roofing felt and non-metal gutters (3.63).
Approved Document G's worked example: a G½ temperature relief valve with 7 m of 22 mm D2 and four elbows. Straight 22 mm pipe is allowed up to 9 m, less 0.8 m per elbow, which leaves 5.8 m, too short. In 28 mm the allowance is 18 m less 1.0 m per elbow, which leaves 14 m, so 28 mm copper is satisfactory.
What an annual service covers
The service schedule comes from the manufacturer. Two examples, both read on the makers' own websites:
| Item | Mixergy (installation and servicing instructions MDC0001-16) | Dutypoint Element UHP (operation and maintenance manual DOC-EUHPCOM1801) |
|---|---|---|
| Frequency and who | Annual maintenance by a competent operative; keep the service record in the booklet | At least annually, by a competent unvented hot water installer or Dutypoint authorised personnel |
| Expansion relief valve | Open the twist cap, check water discharges and runs clearly through the tundish to the final discharge point, and that the valve reseats | Part of the commissioning checks repeated at service; if it does not reseat, lift it once or twice to clear debris, then replace |
| Temperature and pressure relief valve | Repeat the same test and check it reseats | As above |
| Strainer | Turn off the mains, remove the cartridge from the pressure reducing valve, clean the strainer and refit | Strainer cleared of debris, where fitted |
| Expansion vessel or air gap | Check the vessel pressure is 3 bar and recharge if needed | Check the vessel or internal air space and recharge it as section 6.2 of the manual sets out; the figures printed there match the relief valve settings, so confirm the charge pressure for the model with Dutypoint before recharging |
| Other checks | Not listed for the annual service | Inlet pressure, thermostat setting, PRV setting, tundish and discharge pipework to Part G, cut-outs on every energy source |
Pressures and settings differ between makes and models, so the right figures are always the ones in the manual for the cylinder on site.
A cylinder also stores hot water for the building, so the water hygiene side applies too. HSE's HSG274 Part 2 says hot water should be stored at 60°C at least and reach 50°C within one minute at outlets (55°C in healthcare premises) (para 2.82). Our guide to Legionella checks on hot and cold water covers the monitoring, and calorifier maintenance covers larger vessels and when the Pressure Systems Safety Regulations come in.
Worked example: a 300-litre cylinder in a dental practice
- Before the visit. The office attaches last year's record: expansion vessel at 3.0 bar, both valves reseated, strainer clean.
- Discharge. The engineer finds that a new suspended ceiling hides the D2 run but the tundish is still visible in the cylinder cupboard. The termination at the gully outside has been covered by a planter. That is recorded as a defect with photos, and the practice manager is told the same day.
- Valves. The expansion relief valve discharges and reseats. The temperature and pressure relief valve discharges but drips afterwards; lifting it twice does not stop it, so it is recorded for replacement with a genuine part.
- Strainer and vessel. The strainer is cleaned. The vessel reads 2.4 bar and is recharged to the manual's figure.
- Temperature. Stored water reads 61°C at the flow, which goes on the Legionella log as well.
Who does what
- The building owner or occupier chooses an installer with the necessary skills (AD G 3.40) and arranges servicing to the manufacturer's instructions.
- The installer fits a proprietary unit, notifies the building control body or self-certifies through a competent person scheme, and hands over the paperwork.
- The maintenance contractor services the cylinder to the manual, checks the discharge arrangement, and reports defects with their urgency.
- The manufacturer's service team handles parts and faults its manual reserves for it.
What to record
- Cylinder make, model, serial number, capacity and location
- Inlet pressure, pressure reducing valve setting and thermostat setting
- Expansion vessel or air gap pressure as found and as left
- Expansion relief valve and temperature and pressure relief valve: discharged, visible at the tundish, reseated
- Strainer cleaned
- Discharge pipe: tundish visible, D2 fall and termination safe
- Stored and outlet temperatures
- Defects, urgency, parts, photos and the engineer's name
Running cylinder servicing in Field Suite
- Your own checklist: the form library has no unvented cylinder form today. Build one with the form builder: number questions with units for pressures and temperatures, Pass/Fail/N/A for the valves and discharge, and a photo of the termination.
- Equipment register: each cylinder sits at its site with make, model, serial number, service interval and job history.
- Contracts: an annual visit, created as a job a set number of days before it is due, with the form attached.
- Defects: a failed valve or unsafe termination is recorded with its urgency, and the office turns it into a quote.
See planned maintenance software, pump and water services software, prices, or book a demo.
Field Suite is job management software. The manufacturer's instructions and Approved Document G decide how a cylinder is installed and serviced.
Questions
Is an annual service of an unvented cylinder a legal requirement?
The Building Regulations govern the installation, not servicing. Manufacturers ask for an annual service: Mixergy, for example, says failing to maintain the cylinder to its instructions invalidates its warranty.
Who can work on an unvented cylinder?
Approved Document G asks for installers with the necessary skills, shown for example by competent person scheme registration or a registered operative skills card for unvented hot water systems. Manufacturers ask for a competent person for servicing.
Does replacing a cylinder need building control?
Installing an unvented system is notifiable, unless a competent person scheme installer self-certifies it (AD G 3.41 and 3.42).
Sources
- HM Government, Approved Document G: sanitation, hot water safety and water efficiency, 2015 edition incorporating 2016 and 2024 amendments, requirement G3 and paragraphs 3.17 to 3.65. https://www.gov.uk/government/publications/sanitation-hot-water-safety-and-water-efficiency-approved-document-g
- Approved Document G, PDF with 2024 amendments. https://assets.publishing.service.gov.uk/media/66f6c6ce3b919067bb4828cc/ADG_with_2024_amendments.pdf
- Mixergy, Installation and servicing instructions for stainless steel hot water cylinders, MDC0001-16, page 38. https://support.mixergy.co.uk/hubfs/Mixergy-Installation-and-Servicing-Guide.pdf
- Dutypoint, Element Ultra High Performance indirect hot water cylinders, operation and maintenance manual, DOC-EUHPCOM1801, sections 5, 6 and 7. https://www.dutypoint.com/app/uploads/2023/04/Dutypoint-Hot-Water-Cylinder-OM-Manual.pdf
- HSE, Legionnaires' disease: Technical guidance, HSG274 Part 2 (2024), para 2.82. https://www.hse.gov.uk/pubns/priced/hsg274part2.pdf
Last checked: 5 November 2026 against the sources above.
Checked in detail: Approved Document G PDF with 2024 amendments downloaded from GOV.UK (requirement G3, paras 3.17 to 3.42, 3.50 to 3.66, Table 3.1 and worked example) and the GOV.UK publication page (England, excepted energy buildings in Wales); Mixergy MDC0001-16 page 38 and Dutypoint DOC-EUHPCOM1801 sections 5 to 7 downloaded from the makers' sites; HSE HSG274 Part 2 (second edition 2024) para 2.82.
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.