Unvented cylinder discharge pipe regulations in England come from Part G3 of the Building Regulations 2010, with the detail set out in Approved Document G. They require a short metal pipe (D1) from the safety valve to a tundish within 600 mm, then a larger pipe (D2) that drops at least 300 mm vertically, falls continuously at 1:200 or steeper, and ends where any discharge is visible but harms nobody. A tundish is the open funnel you can see near the cylinder, so a discharge is noticed rather than hidden. This guide explains the dimensions, materials, length limits and termination points, and who is allowed to carry out the work.
What do the discharge pipe regulations for an unvented cylinder require in England?
Unvented cylinder discharge pipework is covered by Part G3 of the Building Regulations 2010, Schedule 1, which applies in England. G3 requires a hot water storage system to withstand the temperatures and pressures of normal use and of reasonably foreseeable failures. Any discharge from a safety device must be taken safely to a place where it is visible but creates no danger to people in or around the building. Approved Document G is the official guidance on meeting that requirement.
Approved Document G, in the edition with 2024 amendments, expects an unvented system to have at least two independent safety devices in addition to the control thermostat. The Water Supply (Water Fittings) Regulations 1999, which apply in England and Wales, require a temperature relief valve or a combined temperature and pressure relief valve on every unvented storage vessel over 15 litres. That valve must be fitted directly to the vessel and discharge enough to keep the stored water below 100°C.
The discharge route is read as two pipes. D1 is the short metal pipe from the safety valve to the tundish. D2 is everything from the tundish to the point where the water leaves the building. Almost every dimension people search for belongs to D2.
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How close to the safety valve must the tundish on an unvented cylinder sit?
The tundish on an unvented cylinder must sit within 600 mm of the safety valve outlet, measured along the D1 pipe. Approved Document G also asks for it to be fitted vertically, in the same room as the cylinder, as close to the valve as practical and lower than the valve. It has to stay visible to the person using the building, so a discharge is noticed.
- D1 is a short metal pipe, commonly 15 mm copper on domestic cylinders.
- D1 must be no smaller in diameter than the nominal outlet of the safety valve.
- The temperature and pressure relief valve and the expansion valve discharges may be joined by a tee above the tundish.
- Manufacturers ask for the tundish, drain valve and motorised valves to be kept clear of electrical components.
- Siting the tundish where nobody will ever look defeats the purpose of the visibility rule in G3.
Fitting, moving or altering a tundish, a relief valve or D1 is work for a competent person. It is not a job for the householder, because the pipework carries water that can leave the cylinder at scalding temperature, the same risk a thermostatic mixing valve on a bath addresses.
What does Approved Document G require of the D2 discharge pipe below the tundish?
The D2 discharge pipe below the tundish must run vertically for at least 300 mm before any bend or elbow, then fall continuously at a gradient of at least 1:200. D2 must also be at least one pipe size larger than the nominal outlet of the safety device, unless the calculated hydraulic resistance of the route demands more. The 300 mm straight section lets the discharge settle before it turns.
- Material: metal, or another material proven to withstand the temperature of the discharged water and permanently marked with its product and performance standard.
- Plastic is only acceptable where the pipe is tested for at least 110°C.
- No valves, isolating taps or stopcocks may be fitted anywhere in the discharge pipe.
- The pipe must stay open to atmosphere along its whole length.
- The route must fall throughout and be kept out of positions where it could freeze.
A blocked, valved or upward-running D2 removes the safety function the regulations are there to protect. If your cylinder has a D2 pipe you cannot trace, or one that rises after the tundish, have it inspected before the next heating season.
22 mm copper D2 or 28 mm copper D2: how far can an unvented cylinder's discharge pipe run?
Approved Document G limits the length of an unvented cylinder's D2 discharge pipe by its resistance, expressed as straight copper pipe with a deduction for every bend. The figures below apply to a safety valve with a G1/2 outlet.
| Criterion | 22 mm copper D2 | 28 mm copper D2 |
|---|---|---|
| Longest permitted run with no bends | Up to 9 m of straight pipe | Up to 18 m of straight pipe |
| Resistance deducted for each elbow or bend | 0.8 m of the permitted length | 1.0 m of the permitted length |
| Worked example with four bends | 9 m less 3.2 m leaves 5.8 m of pipe | 18 m less 4.0 m leaves 14 m of pipe |
| If the route is still longer than that | Step up to 28 mm and recalculate the resistance | Step up to 35 mm, which allows 27 m with 1.4 m per bend |
The table in Approved Document G is based on copper pipe. Plastic pipes can have a different bore and different resistance, so the copper figures cannot simply be copied across. Where a long or twisting route is unavoidable, the resistance has to be worked out for the actual run rather than assumed.
Where is an unvented cylinder's discharge pipe allowed to end?
An unvented cylinder's discharge pipe may end in a trapped gully, with the pipe outlet below the fixed grating and above the water seal. Approved Document G also accepts a low-level discharge pointing downwards, up to 100 mm above an external surface such as a car parking area. Manufacturers add that a low outlet should be guarded so children cannot reach the hot water.
Connection to a soil stack is possible, but only where the stack can safely take the temperature of the discharged water. Metal stacks such as cast iron are suitable. The Approved Document G FAQ allows discharge into PVC-U stacks for domestic unvented cylinders on small systems, but not for combination units. Where the discharge joins a stack, polybutylene, cross-linked polyethylene or polypropylene to BS EN 1451-1 are named as acceptable pipe materials, provided the other requirements are still met.
Discharging into a soakaway, a sealed drain or a hidden void does not meet G3, because the discharge would not be visible. If you are planning a new cylinder and the only convenient drain is on the wrong side of the house, the route needs to be settled before the unvented cylinder is positioned.
Who may work on an unvented cylinder's discharge pipe, and is it notifiable?
Work on an unvented cylinder and its discharge pipe may only be carried out by a person competent to do it, because the system has no vent pipe to atmosphere. Installing or replacing an unvented hot water cylinder is notifiable building work in England, and the records behind it are the ones covered in how to check if your boiler has been registered. Where the installer belongs to a Competent Person Scheme covering unvented hot water, no notice to building control is needed in advance.
The installer then self-certifies the work and the scheme operator issues the homeowner a certificate of compliance. That certificate is the paperwork a buyer's solicitor asks for later, as our guide on which boiler certificate you need when selling a house sets out. More on the people and the paperwork is set out in our guides on who can install an unvented cylinder and on the building regulations notification a new cylinder needs.
Where the cylinder is heated by a gas boiler, any work on the gas supply, the flue or inside the boiler casing is separate. That work is for a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998 as amended, in England.
Water is running from the tundish on my unvented cylinder: what does it mean?
Water running from the tundish on an unvented cylinder means a safety device has discharged, and the cause needs checking by a competent person. The tundish is doing exactly what the regulations intend: making that discharge visible. A brief discharge during a heating cycle and a steady trickle point to different faults, so it helps to note which you have, just as brown water from the hot tap only points to the hot water side.
- Note whether the discharge is occasional, or continuous day and night.
- Read the boiler pressure gauge and note what it shows when the system is cold.
- Leave the relief valves, tundish, discharge pipe and expansion vessel alone: these are for a competent person.
- Do not fit anything into the discharge pipe to stop the drip, as valves there are not permitted.
Common causes an engineer looks at include a relief valve held open by debris or scale in a hard water area, the same scale behind a boiler banging when it heats water, and an expansion vessel that has lost its air charge. Our guide on the expansion vessel losing its charge explains that second case. If you want the cylinder, its valves and the discharge pipework looked at by an engineer, you can arrange a visit through our boiler service and repairs page.
Sources
- Approved Document G with 2024 amendments (for use in England)
- Building Regulations 2010, Schedule 1 (Part G3)
- Approved Document G Frequently Asked Questions
- The Water Supply (Water Fittings) Regulations 1999
- AD G amendment booklet 2024
- Heatrae Sadia Megaflo Eco installation manual
- Worcester Bosch Greenstore installation and maintenance instructions
- Glow-worm Hydracyl unvented cylinders manual