In England, a vented hot water cylinder should have a suitable vent pipe running from the top of the cylinder to a point open to the atmosphere, above the water level of the cold water storage cistern and above the cistern itself. That comes from Requirement G3 of Schedule 1 to the Building Regulations 2010, with the detail in Approved Document G. An unvented cylinder is different: it is fed straight from the mains and has no vent pipe at all, so safety devices, a tundish and a discharge pipe do the same job. Water law applies on top, through the Water Supply (Water Fittings) Regulations 1999. This page sets out both routes, who may work on them, and what you can safely check at home.
What do hot water cylinder vent pipe regulations require in England?
The rule for hot water cylinder vent pipes in England is Requirement G3 of Schedule 1 to the Building Regulations 2010. The supporting guidance is Approved Document G, Sanitation, hot water safety and water efficiency. For a vented storage vessel it says there should be a suitable vent pipe. The pipe connects the top of the vessel to a point open to the atmosphere, above the water level of the cold water storage cistern and above that cistern.
Approved Document G also expects a second line of defence. Besides the thermostat, the heat source or the storage vessel should carry a device that stops the stored water exceeding 100°C. The system should have pipework that takes hot water from those safety devices to a safe place open to the atmosphere, where nobody in or around the building is put at risk.
Water law adds a parallel duty. Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 requires appropriate vent pipes, temperature control devices and combined temperature and pressure relief valves, so that water in a secondary hot water system does not exceed 100°C. A secondary system is simply the hot water you draw at the taps, as opposed to the primary circuit that heats it.
Why an unvented hot water cylinder has no vent pipe at all
An unvented hot water cylinder has no vent pipe by design. Approved Document G describes unvented systems as storage vessels supplied directly from the cold water mains, with no vent pipe to the atmosphere to relieve pressure. Expansion is taken up inside the system, and pressure is released through valves rather than through an open pipe over a loft cistern.
In place of the vent pipe, Requirement G3(3) is treated as met when the unvented vessel has at least two independent safety devices in addition to the thermostat. Approved Document G is blunt about the reason. A badly installed unvented cylinder can burst, or cause harm through escaping hot water.
That is why an unvented system may only be installed by a person competent to do the work, with arrangements that keep the stored water below 100°C and carry any discharge away safely. If you are thinking about moving from a cistern-fed cylinder to a mains-pressure one, the practical differences are set out in our guide to converting a vented system to an unvented cylinder.
Where a cylinder vent pipe may end, and what must never sit inside it
A vent pipe on a primary circuit must not terminate over a cistern that holds wholesome water for domestic supply or for a secondary system. That prohibition is in Schedule 2 of the Water Supply (Water Fittings) Regulations 1999. The same schedule requires discharges from temperature relief valves, combined temperature and pressure relief valves and expansion valves to happen in a safe and conspicuous manner. Conspicuous matters: an outlet nobody can see is an outlet nobody notices when it starts running.
Manufacturer instructions add the working dimensions for a cistern-fed cylinder. Heatrae Sadia's Megalife manual, for example, sets out the following for its vented water heaters.
- A vent pipe of at least 19 mm nominal bore
- A pipe that rises continuously along its whole length, with no dips
- A vent that discharges into the feed cistern
- No valves of any kind fitted in the vent pipe
- A cold water cistern on a level, rigid platform that supports the base over its whole area
Those figures come from a manufacturer's installation manual, not from Approved Document G. Always work to the instructions for the cylinder actually fitted.
Tundish and discharge pipe: the D1, D2, 600 mm and 300 mm rules
An unvented cylinder replaces the vent pipe with a tundish and a discharge pipe, and Approved Document G sets out the sizes. A tundish is a small open funnel in the pipe, so that any discharge can be seen. The safety devices discharge directly, or through a manifold, via a short metal pipe called D1 into the tundish. The pipe leaving the tundish is D2.
- D1 is not smaller than the nominal outlet size of the temperature relief valve
- The tundish sits vertical, in the same room as the unvented system, as close as possible to the valve and lower than it
- No more than 600 mm of pipe runs between the valve outlet and the tundish
- At least 300 mm of vertical pipe runs below the tundish before any elbows or bends
- D2 is at least one pipe size larger than the nominal outlet size of the safety device, provided the total equivalent hydraulic resistance does not exceed that of a straight pipe 9 m long
Manufacturer manuals add that any discharge pipe connected to pressure relief devices must fall continuously along its length and be kept clear of frost. Sizing tables for valve sizes G1/2, G3/4 and G1 run from 15 mm up to 54 mm, with maximum lengths and deductions for each bend or elbow.
You should not normally see water in the tundish. Visible discharge points to a problem with a pressure relief valve or the expansion vessel, and it is a job for a qualified engineer. The water passing through is close to boiling.
Who may install a cylinder, and when building control needs to be told
Replacing a hot water storage system in England should be carried out by a competent person, according to Approved Document G. For unvented systems the guidance is firmer still, because of the burst and scalding risk. Notifying building control, or registering the work through a competent person scheme, is the installer's responsibility rather than yours.
GOV.UK publishes the list of authorised competent person schemes and explains how they are approved. Schedule 4 to the Building Regulations 2010 exempts some small work from notification, including hot water storage systems with a storage vessel of not more than 15 litres, where the associated electrical work is also non-notifiable. A typical under-sink water heater falls in that bracket. A full cylinder does not.
Our guide on who can install an unvented hot water cylinder in England goes through the qualification side in more detail. For a cylinder replacement you can arrange a free home survey and a written fixed price before any work starts.
The HSE scalding alert: loft cisterns, immersion heaters and vented systems
HSE has issued a safety alert about scalding risk from domestic hot water systems. It concerns systems with a fixed fully or partly electric immersion heater working with a plastic cold water cistern in the loft. Two similar fatal incidents happened within four years. In both, large volumes of near-boiling water came through bedroom ceilings onto people sleeping below.
The main cause HSE identified was support. The plastic cisterns were not supported across their whole base, and the bottom overhung the bearer platform. HSE says cisterns should sit on adequate support. It also says a safety cut-out independent of the immersion heater thermostat can limit the stored temperature if that thermostat fails.
Where a galvanised cistern is being replaced with a plastic one, HSE advises that the immersion heater thermostat is checked and preferably replaced with one that has a safety cut-out. The risk is greatest where the cistern sits above bedrooms, which HSE notes is common in houses built between 1945 and 1975. Checking or changing an immersion heater and its thermostat is work for a qualified engineer, not a householder task.
What you can check at home, and what belongs to a qualified engineer
On a hot water cylinder and its vent or discharge pipework, your safe checks are limited to four things. Everything else on the system needs a qualified engineer. Anything touching gas, a flue, seals, condensate inside the appliance or the boiler casing needs a Gas Safe registered engineer.
- The boiler pressure gauge reading
- Thermostat and programmer settings, including the cylinder thermostat dial
- The reset button on the boiler
- Bleeding radiators to release trapped air
Operating a relief valve, cleaning a strainer, recharging an expansion vessel or altering discharge pipework are not householder steps. Manufacturers treat them as part of a scheduled service, because a blocked valve or discharge pipe defeats the safety chain. Heatrae Sadia's PremierPlus manual notes that intermittent water from the expansion valve shows the air volume in the expansion vessel is no longer sufficient. Our guide on whether unvented cylinders need an annual service covers what that visit involves.
If your cylinder is discharging, your water runs cold or the boiler behind it has stopped, you can book a boiler service or repair visit and have the system looked at properly.
Sources
- Approved Document G: Sanitation, hot water safety and water efficiency (with 2024 amendments)
- Sanitation, hot water safety and water efficiency: Approved Document G – GOV.UK
- The Building Regulations 2010, Schedule 1
- The Building Regulations 2010, Schedule 4 (non-notifiable work)
- The Water Supply (Water Fittings) Regulations 1999, Schedule 2 – Hot water services
- HSE – Scalding risk from domestic hot water systems: safety alert
- Competent person scheme: current schemes and how schemes are authorised – GOV.UK
- Heatrae Sadia – Megalife vented cistern fed water heater installation manual
- Heatrae Sadia – PremierPlus unvented hot water cylinder installation and user manual