A D2 pipe is the external discharge pipe connected to an unvented hot water cylinder. Its purpose is to safely release hot water if the cylinder’s safety valves activate due to excessive temperature or pressure.
Under Building Regulations Part G, the D2 pipe forms part of the cylinder’s safety discharge pipework and must terminate at a safe point of discharge to prevent the risk of scalding.
If you are planning a new system or replacing an existing cylinder, it is essential that the work is carried out by a G3 certified engineer. A professional hot water cylinder installation or replacement ensures that the unvented hot water cylinder, discharge pipework and safety devices are installed correctly and fully comply with Building Regulations.
An unvented hot water cylinder stores water under high pressure and temperature, which is why several safety devices are built into the system.
These safety components include:
If any of these safety devices activate, water flows through the discharge pipework and tundish, eventually exiting through the D2 pipe.
The D2 pipe ensures that any discharged hot water leaves the building safely and in a controlled manner.
Although discharge events are very rare, the discharge pipe is a critical part of the overall safety design of the system.
The discharge pipework on an unvented cylinder is normally divided into two sections.
D1 Pipe: The D1 pipe runs from the temperature and pressure relief valve (TPRV) on the cylinder to the tundish.
Tundish: The tundish is a visible safety fitting that allows engineers and homeowners to see if the cylinder is discharging water.
D2 Pipe: The D2 pipe runs from the tundish to the final discharge point.
The purpose of the D2 pipe is to carry hot water away from the system safely if the cylinder’s safety valves ever operate.
Under Building Regulations Part G, the D2 discharge pipe must terminate at a safe point of discharge.
In many installations, the simplest and most common arrangement is for the pipe to terminate externally, often with a downward elbow or cowl directing the discharge safely toward the wall or ground.
However, the discharge pipe does not always have to terminate directly outside.
In some installations, the D2 pipe may discharge into a suitable soil stack or drainage pipework system, provided the installation complies with Building Regulations and the receiving pipework can safely withstand the temperature of the discharged water.
This means the discharge point may be connected to a suitable drainage system, such as:
provided that the receiving pipework is capable of safely handling the discharge temperature and the installation complies with the relevant Building Regulations Part G guidance.
The key requirement is that the discharge must occur safely and without creating a scalding risk.
While the discharge pipe is an important safety feature, it is very rare for it to ever operate.
For water to discharge through the D2 pipe, a significant system fault would need to occur or multiple safety mechanisms would need to fail.
Unvented cylinders are protected by several safety devices:
These components prevent dangerous pressure or temperature conditions from developing within the cylinder.
Because of this multi-layer safety design, D2 discharge events are extremely uncommon when systems are installed and maintained correctly.
A tundish is a small fitting installed on the discharge pipework of an unvented cylinder.
It provides a visible air gap between the safety valve and the discharge pipe.
The tundish allows homeowners and engineers to see if the system is discharging water.
If water is visible flowing through the tundish, it usually means the cylinder is releasing pressure through the safety valves and the system should be inspected by a qualified engineer.
You can read more about this in our detailed guide explaining
why an unvented hot water cylinder may leak into the tundish or drip onto the floor.
The tundish forms part of the D1 discharge pipework, which then connects to the D2 pipe that carries water safely outside the building.
Because unvented cylinders store water under pressure, their installation is strictly regulated in the UK.
Unvented cylinder installations must be carried out by engineers who hold a G3 Unvented Hot Water qualification, which is a legal requirement under Building Regulations.
You can learn more about this certification in our guide explaining
what a G3 qualification is for unvented cylinders.
Correct installation includes:
These safety measures ensure the system operates safely and complies with Building Regulations Part G.
Because the D2 pipe is a critical safety component, it must be installed correctly in accordance with Building Regulations Part G.
Common issues include:
If you notice water coming from the pipe outside your property, it may indicate a problem with the system.
You can read more about the causes of this here:
why an unvented cylinder may leak into the tundish.
Repeated discharge may indicate:
While Part G focuses on safety, Building Regulations Part L focuses on energy efficiency.
Part L requires that heating and hot water pipes are insulated where they run outside the heated building envelope, including:
Pipe insulation helps to:
A D2 pipe is the discharge pipe that carries hot water safely away from an unvented hot water cylinder if the system’s safety valves activate.
No. Under normal operation, an unvented cylinder should not discharge water. If water is visible from the pipe outside the property, the system should be inspected by a qualified engineer.
No. While many installations discharge externally, a D2 pipe may also terminate into a suitable soil stack or waste pipe internally where this complies with Building Regulations and the receiving pipework can safely handle the discharge temperature.
Unvented cylinders must be installed by engineers who hold a G3 Unvented Hot Water qualification.
If you are considering installing or replacing an unvented hot water cylinder, it is essential that the system is installed by qualified engineers who understand G3 safety requirements and Building Regulations compliance.
At Boilergurus, our engineers specialise in hot water cylinder installation and replacement, ensuring systems are installed safely and professionally.
Our engineers bring a trio of skills – plumbing, heating and electrics –, ensuring every part of the system is installed correctly.
20th August, 2026
18th March, 2026
