D1 is the short run from relief valve to tundish; D2 is from tundish to termination. D2 must be at least one size larger, fall continuously, stay within a maximum equivalent length that every bend reduces, and terminate somewhere nobody can be scalded.
What it is for
If a relief valve on an unvented cylinder opens, it releases stored water that may be at or above 100 degrees. As the pressure drops at the valve, some of that water flashes instantly to steam, which expands enormously.
The discharge arrangement has three jobs:
- Carry the discharge safely away without it scalding anyone or damaging the building.
- Cope with the volume, including the steam flash, which is why D2 has to be larger than D1.
- Make the discharge visible, so somebody notices it is happening. A relief valve operating means something has gone wrong upstream, and if nobody sees it the underlying fault continues indefinitely.
That third point is why the tundish exists and why it must be visible. A discharge arrangement that works perfectly but is entirely concealed has failed at one of its three jobs.
D1: valve to tundish
- The run from the relief valve outlet to the tundish.
- Maximum 500mm in length.
- Same size as the valve outlet.
- Metal pipe.
- As direct as possible – this is a short connecting run, not a route.
The tundish
- An open air break with a visible gap between the incoming pipe and the outgoing one.
- Fitted vertically. Not on its side, not at an angle.
- Within 500mm of the relief valve.
- Visible, in the same room or space as the cylinder, positioned where somebody would actually see water running through it.
- Not concealed inside a sealed boxing or behind a fixed panel.
Boxing a cylinder in for appearance, with the tundish inside the boxing, is extremely common. It defeats the entire purpose – a relief valve could discharge for months and nobody would know until the underlying fault became something worse. If your cylinder is boxed in, it is worth confirming the tundish can still be seen.
D2: tundish to termination
This is where the requirements are most specific and where most installations fall short.
| Requirement | Detail |
|---|---|
| Diameter | At least one pipe size larger than D1. This accommodates the steam flash |
| Vertical drop below the tundish | A minimum straight vertical section before any horizontal run or bend |
| Fall | Continuous, at not less than the specified minimum gradient. No horizontal or near-horizontal sections |
| Maximum equivalent length | A stated maximum for each pipe size, which every bend reduces by a specified allowance |
| Material | Metal, capable of withstanding water above 100 degrees |
| Support | Properly clipped, so it does not sag and create a low point |
Equivalent length is the requirement most often missed. The maximum is not simply a length of pipe – each elbow counts against the allowance by a defined amount. A run with several bends can exceed the allowance for its size while being physically much shorter than the stated maximum, at which point it must be upsized again. Nobody does this calculation unless they know to.
The consequence of getting it wrong is that the pipe cannot carry the discharge, so it backs up at the tundish and overflows into the room – which is precisely what the arrangement was meant to prevent.
Termination
Where the discharge ends up, and the options in rough order of preference:
- Low level, to a gully below the grating but above the water seal. Contained, visible if you look, and no scald risk.
- Low level externally, below a fixed grating, at low level so the discharge cannot reach a person.
- High level externally, discharging downwards onto a non-habitable surface – a flat roof – with a wire cage or guard where people could come into contact.
- Into an internal soil stack, which is permitted with specific conditions including a metal connection and a trap.
What is not acceptable: terminating over a path, a doorway, a window, a patio or anywhere a person could be standing. Water at boiling point discharging at head height over a back door is a serious scald risk, and it is a finding we see.
Also worth checking: the termination must be visible or at least identifiable, so that a discharge is noticed. A pipe terminating into a buried drain with no visible break means nobody ever knows the valve is operating.
Checking your own installation
- Find the tundishIt should be in the same space as the cylinder, vertical, and visible. If you cannot find it, that is the first finding.
- Compare the two pipesThe one going in and the one coming out. The outgoing pipe should be visibly larger. If they are the same size, D2 is undersized.
- Follow D2 as far as you canLooking for horizontal sections, sags, and counting bends.
- Find the terminationAnd ask whether anybody could be standing there.
- Look for evidence of past dischargeStaining, algae, or a wet patch below the termination. It means a relief valve has been operating and something upstream needs attention.
Most of that is a five-minute visual check. If you find problems, it does not mean the cylinder is unsafe today – it means that if a relief valve ever needs to operate, the arrangement may not handle it safely.
It is also worth resolving before you sell. See what Building Regulations Part G3 requires.
Cylinder boxed in? Check you can still see the tundish. It is the most common non-compliance there is.
Questions we get asked about this
Why must D2 be bigger than D1?
Because the water flashes to steam as it discharges and pressure drops, and steam occupies enormously more volume than water. A D2 the same size as D1 cannot carry it and the discharge backs up at the tundish.
Can the tundish be hidden in a cupboard?
It must be visible and in the same space as the cylinder. The whole point is that somebody sees a discharge happening. A tundish inside sealed boxing does not meet the requirement.
What is equivalent length?
The maximum permitted D2 run, with each bend counting against the allowance by a defined amount. A run with several bends can exceed it while being physically much shorter than the stated maximum, at which point the pipe must be upsized.
Can the discharge go into the drain?
To a gully below the grating but above the water seal, yes, and that is a good arrangement. Into a sealed connection with no visible break is not, because discharge would go unnoticed.
Can I use plastic pipe for D2?
No. The discharge may exceed 100 degrees and the pipework must be metal and capable of withstanding it.
My discharge pipe terminates over the back door. Is that a problem?
Yes. Water at or above boiling point discharging where somebody could be standing is a scald risk and it is not compliant. It needs re-routing to a safe termination.
Regulations and standards referenced on this page
- Approved Document G: sanitation, hot water safety and water efficiency — Ministry of Housing, Communities and Local Government
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