D2 must be at least one size larger than D1, fall continuously, and stay within a maximum equivalent length that reduces with every bend. The tool works that out for a given run.
What it calculates
Given the relief valve outlet size, the proposed D2 pipe size, the run length and the number of bends, the tool tells you whether the arrangement is within the permitted equivalent length – and if not, what size it needs to be.
It also checks the other requirements that go with it: the tundish position, the vertical drop below it, the fall, and the material.
The maximum permitted D2 length 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, which is why so many installations have an undersized D2.
The requirements it checks
| Element | Requirement |
|---|---|
| D1 – valve to tundish | Maximum 500mm, same size as the valve outlet, metal |
| Tundish | Vertical, within 500mm of the valve, visible, in the same space as the cylinder |
| Vertical drop below the tundish | A minimum straight vertical section before any bend or horizontal run |
| D2 diameter | At least one size larger than D1 |
| D2 fall | Continuous, at not less than the minimum gradient. No horizontal sections |
| Equivalent length | Within the maximum for that pipe size, with each bend deducted |
| Material | Metal, capable of withstanding water above 100 degrees |
| Termination | Safe – low level to a gully below a grating, or a visible position where discharge cannot scald |
For the detail behind each of these, see D1 and D2 discharge pipework.
Why it matters
If a relief valve opens, it discharges water at or above boiling point, and some of that water flashes instantly to steam as the pressure drops. Steam occupies enormously more volume than water, which is why D2 has to be larger than D1.
An undersized or over-long D2 cannot carry the discharge. The result is that it backs up at the tundish and overflows into the room – which is precisely what the arrangement exists to prevent.
It is also the thing a building control inspector looks at first on an unvented installation, and it is the most common non-compliance we find on existing systems.
Checking your own installation without the tool
- Find the tundishIt should be in the same space as the cylinder, vertical, and visible. If you cannot find it, that is finding one.
- Compare the two pipesGoing into the tundish and coming out. The outgoing pipe should be visibly larger. Same size means undersized D2.
- Look at the first section below the tundishThere should be a straight vertical drop before any bend.
- Follow the runCounting bends and looking for any horizontal or near-horizontal section.
- Check the materialMetal, not plastic.
- Find the terminationAnd ask whether anybody could be standing there.
- Look for evidence of past dischargeStaining, algae or a wet patch below the termination means a relief valve has been operating and something upstream needs attention.
That is a five-minute visual check and it identifies most non-compliances without any calculation at all.
If it is wrong
A non-compliant D2 does not make the cylinder unsafe today. It means that if a relief valve ever needs to operate, the arrangement may not handle it safely – and relief valves do operate, most commonly because an expansion vessel has lost its charge.
The remedies, in rough order of cost:
- Upsize the pipe, which is frequently all that is needed.
- Reduce the number of bends, or use swept bends rather than tight elbows.
- Shorten the run by choosing a nearer termination.
- Re-route to a better termination.
- Relocate the tundish where it has been concealed.
- Relocate the cylinder, which is the expensive answer and occasionally the only one.
It is also worth resolving before you sell. An unvented installation with no certificate and a visibly non-compliant discharge is a due diligence finding.
The architecture for it is defined and it is not yet live. When it is, it will be free and require no email address. In the meantime the visual check above identifies most problems, and we check the discharge arrangements as standard at every unvented cylinder service.
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 larger than D1?
Because the discharge flashes to steam as the pressure drops, and steam occupies far more volume than water. A D2 the same size as D1 cannot carry it and it backs up at the tundish.
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.
Can I use plastic pipe for D2?
No. The discharge may exceed 100 degrees, so the pipework must be metal and capable of withstanding it.
My tundish is boxed in. Is that a problem?
Yes. It must be visible and in the same space as the cylinder, so that a discharge is noticed. A concealed tundish means a relief valve could discharge for months with nobody knowing.
Is a non-compliant D2 dangerous?
Not today, in itself. It means that if a relief valve does need to operate – which they do, most commonly from a failed expansion vessel – the arrangement may not carry the discharge safely.
Is the tool available now?
Not yet. The visual check on this page identifies most problems, and we check the discharge arrangements as standard at every unvented cylinder service.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.