Running the Discharge Pipe From an Unvented Cylinder: D1, the Tundish and the Elbow Count That Sizes D2

The cylinder is on the landing and the pipe has nowhere to go

Mid-terrace, airing cupboard on the first floor half-landing, cylinder delivered at eight and the old vented one already cut out. The hot and cold connections are twenty minutes of work. The problem standing in the way of a working installation by teatime is that the only external wall within reach of that cupboard is the front elevation, above a public footway, and nothing may terminate there. The route to the back of the house is fourteen metres and passes a purlin, two joist runs and a soil stack.

That is the part of an unvented job that gets designed on the day, and it is the part that has a table behind it. Everything else about the installation is forgiving — a hot main can be 22 mm or 28 mm and both will work, a cylinder can sit a metre either side of where it was drawn. The safety discharge is not forgiving, because the rules governing it are dimensional and prescriptive: this size, this fall, this many metres, minus this much for every bend. You either satisfied the table or you did not, and it is the first thing a Building Control officer or a scheme assessor puts a tape on.

It is also two pipes rather than one, with a safety fitting between them, and each half is sized by a different rule. Calling the whole run "the discharge" is how the mistake starts. The short leg above the tundish is D1 and it is sized off the valve. The long leg below it is D2 and it is sized off the route.

D1 is short, metal, and contains nothing

The leg from the safety device to the tundish is the simplest pipe on the job and it is where the shortcuts happen. It must be metal — this is water that can arrive at close to boiling and flash to steam the moment it meets atmosphere, and a push-fit plastic connector on the valve outlet is a failure waiting for the one day it is asked to work. It must be at least the nominal outlet size of the safety device, so a G½ valve gets 15 mm and a G¾ valve gets 22 mm, and it must run at a continuous fall with no dips a slug of water could sit in.

The tundish goes vertically below the valve and within a few hundred millimetres of it. The exact maximum is dimensioned on Diagram 2 of Approved Document G and repeated, sometimes tighter, in the cylinder manufacturer's own instructions — take it from whichever of those your work is signed off against, because a tundish mounted at the far side of the cupboard on the end of a metre of pipe is a straightforward fail and an easy one to spot from the doorway. It also has to be in the same space as the cylinder and where somebody standing at the cupboard can see it, which rules out the neat solution of hiding it in the floor void.

Then the rule that has no exceptions anywhere in the chain: there is no valve, no isolator, no restrictor, no flexible hose and no strainer between the safety device and the point of discharge. Not in D1, not in D2, not at the termination. A service valve fitted "so it can be isolated for maintenance" converts a safety device into a decoration, and it is the single defect most likely to appear on a system somebody else worked on last.

Where the pipe is allowed to end

"Safe" and "visible" are both requirements and they pull against each other, which is why the termination is negotiated with the householder rather than chosen by the plumber alone. Safe means water at close to 100 °C, possibly for hours, cannot reach a person, a pet or anything it will destroy. Visible means somebody will notice it running — a discharge nobody sees is a fault that continues until the cylinder is condemned or the water bill arrives.

The terminations Approved Document G describes come down to four patterns. Into a trapped gully, with the pipe end below the fixed grating and above the water seal, which is the tidiest answer where a gully exists near the right elevation. Downward at low level, within 100 mm of the ground over a hard standing or a grassed area, with a wire guard preventing contact but not obscuring the view of it — the standard answer for a rear elevation. Upward or sideways at high level into a metal hopper and metal downpipe, with the pipe end clearly visible where it enters. Or onto a roof able to take a high-temperature discharge, kept well clear of any plastic gutter that would collect it, with a stated separation of 3 m.

The material rule follows the water rather than the pipe. Anything the discharge lands in or passes through has to survive it: a plastic hopper, a uPVC downpipe or a plastic gutter under a discharge point is a soft, slow failure that nobody attributes to the cylinder until the section deforms. That is also why a pipe terminating over a gully must not simply be pushed into a plastic soil branch — the connection has to be an air break at an open fitting, not a joint into the drainage system.

Then the constraints the drawing never shows. A discharge over a footway, a doorstep, a fire escape, a car parking bay or a bin store is not a safe place regardless of how the table works out. An external run on a north elevation freezes, and a frozen D2 is a blocked D2 — the mitigation is to keep the exposed length short and the fall generous rather than to lag it, because insulating a discharge pipe hides the thing that has to be seen. And where no lawful termination exists at all, the fallback is a proprietary discharge collection vessel accepted by the cylinder manufacturer, which is a component with its own capacity limit and its own commissioning record, not a bucket.

The same hazard under the IPC, answered a different way

A plumber crossing between the British and North American versions of this job finds the physics identical and the arrangement barely recognisable. There is no tundish. There is no equivalent-length table. The relief valve discharge under the International Plumbing Code at Section 504.6, and the International Residential Code at Section P2804.6.1, runs full-size from the valve outlet by gravity to a floor, an indirect waste receptor or the outdoors, and the air gap is made at the receiving end rather than partway along the pipe.

The list of requirements there is short and worth knowing verbatim if you work both sides: the pipe is not smaller than the valve outlet, serves one relief device only, is not trapped, has no valves or tee fittings in it, flows by gravity, is readily observable, is made of material rated for the discharge temperature — 250 °F is the usual figure — and terminates between two pipe diameters and 6 inches above the receptor. The end is left plain, because a threaded end invites somebody to cap it.

The differences are instructive in both directions. The North American arrangement puts the visible air gap at the point of discharge, which is easier to inspect and worse at telling a householder upstairs that anything is happening. The British arrangement puts the break in the airing cupboard next to the cylinder, which is what makes a dripping tundish such a good diagnostic, and pays for it with a long pipe that has to be sized on a table. What neither permits is a discharge connected into the drainage system, and that is the rule to carry across.

A tundish that is doing something

On a system already in service, water moving through the tundish is a diagnosis waiting to be made, and which valve is passing tells you where to look. A temperature relief valve discharging means a control failure and the cylinder should be isolated the same day. An expansion relief valve discharging means the volume increase of each heating cycle has nowhere to go, and that is the far commoner finding: a waterlogged expansion vessel that has lost its air charge, a bubble-top cylinder whose internal air gap needs re-forming by draining, an inlet pressure reducing valve that has drifted upward, or a standing mains pressure that has risen since commissioning and now sits close to the relief setting.

The check is quick. Read the vessel's schrader valve pressure against the cold system pressure with the system depressurised, put a gauge on the inlet set's reduced pressure, and confirm the relief setting on the badge is genuinely above it with margin. Fix the cause rather than the symptom — a relief valve that has been passing for months has usually picked up enough scale on the seat that it will weep afterwards regardless, and at that point it is a replacement rather than an adjustment. The quantity of water this wastes is not trivial and is covered from the metering side in the water bill guide linked below.

Signing it off and handing it over

Unvented hot water storage above 15 litres is notifiable building work, and the normal route is self-certification by an operative holding the relevant unvented competence and registered with a competent person scheme. The discharge arrangement is the part of that certificate most likely to be looked at, so leave the evidence behind: the route, the size, the bend count and the arithmetic on the commissioning sheet, not in your head.

1. Take the valve stamp, the cylinder capacity and the total heat input from the badge, adding every heat source rather than the largest one.
2. Confirm the relief valve's rated capacity covers that total, and record it — this is the check that fails after somebody adds an immersion or a solar coil later.
3. Walk the D2 route before cutting anything, counting every change of direction and confirming a continuous fall of at least 1 in 200 exists the whole way.
4. Size D2 from the table: straight-pipe allowance for that valve and pipe size, minus the per-bend deduction times the walked count, against the measured length.
5. Set the tundish vertically below the safety device within the distance the document and the manufacturer's instructions allow, in the same space as the cylinder and in plain sight.
6. Drop 300 mm straight below the tundish before the first bend, and keep the whole run free of valves, isolators, strainers and flexible connections.
7. Terminate where near-boiling water can be seen and cannot reach anybody, with a guard at low level, and confirm nothing plastic sits in the discharge path.
8. Commission the expansion provision: vessel charge against system pressure, inlet reduced pressure gauged, and the relief settings checked to sit above it with margin.
9. Operate both relief valves manually against the tundish, watch the discharge run away cleanly, and confirm both reseat dry.
10. Show the householder the tundish, say plainly that anything running through it is a fault, and leave the discharge sizing on the certificate with the annual service interval.