The gas locker rules that fail inspections

Ask anyone who inspects habitation gas systems for a living what they fail most often and you will not hear “the pipework”. You will hear “the locker”. The cylinders themselves are somebody else's certified product; the box you built around them is yours, and it is the part that is usually undersized, unsealed, half-vented, or vented into a hole a bottle is standing on. The rules are short, they are all measurable, and they all descend from a single physical fact.

The one fact that generates every rule

LPG is heavier than air. Propane vapour is roughly 1.5 times the density of air, butane about 2. That is the whole story. A leak of natural gas in a house rises to the ceiling and finds a window. A leak of LPG in your van does the opposite: it falls, pools in the bottom of the locker like an invisible liquid, and if the locker is not sealed it keeps going — under the furniture, along the floor, into the lowest corner of the van, which in a self-build is often exactly where the leisure batteries, the fridge relay, or a foot-level heater intake live.

Read every requirement below as an answer to that one behaviour. Sealed from the living space, because the gas will find any gap. No electrics inside, because pooled vapour needs only a spark. A vent in the floor, because a vent in the wall is useless to a gas that sinks. Bottles restrained, because a cylinder that moves is a cylinder that stresses a connection. Nothing on the list is bureaucratic. It is all the same physics, written down five times.

The drop-out vent is the number they measure

EN 1949 — the European standard for LPG installations in leisure vehicles, and the technical backbone of both the German G 607 check and the French Qualigaz inspection — requires a permanent vent at the lowest point of the gas compartment, opening directly to the outside air. Its free area must be at least the greater of:

100 cm² (10,000 mm²), or 2 % of the locker floor area.

Be honest about which one applies to you. On a normal one- or two-bottle locker, the 100 cm² minimum is what governs, every time. A box sized around a single 11 kg propane bottle has a floor of roughly 370 × 370 mm = 0.14 m²; 2 % of that is 27 cm². Put two bottles side by side — 740 × 370 mm, 0.27 m² — and the share rises to only 55 cm², still barely half the minimum. The 2 % rule does not start to bite until the locker floor passes 0.5 m², which is a bench-sized compartment holding three or four bottles, not a cupboard. If somebody tells you their vent is fine because it is “2 % of the floor”, they have almost certainly built it too small.

Now convert 100 cm² into hardware, because this is where builds quietly fail. A 100 mm round hole is 78.5 cm² — not enough. You need about a 113 mm clear diameter to reach 100 cm², and that is free area, after the grille. A vented cap with louvres, a mesh insect screen, or a skirt in the airflow path all subtract from the hole you drilled. Buy a vent whose manufacturer states a free area, or fit two smaller ones and add their stated areas. Two 80 mm vents at a stated 40 cm² each still leave you 20 cm² short; three clears it.

The vent must also actually reach open air. Discharging downwards into a boxed chassis rail, a wheel-arch pocket, or a sealed sill is the same as no vent at all — the gas simply pools one level lower. And it must never be obstructed: not by the bottle base standing on it, not by the rubber mat somebody adds to stop the rattling, not by the spare hose, the levelling blocks, or the bag of pegs that ends up wedged beside the cylinders because the locker is the only dry outside box on the van. Position the vent clear of the bottle footprint at design time; you cannot police the pegs.

The rest of the box

Sealed from the living space. Continuous, gas-tight construction — no service holes to the interior, no cable glands, no shared cavity with a wardrobe. If a wire or a gas pipe passes through the locker wall, the penetration is sealed. The door seals and opens to the outside.

No electrical fittings inside. No socket, no light, no fridge junction, no battery, no switch, no cable route that is not itself sealed. There is no ignition source inside a gas locker, full stop.

Bottles restrained top and bottom. Two straps, not one: a low strap stops the base walking, a high strap stops it toppling. The test in your head is not the driveway, it is an emergency stop or a hairpin descent with a full 26 kg bottle trying to continue in the old direction. Bolt the anchors to structure, not to 4 mm ply.

Sizing the box around the bottle you actually own

Work outwards from the cylinder. Allow about 30 mm of clearance around it for straps, fingers, and getting it in and out with cold hands, and roughly 150 mm of headroom above the shoulder for the regulator and pigtail. Common EU bottles, with the weight that matters — full, not nominal:

Two bottles side by side means two cells wide and one deep: an 11 kg pair needs an internal 740 × 370 × 730 mm before you add wall thickness and a door. That is a large object to discover late, which is why the locker belongs in the layout at the same stage as the bed and the wheel arches — see the order to build a conversion in. The 730 mm height in particular is what forces a rethink: it will not live under a 450 mm bench seat, no matter how the sketch looked.

Regulator, pipe, hose, valves

30 mbar is the working pressure of a fixed installation in an EU leisure vehicle — the same regulator outlet whether you run propane or butane, which is why modern appliances are marked for it. The regulator is replaced 10 years after the manufacturing date stamped on its body, not ten years after you fitted it; buy one that has been sitting in a warehouse for six years and you have bought four years of service. Inspectors read that stamp.

Flexible hose only where the appliance maker specifies it — typically the short high-pressure pigtail from cylinder to regulator, and a deliberately flexible section at a slide-out appliance. It must be visible, in date, unstrained, and it must never be routed through a wall, partition, or floor. Everything else is rigid pipe — copper or approved steel — clipped at roughly every 50 cm (the exact spacing depends on pipe diameter and run orientation), supported so it cannot vibrate against an edge, and protected wherever it passes through a panel. Every appliance gets its own accessible isolation valve, and every appliance needs a flame-supervision device that shuts the gas off if the flame goes out. A cooker without an FSD is a common failure on imported or second-hand hobs.

More than two bottles is outside normal EN 1949 practice. It is not a criminal offence to carry three, but the standard and every inspection routine built on it assume one or two, and a three-bottle bench will attract questions about restraint, vent area, and whether the compartment is still a gas locker or has become a storage cupboard with cylinders in it.

The payload bite nobody budgets for

Two full 11 kg propane bottles weigh 52 kg. Two 13 kg bottles are 60 kg. That is not a rounding error on a 3.5 t van — it is a fifth of a tight payload allowance, spent before you load a single person. Worse, the classic self-build location is a rear cupboard behind the axle, so the mass lands high and far back, loading the rear axle by more than its own weight and lifting the front. If your payload budget is already thin, one 11 kg bottle plus a spare 5 kg buys back 14 kg for the cost of a slightly more frequent swap.

Who actually checks, and where

Germany: the DVGW G 607 gas system check, carried out by a certified tester every two years. It was re-anchored in law in June 2025, and a valid certificate is now a prerequisite for the HU roadworthiness test on vehicles with a fixed LPG installation — meaning a bad locker no longer just fails a gas check, it stops the van being road-legal.

France: a Qualigaz inspection of the installation is part of obtaining VASP homologation for a converted vehicle. Fail it and the conversion does not get registered as a camping-car.

UK: a private self-build used by its owner has no legal requirement for a gas safety certificate — that obligation attaches to work done for others and to vehicles let for hire. In practice insurers routinely ask for a habitation check or gas safety report before they will write a policy, and a claim after a gas fire is exactly when the absence of one becomes expensive.

Which leads to the only piece of advice in this article that is not a measurement: have the gas signed off by a qualified person, whatever the letter of the law where you live. You can wire your own 12 V, plumb your own water, and cut your own furniture with nothing worse than rework at stake. Gas is the one system whose failure mode is your family asleep in a sealed metal box. An hour of a certified tester's time is the cheapest component in the build.

Get the box right on screen first

Every number above is a design-stage decision and a demolition job afterwards. Place the bottle you actually intend to buy in your layout and let the locker size itself around it: VanPlot derives the internal box from the cylinder's real diameter and height plus the 30 mm clearance and 150 mm regulator headroom, tells you the drop-out vent free area you owe and which rule set it, and carries the full bottle weight into the payload and axle-load readout, so the rear-cupboard mistake shows up as a number rather than a surprise on a weighbridge. If you only want the arithmetic, the gas locker calculator is free and needs no account — check your vent against 100 cm² before you cut the hole, not after the inspector measures it.