Gas Water Heaters & Unvented Safety: A CENWAT Guide

"Water heater" covers everything from a small over-sink unit to a pressurised cylinder serving a whole house. The gas side is familiar — flueing, ventilation, gas rate, combustion — but hot water adds two big safety themes: Legionella and, on pressurised systems, unvented safety. This guide pulls them together. It's study material; only a Gas Safe registered engineer may work on the gas — and unvented cylinder work additionally requires G3 competence.

Types of gas water heater

• Instantaneous — heats water on demand as it flows, with no storage (e.g. a combi's hot water).
• Single-point — an instantaneous heater serving one outlet (e.g. an over-sink heater).
• Multipoint — an instantaneous heater serving several outlets from one unit.
• Storage — heats and stores a volume of hot water in a cylinder, ready for use.

Legionella: store hot, deliver safe

Stored water in the 20–45 °C range lets Legionella bacteria multiply. The control is temperature: store at ≥60 °C and distribute so the water is ≥50 °C at the outlets within about a minute. The cylinder thermostat is therefore set to a minimum of 60 °C — never reduce it below that on a stored system.

The scald conflict — and the TMV

Storing at 60 °C protects against Legionella but is hot enough to scald. The resolution is a thermostatic mixing valve (TMV) downstream of the cylinder, which blends in cold to limit the temperature at the tap — commonly to a maximum of around 48 °C at baths — while the stored water stays hot. Store hot, deliver safe.

Unvented hot water: the layers of protection

An unvented cylinder is sealed and fed at mains pressure — no open vent or loft tank. That convenience brings stored energy that must be controlled, so G3 requires layered safety devices, typically:

• A pressure-reducing valve and an expansion vessel (or air gap) to handle pressure and the expansion of heated water.
• An expansion relief valve for over-pressure.
• A temperature & pressure relief valve (T&P) — the last line — typically releasing at around 90–95 °C or about 7 bar.
• A thermal cut-out (high-limit) that cuts the heat source if the thermostat fails.

Tundish and discharge (D1 / D2)

If a relief valve operates, the hot water must go somewhere safe and visible. A tundish (a visible air break) is fitted within 500 mm of the safety valve so a discharge can be seen. From there the discharge pipe runs: D1 from the valve to the tundish, then D2 from the tundish to a safe termination — D2 being at least one pipe size larger than D1 and ending where a sudden discharge of near-boiling water can't scald anyone.

1. Types: instantaneous, single-point, multipoint, storage.
2. Legionella: store ≥60 °C, deliver ≥50 °C at outlets within ~1 min; risk zone 20–45 °C.
3. TMV limits outlet temperature (≈48 °C at baths) for scald protection — store hot, deliver safe.
4. Unvented = sealed, mains pressure; needs layered G3 safety devices.
5. T&P relief valve ≈90–95 °C / ≈7 bar is the last line; thermal cut-out backs up the thermostat.
6. Tundish (visible air break) within 500 mm; D1→D2, D2 a size larger, to a safe place.
7. G3 competence required for unvented work; a weeping tundish = a fault to investigate.