The Complete CENWAT Guide: Boilers & Water Heaters

CENWAT covers central heating boilers and water heaters — the bread and butter of domestic gas work. It pulls in nearly everything from CCN1 (flueing, ventilation, gas rate, combustion) and adds the system side: controls, condensate and commissioning. This guide maps the topic and links to a full guide on each part. You need CCN1 plus CENWAT, and only a Gas Safe registered engineer may carry out the work.

Boiler types

The main families: a combi gives heating and instantaneous hot water from one unit; a system boiler has the pump and expansion vessel built in and feeds a cylinder; a regular (heat-only) boiler uses external components. Almost all new boilers are condensing and room-sealed. Read the full guide to boiler types →

Sealed vs open-vented systems

A sealed system is pressurised, with a diaphragm expansion vessel and a pressure relief valve (typically set at 3 bar). An open-vented system uses a feed-and-expansion cistern and an open vent pipe — it admits air, so is more corrosion-prone. Knowing which you're on shapes filling, expansion and safety.

Condensate

A condensing boiler cools the flue products below their dew point (around 55 °C) to recover latent heat, producing an acidic condensate (pH around 3–6) that must be drained in corrosion-resistant plastic — never copper or steel. Read the full guide to condensate handling →

Interlock and controls

Boiler interlock means the boiler can't fire when there's no heat demand — achieved by the room thermostat, cylinder thermostat and motorised valves working together. TRVs alone do not provide interlock. Read the full guide to boiler interlock & controls →

Benchmark commissioning

Commissioning to the MIs and the Benchmark checklist proves the boiler is set up correctly — gas rate, combustion (links to CPA1), controls and water treatment — and the signed log supports the warranty. Read the full guide to Benchmark commissioning →

Water heaters

Instantaneous, storage and multipoint water heaters each deliver hot water differently, with their own flueing, ventilation and safety needs. Stored water is kept around 60 °C to limit Legionella; mains-pressure unvented systems need layered safety controls and a competent (G3) installer. Read the full guide to water heaters →

1. Standard: BS 6798; water treatment BS 7593; commission to MIs + Benchmark.
2. Types: combi, system, regular — almost all condensing and room-sealed.
3. Sealed system: expansion vessel + PRV (typically 3 bar). Open-vented: F&E cistern + vent.
4. Condensate: acidic (pH ~3–6); plastic only; external runs short, insulated, ≥32 mm.
5. Interlock: room/cylinder stats + motorised valves. TRVs alone don't provide it.
6. Cylinder stored ~60 °C to limit Legionella; unvented needs G3.
7. Commission to Benchmark; check gas rate and combustion (CPA1).