CENWAT Study Guide 2026

A draught diverter prevents excessive or reversed draught from affecting combustion; it mixes room air with flue gases to reduce corrosive condensation and protect the heat exchanger.

LTHW (Low Temperature Hot Water) refers to heating systems operating at flow temperatures typically below 90°C, which is standard for domestic central heating.

When notifying Building Control of a replacement boiler installation via a Competent Persons Scheme such as Gas Safe Register, the notification must include the installer Gas Safe registration number, boiler make, model, serial number, efficiency rating, type and details of controls fitted, and commissioning data including flow and return temperatures and gas rate readings.

A new central heating system should be pressure tested at 1.5 times the maximum working pressure, or a minimum of 3 bar for sealed systems, for a defined test period. The system should hold pressure with no detectable drop and all joints should be visually inspected.

An S-plan system uses two separate 2-port motorised zone valves, one controlling the heating circuit and one controlling the hot water circuit. This allows completely independent control of each circuit, unlike the Y-plan which uses a single 3-port valve.

In a Y-plan system, when both thermostats call for heat simultaneously, the 3-port mid-position valve moves to its mid-position, allowing water to flow to both the heating and hot water circuits at the same time. This is the normal operating condition during cold weather.