CENWAT revision notes
Boilers and water heaters — controls, commissioning sequence and spotting an unsafe install.
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- 10topics
BS 7967: Flue Gas Analyser Operation & CO/CO2 Ratio
Approved Document L: Controls & Boiler Interlocks
- Boiler controls and the interlock principleWhat each heating control does, which devices are safety controls, and what a correctly wired boiler interlock does and does not do.
- Dew point temperatures and boiler condensing efficiencyWhy a condensing boiler needs return water below about 55 °C, what keeps the return too hot, and how weather compensation helps.
- Flame supervision: thermocouples and flame rectificationHow the device that shuts the gas off when a flame goes out actually senses the flame.
- Manual reset overheat cut-out safety controlsHow the high-limit thermostat backs up the control thermostat, why a latched trip needs a deliberate reset, and what prevents overheating.
- Motorised valve controls: S-plan and Y-plan auxiliary switchesHow a thermostat's call opens a motorised valve, how the valve's end switch fires the boiler, and what the wiring centre does.
- S-plan and Y-plan: two common heating control layoutsHow the S-plan and Y-plan layouts route heating and hot water, and how they differ.
BS 6798: Boiler Commissioning & Servicing
Calculations: heat output & efficiency
- Boiler efficiency from input and output (worked example)Work out boiler efficiency from input and output, rearrange it to find output, and use the result to check the boiler meets the load.
- Cylinder reheat time from heat input (worked example)A worked estimate of how long a heat source takes to reheat a hot water cylinder, from energy and power.
- Hot-water heat output from flow rate and temperature riseA worked calculation of the heat delivered into domestic hot water from flow and temperature rise, and what it means for boiler choice.
Approved Document L: Boiler Flue & Condensate Design
- Condensate discharge: external routing & frost protectionRoute condensate safely: plastic pipe, a sealed trap, 19 mm internal pipe falling 45 mm per metre, and a short, upsized, insulated external run.
- Condensing boiler pluming and plume management kitsWhy condensing boilers plume, when a plume becomes a siting problem, and how repositioning the terminal or an approved plume kit fixes it.
- Flue terminals, plume and condensate basicsTerminal clearances and guards, how a condensing flue is laid and supported, and the one-line rules for plume and condensate.
- Preventing and clearing a frozen condensate pipeWhy a condensate pipe can freeze, how to reduce the risk, and how to deal with one that has.
Calculations: efficiency
Boiler Types: Open-Flued, Room-Sealed and Condensing Concepts
Fault-Finding and Unsafe Installation Signs
- A structured approach to boiler fault findingReason from symptom to cause, work from the supply inwards, and learn the classic patterns: dead, short-cycling, kettling or sooting boilers.
- No hot water from a combi: a fault-finding checklistAn ordered set of checks for the common complaint of a combination boiler giving heating but no hot water, or none at all.
- Why radiators run cold: air, sludge or balancingA way to reason about cold radiators from where the cold is, pointing to air, sludge or a flow problem.
Gas Water Heaters: Multipoint and Continuous-Flow
Sealed System Pressure, Expansion & Filling Faults
Prepare for CENWAT questions by following the appliance
For CENWAT revision, follow a boiler or water heater through siting, supply, flueing, controls and commissioning. In a fault scenario, distinguish an observation from a diagnosis: a symptom may have several possible causes. Explain which check would help separate them.
The CENWAT qualification guide connects the appliance topics to the core gas assessment. Revisit CCN1 practice for the shared safety principles, then use the boiler gas-rate examples to work through input, units and comparison with appliance data. The Approved Document L guide covers the efficiency, 55 °C design and control requirements a boiler replacement has to meet.
In-depth guides for this module
Longer walkthroughs that sit alongside the revision notes.
CENWAT revision guide: boilers, water heaters, controls and flues
Revise CENWAT in job stages: can the appliance be installed here, supplied and flued safely, commissioned properly (gas rate, combustion checks) and controlled with full boiler interlock — and what must be proved before handover. Gas rate and heat input are the high-value method questions.
Flue flow and spillage test procedure: open-flued gas revision guide
Flue flow checks the chimney route; spillage checks that products of combustion are not spilling back into the room while the appliance runs.
More Gas modules
Every module has its own revision notes and question bank.