CENWAT revision notes
Boilers and water heaters — controls, commissioning sequence and spotting an unsafe install.
- 21notes with free previews
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- 9topics
BS 7967: Flue Gas Analyser Operation & CO/CO2 Ratio
Approved Document L: Controls & Boiler Interlocks
Boiler controls and the interlock principle
The difference between operating and safety controls and what a boiler interlock is for.
Dew point temperatures and boiler condensing efficiency
Understanding return water temperatures for condensing boilers.
Flame supervision: thermocouples and flame rectification
How the device that shuts the gas off when a flame goes out actually senses the flame.
Manual reset overheat cut-out safety controls
How a latched overheat cut-out differs from normal temperature control, and why resetting is not a repair.
Motorised valve controls: S-plan and Y-plan auxiliary switches
Understanding switched live wiring and auxiliary microswitches on central heating valves.
S-plan and Y-plan: two common heating control layouts
How 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)
A worked efficiency calculation linking gas energy in to useful heat out, with the gross and net basis explained.
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 rise
A worked calculation of the heat delivered into domestic hot water from flow and temperature rise.
Approved Document L: Boiler Flue & Condensate Design
Condensate discharge: external routing & frost protection
Choosing a condensate route and protecting exposed pipework, including runs through unheated indoor spaces.
Condensing boiler pluming and plume management kits
Visible condensation does not establish safe flue discharge; plume management must suit the appliance and its surroundings.
Flue terminals, plume and condensate basics
The three flue-outlet details that cause complaints: terminal position, visible plume and condensate.
Preventing and clearing a frozen condensate pipe
Why 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 finding
A repeatable way of reasoning from symptom to cause instead of swapping parts at random.
No hot water from a combi: a fault-finding checklist
An 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 balancing
A way to reason about cold radiators from where the cold is, pointing to air, sludge or a flow problem.
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
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