CENWAT Cheat Sheet 2026

The 30 highest-yield CENWAT facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. What is the purpose of the boiler interlock in a central heating control system? → To prevent the boiler firing when there is no demand for heat
2. What is the primary function of a thermostatic radiator valve (TRV)? → To control water flow through a radiator based on room air temperature
3. What is the maximum CO level in a flue gas (as measured at the flue terminal) that is generally considered acceptable for a gas boiler? → CO:CO2 ratio below 0.004 (or as specified by the manufacturer's commissioning data)
4. What is the purpose of a lockshield valve on a radiator, and who should adjust it? → To balance system flow during commissioning; adjusted by the installer using a tool
5. What type of material is NOT suitable for condensate pipework due to its reaction with acidic condensate? → Copper or galvanised steel
6. Under the Gas Safety (Installation and Use) Regulations 1998, how often must a landlord have gas appliances safety checked in a rented property? → Every 12 months
7. What qualification must a plumber hold to install an unvented hot water storage system in the UK? → G3 Unvented Hot Water Storage qualification
8. What is a floor-standing boiler typically used for in UK installations? → High-output applications or older properties where wall-hung installation is not practical
9. What is the function of the overheat thermostat (limit thermostat) on a boiler? → To provide a safety lockout if water temperature exceeds safe limits
10. Which British Standard covers the specification for forced circulation hot water central heating systems in domestic premises? → BS 5449
11. Under the Water Supply (Water Fittings) Regulations 1999, what is required when connecting a central heating system to the mains water supply? → An approved backflow prevention device must be installed
12. What is the minimum horizontal distance a room-sealed flue terminal must be from an openable window or door? → 300 mm
13. What is the main advantage of an S-plan system over a Y-plan system? → S-plan provides fully independent time and temperature control for each zone
14. What does LTHW stand for in heating system terminology? → Low Temperature Hot Water
15. How often should the inhibitor concentration in a central heating system be checked? → Annually during a boiler service, and after any significant water loss or system work
16. What is the maximum recommended length of the discharge pipe from a tundish to the termination point for D2 (22mm) pipe sizing? → 9 metres
17. What is the recommended temperature range for setting a room thermostat in a domestic property? → 18–21°C
18. What document must be completed when commissioning an unvented hot water cylinder? → Building Regulations Part G compliance notification and Benchmark commissioning checklist
19. What term describes the difference in temperature between flow and return pipes in a central heating system? → Delta T (ΔT)
20. What is a 'Type C' gas appliance classification? → A room-sealed appliance with both air supply and flue gas ducts connected to the outside
21. What regulation governs the installation and use of gas water heaters in the UK? → Gas Safety (Installation and Use) Regulations 1998
22. What is a back boiler? → A boiler installed behind a gas fire in a fireplace recess
23. What does a high CO/CO2 ratio reading during combustion analysis typically indicate? → Incomplete combustion, possibly due to insufficient air supply or burner fault
24. What is the primary purpose of a central heating system in a UK domestic property? → To distribute heat from a single source throughout the building
25. What is the function of a thermostatic radiator valve (TRV)? → To regulate the flow of water through the radiator based on room temperature
26. What instrument is used to perform a gas tightness test? → A manometer (U-gauge or digital manometer)
27. What is the standard calorific value (CV) of natural gas used for gas rate calculations in the UK? → Approximately 38.76 MJ/m³ (gross) — used with meter readings to calculate heat input
28. What is the minimum internal diameter recommended for a condensate discharge pipe? → 22 mm for external runs, 19 mm for internal
29. What is the minimum insulation thickness required for central heating pipework in unheated areas under Building Regulations Part L? → 19mm for pipes up to 22mm diameter
30. Which component in a condensing boiler is specifically designed to extract additional heat from flue gases? → Secondary heat exchanger