CENWAT Central Heating System Design 4 — Questions and Answers
Question 1: What is the purpose of zone valves in a central heating system?
- To independently control heating and hot water circuits (Correct answer)
- To act as pressure relief devices
- To balance flow between radiators
- To control gas pressure to the boiler
Correct answer: To independently control heating and hot water circuits
Zone valves (motorised valves) open and close to direct hot water flow to separate circuits such as heating and hot water, allowing independent control via separate thermostats.
Question 2: Which component in a central heating system controls the time that heating comes on and off?
- Programmer or timer (Correct answer)
- Room thermostat
- TRV
- Boiler thermostat
Correct answer: Programmer or timer
A programmer or timer controls when the heating and hot water circuits are active, setting on/off periods throughout the day.
Question 3: What does 'commissioning' a central heating system involve?
- Setting up, testing and adjusting the system to operate correctly to design specification (Correct answer)
- Simply filling the system with water
- Only testing the boiler ignition
- Registering the installation with Gas Safe Register
Correct answer: Setting up, testing and adjusting the system to operate correctly to design specification
Commissioning includes flushing, filling, venting, pressure testing, setting controls, balancing radiators, and verifying the system meets design performance before handover.
Question 4: What is the purpose of power flushing a central heating system?
- To remove sludge, corrosion products and scale from the pipework and components (Correct answer)
- To increase the system pressure
- To remove air from radiators
- To test for gas leaks
Correct answer: To remove sludge, corrosion products and scale from the pipework and components
Power flushing uses a high-flow, low-pressure machine with chemicals to dislodge and remove accumulated sludge, rust and scale, restoring system efficiency.
Question 5: What chemical is commonly added to a central heating system to prevent corrosion?
- Corrosion inhibitor (e.g., Sentinel X100 or Fernox F1) (Correct answer)
- Descaling acid
- Chlorine solution
- Antifreeze alone
Correct answer: Corrosion inhibitor (e.g., Sentinel X100 or Fernox F1)
A corrosion inhibitor such as Sentinel X100 or Fernox F1 is dosed into the system water to protect metals from corrosion and prevent the formation of magnetite sludge.
Question 6: What is the role of the boiler interlock in a modern central heating system?
- To prevent the boiler firing when there is no demand for heat (Correct answer)
- To lock the boiler for servicing
- To secure the boiler to the wall
- To interlock the gas valve with the flue damper
Correct answer: To prevent the boiler firing when there is no demand for heat
A boiler interlock ensures the boiler only fires when there is a genuine demand for heat, preventing unnecessary cycling and improving efficiency — achieved by wiring zone valves' end switches to the boiler.
CENWAT Central Heating System Design 4
What is the purpose of zone valves in a central heating system?