CENWAT Inhibitor & Water Treatment 1 — Questions and Answers
Question 1: What is the main purpose of adding a corrosion inhibitor to a central heating system?
- To protect metal components from corrosion and prevent the formation of magnetite (black sludge) (Correct answer)
- To act as an antifreeze
- To reduce system noise
- To descale the heat exchanger
Correct answer: To protect metal components from corrosion and prevent the formation of magnetite (black sludge)
Corrosion inhibitor chemicals (such as Fernox F1 or Sentinel X100) form a protective film on metal surfaces, preventing electrochemical corrosion and the formation of iron oxide sludge (magnetite).
Question 2: What is magnetite and why is it harmful in a central heating system?
- Black iron oxide sludge formed by corrosion that blocks radiators, damages pumps and coats boiler heat exchangers, reducing efficiency (Correct answer)
- A naturally occurring scale from hard water
- A type of corrosion inhibitor
- A filter media used in system filters
Correct answer: Black iron oxide sludge formed by corrosion that blocks radiators, damages pumps and coats boiler heat exchangers, reducing efficiency
Magnetite (Fe3O4) is black iron oxide sludge generated by corrosion of ferrous components. It settles in radiators causing cold spots, damages pump impellers and coats heat exchangers reducing efficiency.
Question 3: How should inhibitor be introduced into a sealed central heating system?
- Via the filling loop, through a purpose-made dosing pot, or directly through a radiator after removing the bleed valve (Correct answer)
- By adding it to the cold water cistern
- Through the gas inlet
- By injecting it into the boiler heat exchanger directly
Correct answer: Via the filling loop, through a purpose-made dosing pot, or directly through a radiator after removing the bleed valve
Inhibitor can be introduced through the filling loop using a dosing pump, through a purpose-made dosing pot on the system, or directly through a radiator bleed valve port after removing the bleed screw.
Question 4: 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 (Correct answer)
- Every 5 years
- Only when adding new inhibitor
- Only when power flushing
Correct answer: Annually during a boiler service, and after any significant water loss or system work
Inhibitor concentration should be checked annually using test strips or a refractometer. After any significant water loss, top-up or system work, the inhibitor must be replenished.
Question 5: What is the correct concentration range for a typical liquid corrosion inhibitor in a domestic central heating system?
- As specified by the manufacturer — typically 1% of system volume (e.g., 500 ml per 50 litres) (Correct answer)
- 10% of system volume
- As strong as possible
- The concentration is irrelevant — more is always better
Correct answer: As specified by the manufacturer — typically 1% of system volume (e.g., 500 ml per 50 litres)
Manufacturer instructions specify the correct dose, typically around 1% of system volume. Too little provides insufficient protection; over-dosing is wasteful and can in some cases affect water treatment components.
Question 6: What must be done to the system water before adding a new dose of corrosion inhibitor after power flushing?
- The system must be thoroughly flushed with clean water to remove the flushing chemicals before the inhibitor is added (Correct answer)
- The inhibitor can be added directly to the flushing chemical mix
- The inhibitor is added before flushing
- No preparation is needed
Correct answer: The system must be thoroughly flushed with clean water to remove the flushing chemicals before the inhibitor is added
After power flushing, the system must be flushed thoroughly with clean water to remove all flushing and descaling chemicals before the corrosion inhibitor is dosed, as mixing chemicals can reduce effectiveness.
CENWAT Inhibitor & Water Treatment 1
What is the main purpose of adding a corrosion inhibitor to a central heating system?