CENWAT Inhibitor & Water Treatment 2 — Questions and Answers
Question 1: What is a system cleanser (flush chemical) used for?
- To break down and remove sludge, scale and corrosion products during a power flush or chemical flush (Correct answer)
- To protect the system from future corrosion
- To neutralise acidic condensate
- To descale the gas injectors
Correct answer: To break down and remove sludge, scale and corrosion products during a power flush or chemical flush
System cleansers (e.g., Fernox F3 or Sentinel X400) are dosed into the system water to loosen and dissolve sludge, scale and corrosion deposits, which are then removed during a power flush.
Question 2: What is a scale inhibitor and when is it particularly important?
- A chemical or physical device that reduces limescale formation, particularly important in hard water areas (Correct answer)
- A device that prevents system pressure from rising
- An additive to prevent sludge only
- A device to reduce noise caused by scale
Correct answer: A chemical or physical device that reduces limescale formation, particularly important in hard water areas
Scale inhibitors (e.g., polyphosphate dosers or chemical scale reducers) are used in hard water areas to prevent limescale from forming on heat exchanger surfaces, maintaining efficiency and preventing premature failure.
Question 3: What is the water hardness level above which a scale inhibitor or water softener is typically recommended for a boiler?
- Above 200 mg/l (as CaCO3) or as specified by the boiler manufacturer (Correct answer)
- Above 50 mg/l
- Above 500 mg/l
- Scale inhibitors are required regardless of water hardness
Correct answer: Above 200 mg/l (as CaCO3) or as specified by the boiler manufacturer
Most boiler manufacturers and industry guidance (e.g., HHIC, BS 7593) recommend scale protection measures when the water hardness exceeds 200 mg/l (20 Clarke degrees), which is common in south-east England.
Question 4: What standard provides guidance on water treatment in central heating systems?
- BS 7593 (Correct answer)
- BS 7671
- BS 5440
- BS 8300
Correct answer: BS 7593
BS 7593 (Code of practice for treatment of water in domestic hot water central heating systems) provides guidance on flushing, inhibitor dosing, water quality and maintenance of central heating system water.
Question 5: What is antifreeze and when is it used in a central heating system?
- A glycol-based additive that lowers the freezing point of system water, used in vulnerable systems such as holiday homes or systems with external pipework (Correct answer)
- An inhibitor that prevents rust only
- A descaling chemical
- A boiler cleaning fluid
Correct answer: A glycol-based additive that lowers the freezing point of system water, used in vulnerable systems such as holiday homes or systems with external pipework
Antifreeze (typically propylene glycol based for heating systems) is added to system water to prevent freezing in vulnerable systems such as holiday cottages, or where external or unheated pipework runs are present.
Question 6: What precaution must be taken when using antifreeze in a central heating system with a combi boiler?
- Check the boiler manufacturer's approval — some plate heat exchangers can be damaged by glycol, and the DHW circuit must be protected from contamination (Correct answer)
- Antifreeze can be used without restriction in any system
- Double the inhibitor dose when using antifreeze
- Only propylene glycol may ever be used
Correct answer: Check the boiler manufacturer's approval — some plate heat exchangers can be damaged by glycol, and the DHW circuit must be protected from contamination
Combi boilers use a plate heat exchanger where the primary (heating) circuit is in close proximity to potable water; glycol contamination of the DHW circuit is a health risk, so manufacturer guidance must always be checked.
CENWAT Inhibitor & Water Treatment 2
What is a system cleanser (flush chemical) used for?