CENWAT Inhibitor & Water Treatment 4 — Questions and Answers
Question 1: What is a polyphosphate scale inhibitor dosing unit?
- A device that slowly releases food-grade polyphosphate crystals into the water supply to inhibit scale and corrosion on heat exchanger surfaces (Correct answer)
- A device that removes polyphosphates from water
- A chemical filter for heating system water
- An electrolytic scale reducer
Correct answer: A device that slowly releases food-grade polyphosphate crystals into the water supply to inhibit scale and corrosion on heat exchanger surfaces
A polyphosphate dosing unit releases small amounts of polyphosphate into the water stream, sequestering calcium and magnesium ions to inhibit scale formation on boiler and heater surfaces in hard water areas.
Question 2: What is an electrolytic scale reducer?
- A device using a zinc or titanium alloy rod to alter the crystal structure of calcium carbonate, reducing adhesion to surfaces (Correct answer)
- A device that electrically heats water to prevent scale
- A water softener using electrolysis
- An ionisation device for the gas supply
Correct answer: A device using a zinc or titanium alloy rod to alter the crystal structure of calcium carbonate, reducing adhesion to surfaces
An electrolytic scale reducer uses dissimilar metals to create a small galvanic current that changes the crystalline form of calcium carbonate from adherent calcite to non-adherent aragonite, reducing scale deposits.
Question 3: What effect does limescale have on the efficiency of a boiler heat exchanger?
- It acts as an insulating layer reducing heat transfer, increasing gas consumption and potentially causing overheating and premature failure (Correct answer)
- It has no effect on efficiency
- It improves efficiency by smoothing the heat exchanger surfaces
- It causes the heat exchanger to corrode faster
Correct answer: It acts as an insulating layer reducing heat transfer, increasing gas consumption and potentially causing overheating and premature failure
Even a thin layer of limescale (1 mm) significantly reduces heat transfer efficiency (by approximately 7–10%), increasing fuel consumption and potentially causing the heat exchanger to overheat.
Question 4: What is the recommended approach for water treatment when replacing a boiler in an existing, heavily sludged system?
- Power flush the existing system with a system cleaner before installing the new boiler, then dose with inhibitor (Correct answer)
- Simply install the new boiler without treating the old system
- Add extra inhibitor to protect the new boiler without flushing
- Drain and refill with fresh water only
Correct answer: Power flush the existing system with a system cleaner before installing the new boiler, then dose with inhibitor
Installing a new boiler into an untreated sludged system will rapidly contaminate the new heat exchanger; the existing system must be power flushed and treated with inhibitor before the new boiler is connected.
Question 5: What information should be recorded about water treatment on the boiler's Benchmark logbook?
- The type and dose of inhibitor used, the system volume, water hardness, whether a scale inhibitor was fitted, and date of treatment (Correct answer)
- Only the brand of inhibitor
- Only the date of power flushing
- Water treatment information is not required in the Benchmark
Correct answer: The type and dose of inhibitor used, the system volume, water hardness, whether a scale inhibitor was fitted, and date of treatment
The Benchmark commissioning checklist requires the engineer to record water hardness, the inhibitor product and dose, system volume treated, and whether a scale inhibitor or softener was fitted.
Question 6: What is the purpose of a system separator (plate heat exchanger) between an old sludged system and a new boiler?
- To thermally separate the old system water from the new boiler circuit, protecting the boiler from sludge in the existing pipework (Correct answer)
- To increase system pressure
- To act as a buffer vessel
- To filter the system water magnetically
Correct answer: To thermally separate the old system water from the new boiler circuit, protecting the boiler from sludge in the existing pipework
A hydraulic or thermal separator isolates the new boiler's clean primary circuit from the old contaminated secondary circuit, protecting the boiler heat exchanger while still delivering heat to the existing radiators.
CENWAT Inhibitor & Water Treatment 4
What is a polyphosphate scale inhibitor dosing unit?