CENWAT Hot Water Systems (Unvented/Vented) 2 — Questions and Answers
Question 1: What is the recommended stored hot water temperature to prevent Legionella bacteria growth in domestic systems?
- 40 degrees C
- 50 degrees C
- 60 degrees C (Correct answer)
- 70 degrees C
Correct answer: 60 degrees C
The HSE Approved Code of Practice L8 and HSG274 recommend that hot water is stored at a minimum of 60 degrees C to prevent Legionella bacteria growth. Legionella thrives between 20-45 degrees C and is killed at temperatures above 60 degrees C. Water should be distributed at a minimum of 50 degrees C.
Question 2: What is the purpose of the expansion vessel in an unvented hot water system?
- To store additional cold water
- To absorb the increased volume of water as it is heated (Correct answer)
- To boost the water pressure
- To act as a secondary heat exchanger
Correct answer: To absorb the increased volume of water as it is heated
When water is heated from 10 degrees C to 60 degrees C, it expands by approximately 4% of its volume. In a sealed unvented system, an expansion vessel with a rubber diaphragm and pressurised air charge absorbs this increased volume. Without it, the pressure would rise dangerously.
Question 3: How often should the temperature and pressure relief valve on an unvented cylinder be manually tested by the householder?
- Weekly
- Monthly
- Every 6 months (Correct answer)
- Annually
Correct answer: Every 6 months
Manufacturers typically recommend that the householder manually operates the temperature and pressure relief valve every 6 months to ensure it has not seized or become blocked with limescale. The full annual service and inspection should be carried out by a qualified person.
Question 4: What is the maximum recommended length of the discharge pipe from a tundish to the termination point for D2 (22mm) pipe sizing?
- 3 metres
- 9 metres (Correct answer)
- 18 metres
- 27 metres
Correct answer: 9 metres
For D2 discharge pipework (22mm copper or equivalent), the maximum recommended length from the tundish to the termination point is 9 metres. Beyond this distance, the pipe must be increased to D3 (28mm) to ensure adequate flow capacity, as specified in Building Regulation G3.
Question 5: What must be visible at the tundish of an unvented hot water system discharge arrangement?
- A pressure gauge reading
- An air gap between the relief valve discharge and the downstream pipe (Correct answer)
- A thermometer reading
- A flow meter reading
Correct answer: An air gap between the relief valve discharge and the downstream pipe
The tundish must provide a visible type AA air gap between the discharge from the relief valve and the downstream discharge pipe. This air gap provides a visible warning that the relief valve has operated and prevents backflow contamination from the downstream drainage system.
Question 6: Under Building Regulation G3, what is the maximum storage capacity of an unvented hot water system that does NOT require Building Control notification?
- No exemption, all sizes require notification
- 15 litres (Correct answer)
- 75 litres
- 150 litres
Correct answer: 15 litres
Unvented hot water storage systems with a capacity of 15 litres or less are exempt from Building Regulation G3 notification requirements. Systems above 15 litres must be notified to Building Control, either directly or through a Competent Persons Scheme.
CENWAT Hot Water Systems (Unvented/Vented) 2
What is the recommended stored hot water temperature to prevent Legionella bacteria growth in domestic systems?