CENWAT Hot Water Systems (Unvented/Vented) 2

Question 1

What is the recommended stored hot water temperature to prevent Legionella bacteria growth in domestic systems?

Accepted Answer

60 degrees C

Answer

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?

Accepted Answer

To absorb the increased volume of water as it is heated

Answer

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?

Accepted Answer

Every 6 months

Answer

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?

Accepted Answer

9 metres

Answer

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?

Accepted Answer

An air gap between the relief valve discharge and the downstream pipe

Answer

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.

ACS CENWAT Practice Test

ACS CENWAT Practice Test

CENWAT Hot Water Systems (Unvented/Vented) 2