CENWAT Hot Water Systems (Unvented/Vented) — Questions and Answers
Question 1: What qualification must a plumber hold to install an unvented hot water storage system in the UK?
- ACS domestic core only
- City and Guilds Level 2 Plumbing
- G3 Unvented Hot Water Storage qualification (Correct answer)
- Part P Electrical certification only
Correct answer: G3 Unvented Hot Water Storage qualification
Under Building Regulation G3, any person installing an unvented hot water storage system over 15 litres must hold a current G3 Unvented Hot Water Storage qualification. This ensures the installer understands the specific safety requirements, as unvented systems operate under mains pressure.
Question 2: What is the correct order of safety devices on an unvented hot water cylinder from the water supply inlet?
- Expansion vessel, pressure reducing valve, temperature and pressure relief valve
- Pressure reducing valve, check valve, expansion vessel, temperature and pressure relief valve (Correct answer)
- Temperature and pressure relief valve, expansion vessel, check valve
- Check valve, temperature and pressure relief valve, expansion vessel
Correct answer: Pressure reducing valve, check valve, expansion vessel, temperature and pressure relief valve
The correct order on an unvented system is: pressure reducing valve, check valve (or combined set), then the expansion vessel, and finally the temperature and pressure relief valve on the cylinder itself. This ensures incoming pressure is controlled, backflow is prevented, expansion is accommodated, and overpressure protection is in place.
Question 3: At what temperature should the temperature and pressure relief valve on an unvented cylinder operate?
- 75 degrees C
- 85 degrees C
- 90-95 degrees C (Correct answer)
- 100 degrees C
Correct answer: 90-95 degrees C
The temperature and pressure relief valve on an unvented hot water cylinder is factory set to operate at 90-95 degrees C. This is above the normal thermostat setting of typically 60-65 degrees C and the overheat thermostat at typically 80-85 degrees C, making it the final line of defence.
Question 4: What is the maximum working pressure rating typically specified for an unvented hot water cylinder?
- 3 bar
- 6 bar
- 10 bar (Correct answer)
- 16 bar
Correct answer: 10 bar
Unvented hot water cylinders are typically rated to a maximum working pressure of 10 bar, though they are normally operated at 3-3.5 bar via a pressure reducing valve. The 10 bar rating provides a significant safety margin. The pressure relief valve is typically set at 6-7 bar.
Question 5: What is the minimum discharge pipe size from the temperature and pressure relief valve to the tundish on an unvented cylinder?
- 15mm (Correct answer)
- 22mm
- 28mm
- 35mm
Correct answer: 15mm
The discharge pipe from the T&P relief valve to the tundish must be a minimum of 15mm, the same size as the relief valve outlet. From the tundish onwards, the pipe must increase by at least one pipe size to minimum 22mm and must have a continuous fall to a safe, visible discharge point.
Question 6: In a vented hot water system, what is the purpose of the vent pipe from the top of the hot water cylinder?
- To supply cold water to the cylinder
- To provide a path for air and steam to escape safely to atmosphere (Correct answer)
- To connect to the central heating circuit
- To supply water to the shower pump
Correct answer: To provide a path for air and steam to escape safely to atmosphere
The open vent pipe rises from the top of the hot water cylinder and terminates over the cold water storage cistern. It provides a safe path for air and steam to escape to atmosphere if the water overheats, establishes maximum system pressure, and prevents dangerous pressure build-up.
CENWAT Hot Water Systems (Unvented/Vented)
What qualification must a plumber hold to install an unvented hot water storage system in the UK?