CENWAT Central Heating System Types 2 — Questions and Answers
Question 1: What is the purpose of the automatic bypass valve in a fully pumped central heating system?
- To fill the system with water
- To prevent boiler short cycling by maintaining minimum flow (Correct answer)
- To control the hot water temperature
- To balance the radiator circuits
Correct answer: To prevent boiler short cycling by maintaining minimum flow
The automatic bypass valve ensures a minimum flow rate through the boiler when zone valves close. Without adequate flow, the boiler could overheat or short cycle. The bypass valve opens progressively as system resistance increases when zones close, maintaining a safe minimum flow through the heat exchanger.
Question 2: Which type of central heating system requires both a feed and expansion cistern and a hot water storage cylinder?
- Combi boiler system
- Open-vented regular boiler system (Correct answer)
- System boiler with unvented cylinder
- Instantaneous water heater system
Correct answer: Open-vented regular boiler system
An open-vented regular (heat-only) boiler system requires a feed and expansion cistern in the loft to accommodate expansion and maintain system water level, plus a separate hot water storage cylinder to store domestic hot water heated indirectly by the boiler.
Question 3: What is the maximum recommended static head for a domestic open-vented central heating system?
- 5 metres
- 10 metres (Correct answer)
- 15 metres
- 20 metres
Correct answer: 10 metres
The maximum recommended static head for a domestic open-vented system is typically 10 metres. Beyond this, the increased pressure at the lowest point of the system can cause problems with seals, joints, and components not rated for higher pressures.
Question 4: In a Y-plan system, what happens when both the room thermostat and cylinder thermostat are calling for heat?
- The 3-port valve closes completely
- The 3-port valve moves to the mid-position allowing flow to both circuits (Correct answer)
- The 3-port valve prioritises hot water only
- The system shuts down to prevent overload
Correct answer: The 3-port valve moves to the mid-position allowing flow to both circuits
In a Y-plan system, when both thermostats call for heat simultaneously, the 3-port mid-position valve moves to its mid-position, allowing water to flow to both the heating and hot water circuits at the same time. This is the normal operating condition during cold weather.
Question 5: What is the recommended minimum pipe size for the cold feed to a domestic open-vented central heating system?
- 8mm
- 15mm (Correct answer)
- 22mm
- 28mm
Correct answer: 15mm
The cold feed pipe from the feed and expansion cistern to the primary circuit should be a minimum of 15mm diameter for domestic systems. This ensures adequate water can flow into the system to compensate for losses and accommodates expansion of water as it heats up.
Question 6: Which Building Regulation specifically deals with the conservation of fuel and power in dwellings, affecting central heating system design?
- Part J
- Part G
- Part L (Correct answer)
- Part P
Correct answer: Part L
Building Regulations Part L (Conservation of Fuel and Power) directly affects central heating system design. It mandates minimum boiler efficiencies of 92% ErP, heating controls requirements including room thermostat, programmer, and TRVs, and system insulation standards to reduce energy consumption.
CENWAT Central Heating System Types 2
What is the purpose of the automatic bypass valve in a fully pumped central heating system?