CENWAT Central Heating System Types — Questions and Answers
Question 1: In a fully pumped central heating system, how is water circulated to both the heating and hot water circuits?
- By gravity to both circuits
- By a single pump with motorised valves directing flow (Correct answer)
- By two separate boilers
- By thermosiphon action only
Correct answer: By a single pump with motorised valves directing flow
A fully pumped system uses a single circulating pump combined with motorised valves (typically two 2-port or one 3-port valve) to direct flow to the heating circuit, hot water circuit, or both as required by the room thermostat and cylinder thermostat.
Question 2: What is the primary disadvantage of a gravity-fed hot water circuit compared to a fully pumped system?
- It requires a larger cold water storage cistern
- It has slower hot water recovery times due to lower flow rates (Correct answer)
- It cannot be used with a conventional boiler
- It requires a pressurised expansion vessel
Correct answer: It has slower hot water recovery times due to lower flow rates
Gravity circulation relies on the natural convection of heated water rising, which produces much lower flow rates than a pump. This results in significantly slower hot water recovery times, meaning the cylinder takes longer to reheat after hot water has been drawn off.
Question 3: Which British Standard covers the specification for forced circulation hot water central heating systems in domestic premises?
- BS 7593
- BS 6798
- BS 5449 (Correct answer)
- BS 7074
Correct answer: BS 5449
BS 5449 covers the specification for forced circulation hot water central heating systems for domestic premises. It sets out requirements for system design, installation, and commissioning of open-vented wet central heating systems.
Question 4: In an S-plan central heating system, which valve configuration is used?
- One 3-port mid-position valve
- Two 2-port motorised valves (Correct answer)
- One 2-port motorised valve
- Three 2-port motorised valves
Correct answer: Two 2-port motorised valves
An S-plan system uses two separate 2-port motorised zone valves, one controlling the heating circuit and one controlling the hot water circuit. This allows completely independent control of each circuit, unlike the Y-plan which uses a single 3-port valve.
Question 5: What is the minimum height requirement for the open vent pipe above the water level in the feed and expansion cistern?
- 150mm
- 450mm (Correct answer)
- One pipe diameter above
- Twice pipe diameter above
Correct answer: 450mm
The open vent pipe must terminate at least 450mm above the water level in the feed and expansion cistern. This prevents water from discharging from the vent during normal pump operation and provides adequate protection against system pressurisation.
Question 6: In a W-plan central heating system, what distinguishes it from an S-plan?
- It uses wireless thermostatic radiator valves for zone control (Correct answer)
- It uses a single 3-port diverter valve
- It has no motorised valves
- It uses gravity hot water with pumped heating
Correct answer: It uses wireless thermostatic radiator valves for zone control
A W-plan system is essentially an S-plan system but replaces conventional wired zone valves and thermostats with wireless thermostatic radiator valves for individual room temperature control. This provides more granular zone control without extensive wiring.
CENWAT Central Heating System Types
In a fully pumped central heating system, how is water circulated to both the heating and hot water circuits?