CENWAT Boiler Types 2 — Questions and Answers
Question 1: What is a back boiler?
- A boiler installed behind a gas fire in a fireplace recess (Correct answer)
- A boiler installed at the back of a garage
- A boiler with a rear flue connection only
- A redundant term for a system boiler
Correct answer: A boiler installed behind a gas fire in a fireplace recess
A back boiler is installed behind a decorative gas fire in a fireplace recess. They are no longer manufactured new but many remain in existing properties.
Question 2: What is a floor-standing boiler typically used for in UK installations?
- High-output applications or older properties where wall-hung installation is not practical (Correct answer)
- Only commercial properties
- Properties with no gas supply
- Underfloor heating systems exclusively
Correct answer: High-output applications or older properties where wall-hung installation is not practical
Floor-standing boilers are used for higher output requirements or in older properties where wall-hung installation is not suitable. They tend to be larger and heavier than wall-hung models.
Question 3: What does 'modulating' mean in relation to modern condensing boilers?
- The boiler varies its heat output to match demand rather than simply switching on and off at full power (Correct answer)
- The boiler modulates the flue gas composition
- The boiler alternates between gas and electric heating
- The boiler adjusts the water pressure automatically
Correct answer: The boiler varies its heat output to match demand rather than simply switching on and off at full power
Modulating boilers adjust their burner firing rate to match the actual heat demand, reducing cycling and improving efficiency compared to on/off boilers.
Question 4: What is the purpose of the primary heat exchanger in a boiler?
- To transfer heat from the burner flame to the heating circuit water (Correct answer)
- To condense water vapour from flue gases
- To pre-heat the incoming cold water supply
- To exchange heat between the heating and hot water circuits
Correct answer: To transfer heat from the burner flame to the heating circuit water
The primary heat exchanger absorbs heat from the burner flame and transfers it to the water in the heating circuit; in condensing boilers a secondary heat exchanger also captures latent heat from flue gases.
Question 5: In a combi boiler, what happens to the central heating circuit when hot water is demanded?
- The central heating circuit is temporarily diverted so all heat is prioritised for domestic hot water (Correct answer)
- The central heating continues to operate normally
- The gas valve closes completely
- The pump stops
Correct answer: The central heating circuit is temporarily diverted so all heat is prioritised for domestic hot water
Most combi boilers prioritise domestic hot water demand by diverting the system to the plate heat exchanger for DHW, temporarily suspending the central heating circuit.
Question 6: What is a plate heat exchanger in a combi boiler used for?
- To heat domestic hot water instantaneously by transferring heat from the primary circuit (Correct answer)
- To transfer heat from flue gases to the primary circuit
- To act as the main combustion heat exchanger
- To store a small volume of hot water
Correct answer: To heat domestic hot water instantaneously by transferring heat from the primary circuit
The plate heat exchanger in a combi boiler uses the primary circuit water (already heated by the main heat exchanger) to heat cold mains water instantaneously for domestic use.
CENWAT Boiler Types 2
What is a back boiler?