CENWAT Boiler Operation and Controls — Questions and Answers
Question 1: What is the minimum efficiency rating required for a new gas boiler installation under current Building Regulations Part L in England?
- 86% ErP
- 89% ErP
- 92% ErP (Correct answer)
- 95% ErP
Correct answer: 92% ErP
Under current Building Regulations Part L (2021 edition for England), replacement gas boilers must achieve a minimum seasonal efficiency of 92% ErP. This effectively means only condensing boilers can be installed, as non-condensing boilers cannot achieve this level of efficiency.
Question 2: At what flue gas temperature does a condensing boiler begin to recover latent heat from the products of combustion?
- 100 degrees C
- 80 degrees C
- 57 degrees C (dew point) (Correct answer)
- 40 degrees C
Correct answer: 57 degrees C (dew point)
A condensing boiler recovers latent heat when the return water temperature is low enough to cool the flue gases below the dew point of approximately 57 degrees C. Below this temperature, water vapour in the combustion products condenses on the secondary heat exchanger, releasing its latent heat energy back into the system.
Question 3: What is the purpose of the boiler interlock in a central heating control system?
- To lock the boiler room door for safety
- To prevent the boiler firing when there is no demand for heat (Correct answer)
- To connect multiple boilers together
- To lock the gas supply during servicing
Correct answer: To prevent the boiler firing when there is no demand for heat
Boiler interlock is a control strategy required by Building Regulations that ensures the boiler does not fire when there is no demand for heat from either the room thermostat or cylinder thermostat. It is achieved through the correct wiring of thermostats, programmer, and motorised valves so that the boiler circuit is only energised when heat is actually needed.
Question 4: Which component in a condensing boiler is specifically designed to extract additional heat from flue gases?
- Primary heat exchanger
- Fan assembly
- Secondary heat exchanger (Correct answer)
- Gas valve
Correct answer: Secondary heat exchanger
The secondary heat exchanger (also called the condensing heat exchanger) is specifically designed to extract latent heat from the flue gases. Cool return water passes through this heat exchanger first, cooling the flue gases below dew point and causing condensation, which releases additional energy into the system water.
Question 5: According to the Boiler Plus legislation (England), what additional control is required when installing a regular boiler?
- A smart thermostat only
- Time and temperature control of heating and hot water as separate zones (Correct answer)
- A weather compensation controller only
- Flue gas heat recovery only
Correct answer: Time and temperature control of heating and hot water as separate zones
Boiler Plus (2018, England) requires that regular (heat-only) boiler installations must have time and temperature control for both space heating and hot water as independently controlled zones. This means separate thermostats and timing for heating and hot water circuits, ensuring efficient operation.
Question 6: What is the correct procedure if the condensate pipe from a condensing boiler freezes?
- Replace the boiler immediately
- Apply warm water to the frozen section of the external condensate pipe (Correct answer)
- Disconnect the condensate pipe and let it drain freely
- Bypass the condensate trap
Correct answer: Apply warm water to the frozen section of the external condensate pipe
If the condensate pipe freezes, the recommended procedure is to apply warm (not boiling) water to the frozen external section to thaw it, then reset the boiler. To prevent future freezing, the external condensate pipe should be insulated, kept as short as possible, and be a minimum of 32mm diameter externally.
CENWAT Boiler Operation and Controls
What is the minimum efficiency rating required for a new gas boiler installation under current Building Regulations Part L in England?