Boiler Operation and Controls Flashcards

What is the minimum efficiency rating required for a new gas boiler installation under current Building Regulations Part L in England?

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.

At what flue gas temperature does a condensing boiler begin to recover latent heat from the products of combustion?

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.

What is the purpose of the boiler interlock in a central heating control system?

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.

Which component in a condensing boiler is specifically designed to extract additional heat from flue gases?

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.

According to the Boiler Plus legislation (England), what additional control is required when installing a regular boiler?

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.

What is the correct procedure if the condensate pipe from a condensing boiler freezes?

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.