CCN1 Combustion and Ventilation 2 — Questions and Answers
Question 1: When testing the combustion performance of a gas appliance, what does a high CO/COâ‚‚ ratio indicate?
- Perfect combustion conditions
- Excess air is entering the appliance
- Incomplete combustion is occurring (Correct answer)
- The flue is working efficiently
Correct answer: Incomplete combustion is occurring
A high CO/COâ‚‚ ratio indicates incomplete combustion. The acceptable maximum CO/COâ‚‚ ratio for most appliances is 0.004 (or 1:250). A ratio exceeding this indicates that the appliance is producing dangerous levels of carbon monoxide relative to the carbon dioxide output, and the appliance requires investigation and rectification.
Question 2: What is 'vitiation' in the context of gas appliance operation?
- The process of igniting a gas burner
- The progressive reduction of oxygen in a room due to an appliance consuming air for combustion (Correct answer)
- The corrosion of flue components
- The measurement of gas pressure at the meter
Correct answer: The progressive reduction of oxygen in a room due to an appliance consuming air for combustion
Vitiation occurs when an open-flued or flueless appliance consumes oxygen from the room for combustion, progressively reducing the oxygen level in the space. This can lead to incomplete combustion, increased carbon monoxide production, and ultimately a dangerous situation if adequate ventilation is not provided.
Question 3: What is the minimum free area of a permanent air vent required for a flueless gas appliance such as a cooker with a rated input of 15 kW?
- No ventilation required below 20 kW
- 5 cm² per kW of rated input above 7 kW
- 5 cm² per kW of rated net input (Correct answer)
- Depends on the room volume only
Correct answer: 5 cm² per kW of rated net input
Flueless appliances require ventilation of 5 cm² of free area per kW of rated net input for the total input of the appliance (not just the amount above 7 kW as with open-flued appliances). For a 15 kW flueless appliance, this would require 75 cm² of free area ventilation.
Question 4: What colour flame indicates correct combustion on a natural gas burner?
- Yellow with orange tips
- Predominantly blue with a clearly defined inner cone (Correct answer)
- Red with flickering
- White with sparking
Correct answer: Predominantly blue with a clearly defined inner cone
A correctly adjusted natural gas burner should produce a predominantly blue flame with a clearly defined inner cone (primary combustion zone). Yellow or orange flames indicate incomplete combustion due to insufficient primary air, while a lifting or floating flame suggests too much primary air.
Question 5: What happens to the products of combustion from a decorative fuel effect (DFE) gas fire?
- They are discharged through a conventional chimney or flue (Correct answer)
- They are all released into the room as it is a flueless appliance
- They pass through a catalytic converter
- They are recycled back into the burner
Correct answer: They are discharged through a conventional chimney or flue
A decorative fuel effect (DFE) gas fire is an open-flued appliance that discharges its products of combustion into a chimney or flue. Although the fire may appear similar to a flueless appliance, it relies entirely on the chimney or flue for safe evacuation of combustion products. The flue must be tested for safe operation.
Question 6: What is the purpose of a spillage test on an open-flued gas appliance?
- To check for gas leaks on the supply pipework
- To verify that combustion products are being safely drawn into the flue and not spilling into the room (Correct answer)
- To measure the gas pressure at the burner
- To test the thermocouple response time
Correct answer: To verify that combustion products are being safely drawn into the flue and not spilling into the room
A spillage test (also called a flue flow test) checks that the products of combustion are being safely drawn up and away through the flue system and are not spilling back into the room. This is typically performed using a smoke match or smoke pellet held near the appliance draught diverter. Spillage indicates a flue problem that must be investigated.
CCN1 Combustion and Ventilation 2
When testing the combustion performance of a gas appliance, what does a high CO/COâ‚‚ ratio indicate?