Define 'draft' in the context of fuel gas systems.

In the context of fuel gas systems, 'draft' refers to the flow of air or gases caused by pressure differences. This is critical in ensuring that combustion gases are properly vented from appliances and that fresh air is supplied for combustion. A well-maintained draft promotes efficiency and safety in fuel gas systems by preventing the buildup of harmful gases and ensuring that the combustion process operates effectively.

Draft is essential for maintaining the balance within the system, allowing for the correct amount of air to mix with the fuel for optimal combustion, and facilitating the safe removal of byproducts. Understanding draft is vital for anyone working with fuel gases, as it directly affects performance and safety standards in installations and operations involving gas-fired appliances.

The other concepts mentioned, such as sound produced by heating systems, the temperature of combustion gases, and gas flow measurement, do not capture the essence of how air or gases circulate in relation to pressure differences, thus making them less relevant in defining 'draft' in this context.