Technical

Ventilation and gas appliances

Gas engineers know how important ventilation is for safe combustion. Here, Gas Safe Register’s Technical Team look at what happens when air and gas mix.

Why do we need ventilation?

Natural gas is a carbon fuel and it needs a certain amount of oxygen to ignite and sustain satisfactory combustion. For every one unit of natural gas burnt, two equivalent units of oxygen are required.

The fresh air that surrounds us is made up of several different elements and contains only 21 per cent oxygen. This means that one unit of natural gas will require 10 units of fresh air in order to achieve satisfactory complete combustion.

Two sources of air are required for a gas appliance to burn correctly: primary and secondary. Primary is the air mixed with unburnt gas before the burner, and secondary air feeds the flame after ignition.

If the combustion process is incomplete, increased levels of carbon monoxide (CO) will be present in the products of combustion (POCs). Carbon monoxide is found normally in POCs, but interference in the combustion process will cause it to be produced at a greater rate.

The most common causes of excessive CO production are generally:

• Oxygen starvation

Oxygen starvation means the flame is not receiving enough oxygen to burn correctly. This can be caused by a number of factors, such as insufficient ventilation, lack of primary air, or the air becoming vitiated because of spillage.

• Flame chilling

This occurs when the flame comes in to contact with a surface that has a lower temperature than the flame itself. One example is the coals on a live fuel-effect gas fire before they have reached a certain temperature.

Debris on the appliance burner can have the same effect because of flame interference.

• Flame impingement

This is when one flame comes into contact with another flame,

A vitiated flame is one that is starved of air

creating a zone inside the flame that does not receive any secondary air and so does not burn completely.

All these scenarios will play a part in causing incomplete combustion, why is why correct combustion ventilation is so important. The easiest way to ensure enough combustion air is supplied to a gas appliance is to make sure that purpose-provided ventilation is provided in the room containing the appliance.

The ventilation requirement for a gas appliance is usually determined by the appliance flue type and heat input. Appliances requiring purpose-provided ventilation for combustion air, other than ventilation required for cooking purposes, would generally be conventional open-flue or flueless type appliances.

Fan-assisted open flue and vertex flue appliances may require ventilation and this requirement is generally specified by the appliance manufacturer.

In the absence of clear guidance from the manufacturer’s instructions regarding ventilation for any domestic appliance, follow British Standard 5440-2: 2009 Flueing and ventilation for gas appliances of rated heat input not exceeding 70kW net (1st, 2nd and 3rd family gases) – Part 2: Specification for the installation and maintenance of ventilation provision for gas appliances. ■