Providing ventilation is an essential part of gas installation work – Kim Morris explains that it is the responsibility of the gas engineer to ensure that the relevant requirements of the Gas Safety (Installation and Use) Regulations 1998 (GSIUR) and manufacturer’s instructions are met in full.
Failure to provide adequate ventilation following gas installation work is not only an offence under the Gas Safety (Installation and Use) Regulations 1998 (GSIUR) but could also lead to serious incidents, including carbon monoxide poisoning.
There are two main reasons why ventilation is so important for gas appliances:
• It provides fresh air to enable gas appliances and their associate flues to operate safely.
• It provides regular air changes in an enclosure or compartment so they don’t overheat, which could cause the appliance to fail.
While ventilation is required in every situation where gas appliances or equipment are installed, there are various factors that need to be taken into consideration when sizing the correct amount of free air ventilation. Every situation will be different with every gas appliance fitted, so the gas engineer will need to consider the various types of appliance or equipment that are installed, the method of removing the products of combustion, the effects of air extraction systems, the methods of obtaining the correct amount of ventilation and then making sure that this ventilation can be maintained.
Types of gas appliances
There are three basic types of gas appliance:
1. Open chimney. This type draws air for combustion from inside the room in which it is sited and exhausts products of combustion via a flue to the atmosphere. A supply of permanent ventilation is always essential for this type of appliance to operate safely. An example of this would be a gas fire and some older types of boilers.
2. Balanced flue (room sealed). Air for combustion is drawn from outside and exhausts products of combustion back outside. Under normal circumstances ventilation is required only to ensure the appliance does not overheat when fitted in an enclosure or compartment.
3. Flueless. This type draws air for combustion from within the room in which it is situated and discharges the products of combustion back into the room – for instance a gas cooker and a single point-of-use water heater.
It is important that the gas engineer knows when ventilation is required and ensures that the correct amount of effective ventilation is provided and not obstructed in any way.
Guidance for the requirement for ventilation and flueing of all gas appliances of rated heat input not exceeding 70kW (net) can be found in British Standard BS 5440-1: 2008 and BS 5440-2: 2009. However, in the first instance, manufacturer’s instructions must always be followed.
Why ventilation is important
Every type of gas appliance (open flue, room sealed or flueless) will require varying amounts of ventilation, and all of these will need to be calculated separately. For example, the ventilation required for a 17kW flueless cooker will differ from a 17kW (net heat input) open flue boiler. The flueless cooker ventilation requirement is dependent on the cubic area of the room whereas the open flue boiler requires permanent ventilation irrespective of the area size.
Natural gas needs a certain amount of oxygen to burn correctly. Every cubic metre of gas requires ten cubic metres of air for complete combustion. As air contains 20.9% oxygen, it can also be said that every cubic metre of gas that is going to be burnt requires approximately two cubic metres of oxygen for complete combustion. The rest is made up of nitrogen, argon and various other gases that are all inert and play no part in the combustion process.
Under safe conditions, natural gas will burn with oxygen to produce carbon dioxide and water vapour. If ventilation is poor or flueing is restricted, colourless, odourless and highly toxic carbon monoxide (CO) can be formed. CO can lead to headaches, drowsiness, unconsciousness and ultimately to death.
If available ventilation is less than 90% of what is required, or if ventilation provision is defective in any way, the installation is categorised as ‘At Risk’ as defined in the Gas Industry Unsafe Situations Procedure. However, the gas engineer will need to make their own judgement and categorise the appliance accordingly should signs of spillage be evident.
|
Type of appliance |
Maximum rated heat input limit (net) |
Room volume (m3) |
Permanent vent size (cm2) |
Openable window required or equivalent |
|
Domestic oven, hotplate, grill or any combination thereof* |
None |
<5 5–10 > 10 |
100 50** Nil |
Yes |
|
Instantaneous water heater |
11kW |
<5 5–10
> 20 |
Installation not allowed 100 50 Nil |
Yes |
Calculating ventilation
The majority of manufacturers specify the heat input of gas appliances in kilowatts (kW). This is the figure that the gas engineer will require when calculating ventilation for open flue gas appliances. They also give the heat output figure – this is for calculating the size of gas boiler/fire when working out heat losses for space heating and should not be used for calculating ventilation.
Heat input is a measure of the amount of gas used and could be written as net or gross. All ventilation calculations are now based on kW net so there might be a requirement to convert gross heat input to net heat input by dividing by 1.1 on older appliances.
Ventilation for open flue appliances is always specified by the manufacturer of the appliance. In the absence of any specific data, reference should be made to BS 5400-2 and the following formula:
Ventilation requirement (cm2) = heat input (kW net) x 5
Adventitious air
A room can normally provide adequate ventilation for open flue appliances up to 7kW (net or gross). This is due to natural ventilation coming into the room from outside through cracks in floorboards, windows and doors, etc. This is known as adventitious air. The move towards zero carbon emissions and the widespread fitting of double glazing and draught-proofing could reduce this source of adventitious air.
Therefore, a gas engineer must still carry out basic commissioning tests to confirm whether or not adventitious air is available. If adventitious air is available, the ventilation requirement (for open flue appliances only) can be reduced by 35cm2.
NOTE
Reducing the ventilation by 35cm2 is equivalent to subtracting 7kW from the appliance heat input. Many open flue appliances of heat input 7kW or under do not need purpose-provided ventilation if adventitious air is available.
Decorative fuel effect (DFE) appliances
DFE appliances are a special case of open flue gas fire. They are usually in the form of a basket or tray housed within a chimney opening. Most DFEs need 100cm2 of ventilation regardless of heat input, unless the manufacturer’s state otherwise.
NOTE
This figure already has adventitious air taken into account and should not be reduced further.
Ventilation of flueless appliances
If the manufacturer does not specify the ventilation requirements for flueless gas appliances, reference can be made to BS 5440-2 for additional guidance. For simplicity, the flueless ventilation chart shown in Table 1 only gives details for cookers and water heaters.
Example 1:
A kitchen houses an open flue boiler that was installed 20 years ago in an un-renovated 1975 house. The data badge on the boiler states that the heat input is 23kW. What would the ventilation requirements be if the ventilation is taken direct from outside air?
As the boiler is 20 years old, it can be assumed that the kW input would be given gross and so will first need converting to net heat input in order to calculate the ventilation requirements:
23 kW gross ÷ 1.1 = 20.91kW net
20.91 – 7 = 13.91
13.91 x 5 = 69.55cm2
Therefore an air vent with a minimum effective free area of 70cm2 is required.
Example 2:
A 15kW flueless cooker i installed in a room with a volume of 8.8m3 without a door to outside – what would be the ventilation requirements?
From Table 1, a 50cm2 air vent is required plus an openable window.
Multi-appliance ventilation
There will be instances when there are multiple gas appliances installed within a room – all requiring varying amounts of ventilation. As this article has already addressed, the type of appliance installed will determine the amount of ventilation required. However, it is not simply a task of adding all the kW input values together and making a calculation.
Some appliances are too different in operation to add together. In other words, the ventilation requirement of open flue appliances cannot be added to that of flueless appliances. There is also a difference in usage between space heaters and other gas appliances. Space heaters are appliances designed to heat internal living spaces. They could be running for several hours, while other appliances are used for shorter periods.
To calculate the ventilation free area, the greatest requirement of any of the following would be used:
A: The total flueless space heater requirements (plasma fires, cabinet heaters, etc).
B: The total open flue space heater requirements (boilers, fires, warm air units) – except where two fires are fitted with the interconnecting wall removed. Fires must not exceed 7kW.
C: The largest individual requirement of any other appliance installed in a room (cookers, tumble dryers,
water heaters).
With regards to DFE gas fires, do not use heat input to calculate the ventilation. Instead, add 100cm2 to the total and do not subtract the adventitious air allowance from the total.
Remember, if there were two DFEs installed within the same room, then the adventitious air allowance would need to be replaced back into the second appliance, therefore making the ventilation requirements:
100cm2 + 100cm2 + 35cm2 = 235cm2
Example 3:
A 55m3 room in an un-renovated 1970s house (interconnecting wall not removed) contains the following gas appliances:
• Flueless cooker: 18kW
• Open flue boiler of heat input 23kW net
• Flueless water heater of heat input 10kW net
• Open flue gas fire of heat input 5kW net
• DFE gas fire
• Plasma flueless fire requiring 100cm2
The first logical step is to place each appliance into one of the three categories and calculate the ventilation for each category.
A: Plasma fire.
Total ventilation requirement: 100cm2
B: Open flue boiler – 23kW
Open flue gas fire – 5kW
Total ventilation requirement:
23 + 5 = 28kW
28 – 7 = 21
21 x 5 = 105cm2
C: Flueless cooker – 18kW.
No ventilation required as room area is sufficient (openable window required).
Flueless water heater – 10kW. No ventilation required as room area is sufficient (openable window required).
The second step is to determine which of the three categories has the largest requirement. In this example, Category B would be the ventilation free area that would need to be specified and then fitted. However, there is still the DFE gas fire to consider. We know that the requirements for the DFE are 100cm2 allowing for adventitious air, and this figure will need to be added to Category B to make the total 205cm2.
NOTE
Adventitious air has already been allowed for, and in calculating ventilation can only be used once. Therefore this will need to be put back onto the DFE gas fire requirement. This means that the final total will be:
Category B (105cm2)
+ DFE (100cm2)
+ adventitious air (35cm2)
= 240cm2 plus an openable window
Every gas appliance requires clean, fresh air for complete combustion and to ensure that open-flued chimney systems operate satisfactorily. It is the gas engineer’s responsibility to ensure that sufficient permanent supply of air is freely available at all times in order to achieve the correct combustion of an appliance
and correct operation of the chimney system.
TO SUM UP
Ventilation is required in every situation where gas appliances or equipment is installed.
Guidance for ventilation up to 70kW (net) can be found in British Standard BS 5440-2: 2009.
Multiple appliances located within the same room will require a different approach to calculating effective ventilation.
