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Understand ventilation

Working example

Example: A 16kW open flue boiler is installed in a room.

16Kw – 7Kw = 9 kW.

Ventilation for 9kW = 9 x 5 = 45 cm2

A vent providing 45 cm² of free air should therefore be located on an outside wall to provide air supply for combustion.

To fully understand the requirement for ventilation you must first examine the combustion process as covered in section 2 of this program

As we have already discovered;

The British Standard that gives guidance and advice on ventilation is: BS 5440 Part 2: 2009. It covers appliances up to 70KW in permanent dwellings. Building Regulation Approved Document J gives information which must be complied with and manufacturers’ instructions must always be referred to when installing or inspecting appliances.

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The GSIUR's 1998 - Gas Safety Installation and Use Regulations

States that a operatives must not install a gas appliance unless it can be used without being a danger to any other person.

Therefore an engineer must ensure that a permanent supply of air is available for the appliance.

How is this achieved?

In order to know if there is sufficient air available a few key points need to be addressed

Ventilation is of paramount importance for combustion (i.e. the burning of carbon, such as gas, coal, wood or oil). It is also vital for the efficient operation of flues, for compartment cooling and the prevention of over-heating. Combustion appliances (i.e. boilers, water heaters, fires, stoves and cookers) have different ventilation criteria dependent upon the type, size and placement of the appliance and it’s important to accurately calculate specific venting requirements for each situation. Inadequate ventilation can cause the build-up of carbon monoxide (CO) gas. This poisonous gas has no smell or taste and can seriously affect the health of the building’s occupants, or even prove fatal.

Adventitious air

Buildings have admit draughts from the outside and this is called as ‘adventitious ventilation’ (advent = outside coming in) which equates to about 35cm2 of free air.

When air for combustion is calculated 5cm2, it is required for each kW of gas used which, in effect, means the first 7kW of of the input rating of a gas appliance can be deducted.

Compartment ventilation

Key point

If an open flued appliance is installed in a compartment, adventitious air is never deducted from the calculation.

A compartment is an enclosure designed to house a gas appliance ex: boiler room.

This will require high- and low-level ventilation unless the manufacturer’s instructions state otherwise.

The purpose of vents located on an appliance compartment is to provide air for complete combustion, enabling the correct operation of the flue and for appliance cooling

In a room

One vent required direct to outside air. (5cm²/kW of rated heat input above 7kW)

In a compartment

Ventilated direct to outside air 10cm²/kW of rated heat input at low level and 5 cm²/kW of rated heat input at high level.

In a compartment

Ventilated via a room or internal space 20cm²/kW of rated heat input at low level and 10cm²/kW of rated heat input at high level. The room is ventilated as in 1 above.

Open flue ventilation in series

Sometimes open flued appliances are located some distance away from an outside wall and, in order to provide air for combustion, it will be necessary to provide ventilation via any other wall between the appliance and the outer wall.

This method of accessing the air for combustion is called installing air vents in series, however the following rules must be applied;

When venting through one wall to an appliance, the internal vent needs to be the same size as the one located on the outer wall.
If venting through more than one room, the internal vents need to be 50% bigger than the external vent.

In the case of compartments, the vents in series rule does not apply.

Vents high and low are sized for the compartment as normal. However, both the internal vents room 2 and 3 are still required to be 50 % bigger than vent 1 to outside.

Calculations for room sealed appliances-

Located in a room

No vent is required.

2. Compartment

Ventilated direct to outside air 5cm²/kW of rated heat input at both high and low level.

3. Compartment

Ventilated via a room or internal space 10cm²/kW of rated heat input at both high and low level.

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