Guide to Ventilation

Ventilation for Combustion Appliances

Providing the correct ventilation for combustion appliances is essential to ensure more efficient operation and safe combustion. A combustion appliance is usually a gas, oil or solid fuel burning appliance such as a boiler, fire, stove, warm air heater or cooker (for cooking or heating).

These appliances not only burn fuel but use oxygen in air combustion. If sufficient fresh air is not provided then the unit will use the available oxygen from the air in the room. If the supply to the appliance is inadequate, the flue will fail to clear all of the combustion products and some will spill into the room itself. Ventilating rooms containing combustion appliances is essential and often additional ventilation is required.

Free Area

Free area is achieved by measuring the smallest apertures of a vent or grille to calculate the air flow. For a louvre, the areas should be measured at right angles to the blades or vanes. For an air vent assembly, the free cross-sectional area of the most restrictive component in an assembly should be used to calculate the free area.

For more information refer to the Free Area calculator.

Open Flued & Flueless Appliances

Ventilation for flueless appliances must be direct to the outside of the building. However, ventilation for open flued appliances (which draw their combustion air from the room or space into which it is installed) should be direct to the outside of the building. They may be vented from another room which in turn is vented to the outside in certain circumstances.

A vent supplying air to an open-flue appliance shall not draw from a room or internal space containing a bath or shower.

Any installation should be installed so that it receives sufficient air for the proper combustion of the fuel and is capable of normal operation without the combustion products becoming a health hazard.

Black Hole Ventilation

‘Black Hole’ ventilation products provide the answer to the dangerous practice of ventilators being blocked off by occupiers who are trying to stop draughts. This is an important risk to safety. While this works to reduce draughts, the ventilator may not pass the required amount of air when the appliance is operating. It is important that a spillage test is carried out as specified by the British Standard and Corgi good practice.

‘Black Hole’ Benefits

• Stops wind blasting in: The air flow within the duct is slowed down by the change of direction and by the vortices formed, reducing draughts to a minimum.
• Stops light showing through: The occupier cannot see through the ventilator.
• Stops light shining out: Night flying insects like moths, crane flies and mosquitoes are not attracted in.
• Absorbs outside noise: Reduces the penetration of traffic noise. The internal construction works as a
  sound baffle.
• Practical: Allows air to be pulled through by the appliance when it is operating.
• Safe: With draught and light annoyance problems eliminated, the occupier will not be tempted to block off the vent.

The 'Black Hole' Magic Explained

As outside wind pressure increases, a normal ventilator lets a lot of air through, causing draughts.

Inside the Stadium ‘Black Hole’ ventilator, vortices are formed behind the internal blades as the incoming air is forced to change direction. The gap that the air is able to pass though narrows. As wind pressure increases, the vortices grow. The total air flow is reduced and it finally tumbles and slows to a trickle, which cuts down annoying draughts.

However, when a heating appliance is operating, air is drawn through the vent and actually speeds up within the constricted air flow between the vortices. This maintains the correct amount of air in the room or compartment to allow the fuel to burn efficiently and flue gases to escape.

When the appliance is not working, incoming air is reduced to a minimum, even when wind levels are high. The actual physical difference is that of the air blown or drawn. The energy of the blown air (wind) is quickly dissipated by the restriction of the internal shape and the vortices, while drawn air will flow smoothly round the vortices.

Source: Flambeau.com