Every gas burning appliance requires a supply of clean fresh air to burn the gas, to clear the products of combustion and in many cases to cool the components.
Different appliance take their air in different ways but there are generally 3 categories an appliance will fit into. Those that are flue less such as hobs which take the air they need from the room they're in and also release the products of combustion into that room.
Open flued such as gas fires and older boilers, they also take the air they need from the room they're in but send their products of combustion up their flue or chimney to be dispersed into the atmosphere.
And lastly room sealed appliances which take the air they need for burning gas from outside and disperse their exhaust products outside, most modern boilers fall into the roomsealed category.
Air vent should be non-closable and should not have additional gauze or screens
If the room doesn't have an outside wall it is possible to have to air vents so the air can pass through to the internal room, however the vents on the internal walls must be 50% larger.
Air vents passing through cavity walls should be ducted
In addition to the above flue category all appliances are graded on the amount of gas they burn, this is referred to their KW rating
Open flued appliances bigger 7kw need ventilation that 5 cm2 squared for every KW, for example a 17kw boiler would need 50 cm2 vent (17-7x5=50 cm2)
If you have more than 1 open flued appliance in a room you only minus the 7kw once, it's not per appliance. For example if you have an older 25kw open flued boiler and a 8kw fire, that means you have a combined KW of 33KW and you'll need 130 cm2 (25+8-7x5=130)
When an open flued appliance is in a cupboard or compartments it will need high and low ventilation. Both the high and low ventilation should be taken from the same source, if it is direct to outside in will need 5cm per kw for the high level vent and 10cm per KW for the low level vent.
if the vent is into a room then it will need 10 cm2 per KW high level vent and 20 cm2 per KW low level vent
Flueless appliances almost always be situated in a room that has window or similar, whether they need additional ventilation depends on the type of appliance, the size of the appliance and the size of the room.
Room Sealed appliances usually don't need additional ventilation unless they are located in a cupboard or compartment then they may need ventilation for cooling, check with the manufacturer for requirements.
Not all appliances fall in to the 3 categories of flues mentioned above, a DFE (decorative fuel effect fires) always needs 100 cm2
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