The combustion air vent to outside, and the draughts problem:

Although most warm air boilers installed since about the year 2000 are “room sealed” and have their combustion air supply incorporated into the flue duct, there are still large numbers of warm air boilers fitted in last century still in regular use today. (They simply don’t seem to degrade with age in the same way as conventional water-filled central heating boilers and can run for decades with no breakdowns.) Most of these 20th century warm air heaters have what is called an “open flue”, i.e. a vertical chimney to discharge the burned flue gasses to outside through the roof of the house.

In order for the an open flue to carry away the flue gas, an equal amount of fresh air from outside needs to enter the room or compartment containing the boiler, to replace the flue gas passing up the flue or chimney. So an air vent to outside letting this fresh air in (along with oxygen for combustion), is essential. 

The reason for the fresh air vent is that without it, the flue gasses may not all go up the flue and some flue gas may spill out into the room. Flue gas contains almost no oxygen as the oxygen will have all been used up in the combustion process. This leads to a chain of events as follows: Flue gas escapes into room >>> reduction in oxygen in the room atmosphere >>> too little oxygen for complete combustion >>> carbon monoxide (the dangerous one) in the flue gasses >>> carbon monoxide spilling into the room >>> risk of injury or death of the occupants. So hopefully you can see how the air vent is important. 

As the air vent is so important for safety, finding and checking the effectiveness of air supply is legal requirement for anyone carrying out work on a gas appliance. 

Open vented boilers often have a fresh air vent from outside which is a simple hole through the wall or a window with a grill or grid on each side. More sophisticated installations might have a vertical tube like a flue, running vertically down from the loft space and supplying fresh air into the boiler cupboard. In other cases I’ve seen air supply ducts running horizontally from outside into the boiler cupboard buried under the floor. These hidden ducts are installed to prevent the cold draught associated with the simple ‘hole through the wall’ method of providing a supply of fresh combustion air to the boiler.

A problem with the ‘hole through the wall’ combustion air vent  is that users/builders/kitchen fitters/window installers often don’t realise the vent is part of the boiler installation and block it up, as the incoming fresh air creates a draught. A gas technician will need to unblock any air vent found covered over, and explain to risks to the user. If the air vent to outside is missing, a new air vent will have to be installed to ensure safe operation of the warm air boiler.

This problem of users blocking air vents is sometimes worked around by installers fitting the fresh air vents in quite imaginative locations - in lofts, behind kitchen units, using ducts under the floor from outside and ending inside the boiler cupboard, etc. So even if you are not aware of a fresh air vent there may still be a hidden vent keeping you safe. 

The only definitive way to find out if your warm air unit needs a fresh air vent from outside is to read the manual, but here is a list of the more common warm air boiler models that need one:

Johnson and Starley:

J15-22, J25-32 and all “J” series

JB40-50

JWD38-50

All “HISPEC” models

Brink:

All models

Balmforth:

All models

McClary:

All models

And here are some of the more common warm air heaters which are room-sealed and do NOT need a fresh air/combustion air vent directly to outside:

Johnson and Starley:

All “Janstar” models

All “Economair” models

All “Warmcair” models

All “Aquair” models

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