Gas requires quite a bit of air for proper combustion since the air is made up of mostly nitrogen and only contains about 20% oxygen. For adequate combustion to happen, you require combustion air, excess air, and dilution air when using a draft device.
Combustion air is made up of primary and secondary air when speaking about perfect combustion. Since we strive for complete combustion in the real world, excess air is added to ensure combustion.
Combustion air requires 10ft³ per 1000 Btuh of the appliance. Excess air is supplied to ensure complete combustion can happen. Excess air is provided at 5ft³ per 1000 Btuh for a total combustion air supply of 15ft³ per 1000 Btuh of the appliance rating.
Dilution air is 15ft³ per 1000 Btuh of the appliance rating.
Combustion air is required for complete combustion of the gas. Combustion air is made up of primary, secondary and excess air.
Primary air is air mixed with the gas before combustion takes place. Burners are used to prepare the air:gas mixture for combustion using the venturi effect. Gas flows through a small opening in the orifice. As the gas travels through the small opening in the orifice, it draws air in around the opening with it. Gas passing through the orifice which has a reduction in size causing an increase in speed and draws primary air in with it is the venturi effect.
Secondary air is supplied around the burners at the heat exchanger inlet and is required to complete the combustion process. This air is provided to the flames.
Excess air is over and above what is theoretically required for complete combustion. It’s supplied to ensure enough oxygen for complete combustion. The downside of excess air is since it absorbs heat, it carries that heat through the venting, reducing the efficiency of the appliance. Too much excess air can cause the flame to waver and possible flame impingement, or cool too much below the required flame temperature and in either case cause incomplete combustion.
Dilution air is the air that combines with the flue gases. It assists with venting and dilutes the exhaust. The draft hood on a natural draft furnace was used to separate the flue venting from the furnace and allow air to draft into the heat exchanger naturally for combustion.
Dilution air is provided in appliances with a draft hood or some form of draft control. No dilution air is required otherwise.
Flue Gas Spillage
A small amount of flue gas spillage upon appliance startup is normal. Anything more needs addressing right away. The chimney will fill with a substantial amount of cool air during the appliance off cycle that the appliance hot flue gases will have to overcome. Venting should correct within 30 seconds or so of the appliance burners starting. Once the chimney heats up again, venting will happen normally.
Checking for spillage is fairly easy to check. When the appliance is up and running, hold a smoke pen in front of the dilution air draft hood. The smoke from the pen should be pulled in toward the draft hood indicating proper draft through the chimney. If the smoke is pushed the opposite direction, this indicated the exhaust is spilling.