Combustion Air Systems

Combustion Air Sources

Combustion air may be available for the woodburning appliance with no intervention or action may have to be taken to ensure adequate combustion air. We look at the different means by which combustion air is available.

Passive Methods

Passive combustion air delivery refers to non-mechanical means (no fan). There are two basic means of passive delivery of combustion air:

1. All fireplaces rely to some extent on combustion air being drawn from the room and being replaced by infiltration through gaps in the building envelope. This is the sole source of combustion air if there is no outside air kit or powered air supply.
2. Most factory-built fireplaces are designed with the capability of utilizing an outside air kit to provide air from outside directly to the firebox. The kits consist of ducting, connectors, and termination materials. The duct is connected to a removable plate, or knockout, on the firebox and extends to an approved source of combustion air (see above). There are also aftermarket outside air kits and methods for passive outside air.

Research indicates that passive air supply methods may not always be effective since they depend on air pressure conditions inside and outside the house. In some situations, such as high wind, pressure may be lower at the weatherhood outside the house. Flow reversal of air and possibly combustion by-products and heat into the air inlet duct may occur in such conditions.

Nonetheless, outside air kits may be required in some jurisdictions. Some recommendations that apply to the installation of outside air for combustion assemblies are:

1. The entry point for the air should be located where it will be unlikely to be blocked by snow, leaves and other debris. (Manufacturers' instructions typically specify at least 12 inches above the ground.)
2. The combustion air should never be taken from any area such as a garage, where combustible liquids or gases may be stored, used or could leak from tanks or other containers.
3. The entry point for the air should never be located in an attic or located at a position above (higher than) the flue gas outlet.
4. The length of the connecting air duct should be as as short and as straight as possible to avoid restriction to the flow of air.
5. Duct materials should be in conformance with the manufacturer's instructions. The use of combustible material could be hazardous.
6. Follow the manufacturer's requirements with respect to height and location of the air inlet.
7. Always follow manufacturer's instructions when installing outside air components.

Mechanical Methods of Combustion Air Delivery

In some situations, it is necessary to use a powered system to bring needed air (make-up air) into the house. Such systems must be sized carefully and installed properly.

Powered Make-Up Air System

A powered make-up air system consists of a properly sized fan and ductwork that forces a calculated amount of air into the house from outside. The size of the fan and the ductwork are calculated after a house pressure test reveals the amount of depressurization that occurs in the house when some or all of the power exhaust devices such as kitchen and bathroom exhausts and clothes dryers operate. With a known depressurization limit for the woodburning appliance, the fan and duct size can be calculated. A manual damper on the system allows for further fine tuning of the system. The goal is to provide, but not to exceed, the needed make-up air, or in other words, to provide a balanced make-up air system. Such systems should be designed and installed by qualified professionals.