Why are OWBs a concern?

OWBs are often used for domestic hot water supply, and therefore must operate all year, unlike indoor solid fuel-burning appliances, which are generally only used during the winter months for space heating. This means OWBs pose a risk all year, not just seasonally. In addition, there are many design and operational aspects to OWBs that pose concerns for local air quality:

Combustion Cycle
The on/off cyclical combustion cycle results in large amounts of smoke being released. When the damper reduces air supply to the firebox, the fuel does not have enough oxygen for proper combustion. As a result, the fire smoulders, creating smoke and creosote that condenses on the inside of the smokestack. When the damper opens again, and combustion increases, the accumulated smoke and pollutants are forced out the chimney and into the surrounding atmosphere.

A smouldering fire is an inefficient fire ( smoke is the result of incomplete combustion.) Home heating fuel is literally going up in smoke, meaning increased fuel costs for the home owner in addition to increased smoke in the atmosphere.

Fire boxes
The fireboxes tend to be large, to accommodate large supplies of wood. Operating instructions often suggest the firebox can be completely filled and left to burn for as long as 96 hours. Due to the on/off nature of the combustion cycle, however, completely filling the firebox creates a large smouldering fire that then burns inefficiently for days.

In addition, while OWBs are designed to burn wood, anecdotal evidence in the US suggests that operators also use OWBs to burn waste materials which often contain toxic substances which are released in the smoke particles when burned. These toxic substances can have severe health implications.

Smoke Stack
Another important design feature that increases the impact of OWBs on local air quality is the height of the smoke stack. Generally, OWBs are designed with stacks that reach no more than eight to twelve feet off the ground. As a result, the smoke they generate does not disperse far above ground level, where it remains more concentrated in the immediate vicinity of people, increasing the immediate health impacts.

Lack of control devices
The majority of OWBs on the market do not have any combustion controls, such as catalytic devices or secondary combustion. These types of devices are standard features on new indoor domestic wood-burning appliances that meet ULS or CSA emissions standards. As a result, more smoke -- with its associated toxic substances -- is released without any limitations.

What can be done to reduce
emissions from OWBs?
Unlike indoor solid-fuel appliances, there are currently no federal or provincial emission standards for OWBs, nor are they designed with any emission control features. In addition, the cyclical nature of the combustion process means there is little that can be done to minimize smoke from the operation of these appliances.

Messages to consider in OWB public awareness campaigns

• Harmful health effects of smoke and other toxic substances released from burning
• Don't burn anything other than seasoned firewood in the firebox
• Explain rationale for a bylaw approach