Environmental Engineering Reference
In-Depth Information
Zambonis used in a number of indoor environments. They also include
tobacco smoking and the increasingly popular activities of burning candles
and incense. In developing countries they include indoor cooking fires which
are not vented, or only poorly vented, to the outdoor environment.
I. Vented combustion appliances
Combustion of fuels such as wood and coal produces large quantities of
smoke that humans in many advanced societies have for centuries found to
be unacceptable in their domiciles. Chimneys and flues were developed and
used to carry smoke away from cooking and heating fires. They exhaust by-
products of fires while providing space heat with varying degrees of success
(depending on how well they were designed and the effectiveness of the
natural draft that carries exhaust up and outwards). Energy-inefficient fire-
places were later replaced by well-vented stoves, which provided heat in local
areas, and furnaces, whose energy could be used to heat an entire building.
Vented appliances are designed to provide a mechanism by which com-
bustion by-products are carried through fluepipes or chimneys by natural
or mechanical means. The effectiveness of these appliances varies, as would
be expected. All vented combustion appliances will, from time to time, cause
some degree of direct indoor contamination. With modern gas, propane, and
oil-fired furnaces, indoor contamination is relatively limited except when a
system malfunction occurs. Indoor air contamination from wood- or coal-
burning appliances, such as fireplaces, stoves, and furnaces, is more common
and varies with appliance, building design, and environmental factors.
For the past half century or more, residences in the colder regions of
North America have been heated by natural gas, propane, or oil in well-
designed furnaces with properly designed and installed flue/chimney sys-
tems. Such furnaces produce little smoke but can produce significant CO
emissions, which pose a potentially serious public health risk if they are not
properly vented. Venting of flue gases is achieved by the use of natural or
mechanical draft. In natural draft furnace systems, warm combustion gases
rise by convection from the fire box (combustion chamber) and are carried
upward by building air which flows into a draft hood on the side of the
furnace where it joins and mixes with flue gases. The system is an open one.
Should there be insufficient draft, flue gases will spill into the building
environment surrounding the furnace and quickly be transported through-
out the building. Such draft failures are not uncommon; in most cases they
result in relatively limited flue-gas spillage and are of minor concern.
Mechanical draft systems which have a fan to exhaust flue gases have
been used for many years, particularly in oil-fired furnaces. These systems
are becoming the norm in North America with the development of medium-
to high-efficiency (80 to 90%) gas and propane-fired furnaces. Because flue
gases contain little heat to carry them upward, high-efficiency furnace sys-
tems must be mechanically vented. Such venting is accomplished without
chimneys. Since mechanical draft systems require no draft hood, the prob-
