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Learn About Draft
Most gas hearth appliances use natural draft venting systems. Natural draft is the upward movement of heated gases caused by the difference in their temperature and the surrounding cooler air. As flue gases exit the venting system, they are replaced by combustion air flowing into the appliance. The greater the temperature difference, the faster the rise and the greater the volume of flue gas and combustion air movement.
FACTORS AFFECTING DRAFT
Vent Gas Temperature
The hotter the vent gases, the lighter they are and the more rapidly they exit the venting system.
Heat Loss
As flue gases move through the venting system, they lose heat to the vent or chimney walls. As the gases cool, they become more dense and move more slowly. The ability of the venting system to retain heat is therefore an important design consideration. Single-wall metal and masonry materials allow gases to cool much more quickly than double-wall gas vents. Aluminum heats quickly, and is also used in gas venting.
Vent Size
If the vent is too small, some combustion products will not be able to exit through the vent, and spillage at the draft hood will occur. If the vent is too large, gases cool excessively and condensation takes place in the vent. You should do a spillage test if you suspect leakage.
Vent Height
Increasing vent height increases draft.
Restrictions to Flow
Any restrictions that slow gas flow, such as undersized vents, changes in venting directions, obstructions or blockage, affect draft adversely.
Combustion/Ventilation Air Supply
Proper supply of air is essential to draft and appliance performance. In houses with very tight construction, there may not be adequate air in the house available for combustion and to provide ample air at the draft hood for proper venting action. Air might have to be brought from the outside to supplement the air inside the house. Power ventilators in the home, such as range hoods and exhaust fans, may further increase the need for air. It may be necessary to isolate the space supplying combustion air from these power units, or provide makeup air.
Ambient Temperature
Since draft is a function of the difference in temperature of the gases inside and outside the vent, cooler ambient temperatures increase draft. Excessive cooling of vent walls, however, has a negative effect on draft.
Draft Hood
The draft hood provides dilution air to the venting system in order to reduce stack temperatures and to neutralize and regulate the effect of stack action on the operation of the appliance. It also prevents backdraft from entering the appliance and provides ready escape of flue gases from the appliance in the event of venting blockage or the absence of draft.
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