Page 115

NORA Oilheat Technicians Manual

How draft controls work Static pressure of the cool air exerts pressure on the outside of the furnace or boiler, the breeching, and flue. The pressure difference between the room air and heated gas (air) causes products of combustion to flow (draft) through the unit and rise through the breeching and chimney. Room temperature air enters through the barometric draft control in the precise amount needed to overcome the excess drafts caused by temperature variations, wind fluctuations and barometric pressure changes. Combustion of fuel is complete and the process is stabilized. The velocity of combustion gases through the heat exchanger is slowed so more heat is extracted. Chapter 6—Draft and Venting 6-9 Draft over-the-fire is the most important draft measurement and should always be checked first tion caused by the heat exchanger. This restriction, or the lack of it, is a clue to the design and condition of the heat exchanger. A clean heat exchanger of good design will normally cause the breech draft to be in the range of -.03" to -.07" when the over-thefire draft is from -.01" to -.02". The difference between the breech draft and the over-the fire draft is often called the ‘draft drop’ or ‘pressure drop’ across the heat exchanger. If the breech draft is -.07" and the over-the-fire draft is -.02", the draft drop is .05". Excessive draft drop may indicate heavy soot and scale deposits in the unit. It is important to understand that over-the-fire draft is indirectly controlled. It is a function, not only of the draft created by the chimney, but also of restrictions in the heat exchanger. A burner operating at more than zero smoke will cause soot deposits to build in the heat exchanger. These deposits increase the draft drop, lowering over-the-fire draft, reducing combustion air, and creating more smoke. The increased smoke causes even more soot to build up, increasing draft drop even further. The result is a quickly plugged heat exchanger. Sometimes there is little difference between the breech draft and the over-thefire draft, usually with older units originally built for coal. If a boiler measures a low efficiency with a high stack temperature (600°F or more), it may be possible to install baffles in the heat exchanger passages. However, as with most older equipment, the best alternative is to recommend a new boiler or furnace of higher efficiency. Chimney sizing Proper chimney sizing is important for the safe and efficient operation of all heating appliances. As the requirement can vary depending upon the size and design of Chapter 6 Draft and Venting


NORA Oilheat Technicians Manual
To see the actual publication please follow the link above