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NORA Oilheat Technicians Manual

6-6 Draft and Venting Figure 6-3: Down draft currents to tumble, causing down draft. (Figure 6-3). Improper or variable draft can cause problems Variations in chimney draft can change the amount of combustion air entering the burner. Low draft will cause the burner fan to push against higher air pressure resulting in less air for combustion and a smoky fire. High draft will cause too much air to rush into the combustion zone resulting in reduced efficiency. Excessive draft will also cause excess air to leak into the unit, further increasing stack temperatures. Table 6-1 (previous page) shows how draft is affected by outdoor temperature, chimney height and chimney temperature. Changes in draft can cause severe problems with older units. Variations in draft have a strong effect on non-flame retention burners. These older burner fans produced very little static pressure and relied on the pressure drop created by the chimney to help draw in their combustion air. Because thermal draft is very weak during a cold start up with these burners, they tend to rumble and smoke until the chimney warms up. Effects of draft on air leakage High draft will draw air into the appliance through leaks. This air will cool the combustion products while increasing their volume, reducing the efficiency of the heating system. Ensuring the draft is proper (See Draft Regulators) will help improve efficiency. Additionally, sealing all air leaks with furnace cement or high temperature silicone wherever possible will reduce the amount of air drawn through the appliance. Air flowing up the chimney is replaced by air being drawn into the building through windows, doors or other gaps in the building envelope. The most common locations for these air leaks are around the burner mounting flange, between the base and the floor, between the base and the heat exchanger, between the sections of a boiler, and around clean-out and inspection doors and plates. Figure 6-4 shows outdoor air infiltration caused by the heating system and chimney. Effects of draft on stand-by losses Whenever the air inside the chimney is warmer than the air outdoors, the chimney will create thermal draft. This is good when the burner is running, but not when the burner shuts off. Most pre-1970s heat exchangers have very large, open passages that offer very little restriction to air flow. It is very easy for the draft from the still warm chimney to draw warmed air from the Chapter 6 Draft and Venting


NORA Oilheat Technicians Manual
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