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NORA Oilheat Technicians Manual

Chapter 6—Draft and Venting 6-19 also a good idea when installing a powerventer, that you also install a fresh air kit to bring combustion air to the burner from outdoors. Many power-venters integrate a fresh combustion air intake into their system. If they do not, you should ensure that the pipes are spaced so that they do not interfere with each other—at least 12 inches apart. Fresh air should be brought in from the same wall as the power-venter exhaust to equalize air pressure within the vent and intake system. Direct venting Direct-vent systems use the power of the burner fan to push the products of combustion out of the building. Direct-vent provides sidewall venting without the use of a power-venter, extra motors, fans, or wiring. Direct-vent systems feature sealed combustion, utilizing clean outside air for combustion. The direct vent system normally uses a stainless steel combination vent hood. Figure 6-18 shows an example of a sealed combustion system. Unlike chimney venting and powerventing, with direct-vent the air pressure inside the boiler or furnace and flue pipe is greater than the pressure in the building. If there is a leak in the heat exchanger or the flue pipe, products of combustion will leak into the building. Direct-vent systems are sold as a complete package. You should only use components that are supplied with the manufacturer’s direct-vent system. Mixing and matching, or do-it-yourself engineering may void the manufacturer’s warranty and may create a hazardous condition. Other drawbacks to a direct-vent system are: • Surface discoloration of the building may occur due to improper burner and control adjustment. Figure 6-18: Direct side-wall venting and outside combustion air • The residential units can only push the exhaust gases and pull combustion air from about 20 feet maximum. This means that you must locate the boiler, furnace, or water heater as close to an outside wall as possible. Things to consider when installing direct-vent systems are: • Seal all joints on the venting system with a high temperature sealant. • Combustion efficiency tests should be taken at the port provided on the unit by the manufacturer. Do not puncture the stainless steel vent tubing. Adjust the burner combustion with a ‘window of tolerance.’ Set the air to produce a trace of smoke. Take the CO2 reading at a trace of smoke and reduce the CO2 by 1.5% to 2%. For instance, if the CO2 reading with a trace is 13%, reduce it to 11% to 11.5% CO2. This will compensate for variations in fuel and outdoor temperatures, and other variables over the year. See Chapter 7 on combustion for more details. Chapter 6 Draft and Venting Sealed Combustion Furnace Sealed Combustion Boiler


NORA Oilheat Technicians Manual
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