FC2500 Furnace Installation and Operation Manual
Example of surface calculation according to the number of outlets used in the plenum:
Total surface of plenum outlets used (in 2 )
Diameter of plenum outlet (in)
Surface of plenum outlet (in 2 )
Number of outlets on the plenum
6 170 The acceptable total area of secondary outlets would therefore be: 28 6
Surface of secondary outlet (in 2 )
Total surface of secondary outlets used (in 2 )
Diameter of secondary outlet (in)
Number of secondary outlets
4 5 6
12.5 19.5
9 1 1
112.5
19.5
28
28
The total area of the secondary outlets of this system would be acceptable because it is equal to or slightly less than the total area of the plenum outlets.
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ACORDING TO THE NATIONAL BUILDING CODE, THE BTU/H PROVIDED BY AT REGISTER MUST NOT EXCEED 10,250 BTU/H (3kW) AND THE HOT AIR TEMPERATURE AT THE OUTLET OF A SECONDARY DUCT SHOULD NOT EXCEED 158 °F (70 °C). Once your hot air distribution system is installed, make sure to measure the static pressure in the start-off plenum. Adjust the opening at the end of each of distribution duct with a shutter until you get a minimum static pressure 0.2” W.C. (50 Pa) at the plenum. The recommended air velocity exiting the register should be between 500 and 750 feet per minute. ( Static Pressure Measurement: Drill a hole on the side of the plenum, directly above the furnace, ensuring it is centered both horizontally and vertically. Insert a measurement tube into the hole and connect it to a manometer to read the static pressure inside the plenum.) 5.5 Air return system and filter The installation of an optional air return system connected to the blower housing will improve the air circulation throughout the house (see Appendix 3: Optional Fresh air return adapter (SFA0252) ). The use of the air filter option is recommended. The filter option will significantly reduce dust circulation within the hot air duct and will prevent obstruction of the blower (see Appendix 2: Optional air filters (SFA0250, SFA0251) ). NOTE: TO ENSURE ADEQUATE STATIC PRESSURE, THE SYSTEM SHOULD BE BUILT IN A WAY THAT THE VOLUME OF COLD AIR RETURN IS AT LEAST EQUAL OR SLIGHTLY HIGHER THAN THE VOLUME OF THE HOT AIR DISTRIBUTION.
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