WW-F-2743A
where: 0.244 = specific heat of standard air (70 F (21 C) dry bulb and 50 percent relative humidity)
60 = minutes per hour
13.5 = specific volume of standard air.
3.8.6 Heat transfer rate. The heat transfer rate per square foot (square metre) of heating surface shall be such that at the specified outlet air test temperature rise, the coincident temperature rise at any point on the primary or secondary heating surface is not greater than the allowable heating
element temperature rise specified in UL 727, UL 731, UL 795, ANSI Z21.47, or ANSI Z83.8 as applicable, for the material used.
3.9 Casing. Each unit shall be enclosed in a sheet metal casing to form a passage around the heat exchanger for the stream of air being heated. The casing thickness shall be not less than the thickness in the reference standards of 3.7. The casing on group 1, vertical units shall enclose the burner and associated fuel system components and controls. On the group 2 and group 3 units, the casing shall be braced and reinforced with structural members so that the components of the heat exchanger section will not be directly supported solely by the sheet metal casing. Removable panels shall be provided on all units to provide required access to components requiring cleaning, servicing, or adjustment. The interior of the casing shall be equipped with baffles, shields, heat reflecting insulation or inner-liner to prevent the casing heat losses from exceeding 1.5 percent for group 1 and group 2 and no heat loss for group 3 and all oil fired appliances of the thermal input. The casing temperature shall not exceed the limits necessary for compliance with installation clearances allowed under the UL or A.G.A. listings. Unless otherwise specified (see 6.2), units furnished under this specification shall be listed as designed for installation with the clearances applicable to Table 42.1 of UL 795 or A.G.A. listing.
3.9.1 Air filters. Unless otherwise specified (see 6.2), the manufacturer's standard racks or filter retainers shall be furnished for group 1 units. When specified (see 6.2), air filters for group 2 and group 3 units shall be furnished with integral racks or auxiliary boxes, as specified. Filters may be of the throw-away type or permanent, cleanable type at the option of the supplier unless one type only is specified (see 6.2). Filters shall conform to the requirements of UL 900 for class 1 and class 2. Filters shall be fitted in a manner to preclude air bypassing and to permit ready removal
for cleaning or replacement. The total net face area of all filters on each unit shall be such that the face velocity of the air is a maximum flow rate attainable within normal operating limits of the unit will be not greater than 300 feet per minute (fpm) (1.5 metre per second (m/s)) for standard velocity filters and not greater than 650 fpm (3.3 m/s) for high velocity filters.
3.9.2 Warm air connections. Type I units shall include provisions for connecting discharge and return air ductwork. Type II units shall be equipped with directional discharge louvers. When specified (see 6.2), optional nozzles or deflector hoods shall be furnished.
3.9.3 Automatic vent damper device. When specified (see 6.2), the furnace shall be furnished with an automatic vent damper. The vent damper shall conform to ANSI Z21.66.
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