WW-F-2743A
concealed setpoint dials and plug-in gage ports. The housing shall also be provided with covers with allen head screws, aspirator type wall box with flush plate and locking screws, built-in concealed thermometers, exposed adjustment covers with visible thermometers. The control unit of the thermostat shall consist of a temperature sensing element, control switch, and anticipating heater. The control switch shall be a hermetically-sealed switch. The thermostat shall have provisions for calibrating the unit to the accuracy specified in NEMA DC 3. The design shall preclude calibration adjustment with ordinary tools, such as screwdriver or pliers. When mercury switches are used in the control unit, a leveling surface, which is an integral part of the thermostat, shall be provided for use in leveling the thermostat. Unless otherwise specified (see 6.2), a system selector switch having "heat" and "off" positions, and a fan selector switch having "auto" and
"on" positions shall be provided integral to or mounted on a subbase of the thermostat.
3.14.2 Blower or fan control. The blower or fan control shall prevent operation of the blower or fan after the burner has fired until the discharge air at the bonnet reaches a predetermined temperature. The fan "on" setting shall be adjustable within a range of not less than
90 F to 140 F (32 C to 60 C). The differential setting may be fixed at a point between
24 F and 36 F (- 4 C and 2 C) or shall be adjustable within the range of not less than 20 F to
50 F (7 C to 10 C) at the option of the supplier. The blower or fan control shall include a suitable manual switch or setting to permit continuous operation of the fan for summer ventilation purposes. When a temperature sensing blower or fan control is not provided, a device which will provide for blower or fan operation to prevent abnormal air temperature shall be furnished. On group 2 units with high-low-off controls, the blower or fan shall reduce its speed to maintain the normal outlet temperature of the furnace within the limits of 3.8.4.
3.14.3 Limit control. The limit control shall conform to UL 353. The limit control shall be designed and installed to shut down the burner before the bonnet air temperature reaches no greater than 200 F (93 C) or at the temperature specified by the applicable standard. The differential shall be fixed at not less than 10 F (- 12 C) and not more than 24 F (- 4 C). Controls provided with adjustable limits up to 250 F (121 C) shall be equipped with stops factory-set to shut down the burner when the bonnet air temperature reaches no greater than
200 F (93 C) and will require a manual reset.
3.14.4 Primary safety controls. Primary safety controls (combustion safeguards) shall be furnished on each oil-, gas-, and combination oil-gas-fired unit. The primary control shall monitor the flame and, in the event of ignition failure on startup or unintentional flame extinguishment during a firing cycle, shall cause safety shutdown of burner.
3.14.4.1 Group 1 and group 2 units. Primary controls for group 1 and group 2 oil-fired units shall be of the cadmium sulfide type, which responds to the luminous energy in the flame. Controls shall lock out if the flame is not established in accordance with UL 296 on ignition and shall require manual reset. The automatic ignition and safety shutoff system for group 1 and group 2 gas-fired units shall be of the intermittent or interrupted direct spark, or hot-surface
ignition types which shall function to cause complete shutdown of the gas in case of pilot or main
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