Thomas & Betts RZ-NA 441-OMS Temperature Limit Safety Controls contd, Air Pressure Switches

Figure 11 - ECO

Limit Control, P/N 82414

Setting 305oF

8.Temperature Limit Safety Controls (cont'd)

•Emergency Cut Off Limit Control

Function: The emergency cut off is a fus- ible link high temperature limit which pro- vides one-time redundant protection against overheating. If the temperature sensitive limit control malfunctions, the electrically activated emergency cutoff will shutdown the system.

Service: If this limit activates, the manual limit control has failed and must be re- placed. The cause for activating the emer-

gency cut off limit control must be found and corrected before re- starting the system.

9. Air Pressure Switches

Location: Control Compartment Electrical Box (See Figure 7.) Depending on the options selected, there are two or four switches.

Figure 12 - Air

Pressure Switch

Attach a slope gauge (0 to 1.0" scale) to the tubing connections in the control compartment. The two connections are located below the elec- trical control box. Remove the caps on the 1/8" NPT test connections and attach the slope gauge. (The recommended method for attaching the slope gauge is to use field-supplied 1/8" female NPT x 1/4" O.D. barbed hose connections.)

A)If the system includes an optional discharge damper, before measur- ing burner differential air pressure, check to be sure that the damper is fully open.

B)With the blower operating, the pressure differential on the slope gauge should read between -.5" and -.7" w.c. If the slope gauge reading is within those limits, no adjustments are necessary.

If the slope gauge reading is not within the setpoint limits of the air flow switches (.2" to .9" w.c.), and the system is operating, replace the air pressure switch(es).

If the slope gauge reading is not between -.5" and -.7" w.c., but within the setpoint limits, clean the sensing tubes (Follow the in- structions in Maintenance Section, Paragraph 4).

C)When air pressure is within the proper range, turn the disconnect switch OFF. Disconnect the manometer and the slope gauge. Re- place the caps removed to connect the slope gauge.

Service: If the pressure check determines that an air flow switch is not functioning properly, the switch cannot be serviced and must be re- placed with an identical replacement. Low air pressure switch is P/N 86986; high air pressure switch is P/N 86987; bypass damper switches, P/N 87249 (normally closed, set to open at .5" w.c.) or P/N 87250 (normally open, set to close at .65" w.c.).

• Low Air Flow Switch

Function: The low air flow switch is a velocity pressure switch that monitors air flow across the burner. Until the air flow attains adequate volume for combustion, the switch remains open. When the switch recognizes adequate air volume, it closes, permitting both the pilot and burner to operate. Low pressure switch is normally open; it closes on pressure rise at .2" w.c. Do not alter or adjust setting.

• High Air Flow Switch

Function: The high air flow switch is a velocity pressure switch that monitors air flow across the burner. If the high air flow switch senses air velocity above the prescribed limit, it will shutdown gas flow to the burner. High pressure switch is normally closed; it opens when pres- sure rises above .9" w.c. Do not alter or adjust setting.

•Bypass Damper Air Flow Switches (systems with Air Control Options AR19, AR20, AR22, AR23, AR32, AR33, AR34, AR36, or AR37)

Function: With a bypass damper, the volume of outside air supplied

to the building is adjusted by a manually set potentiometer (Option AR19 and AR22) or automatically by a pressure null switch (Option AR20 or AR23), a photohelic pressure switch (Option AR36 or AR37), or a field-supplied computer signal (Option AR33 or AR34). With Options AR19, AR20, AR33, and AR36 the supply air is varied by adjusting the position of a damper at the blower discharge. With Op- tions AR22, AR23, AR34, and AR37, a return air damper is adjusted to vary the volume of return air. The unit is arranged so that a fixed amount (20%) of the rated volume flows over the burner at a constant velocity. The remainder (80%) of the rated air volume flows either through a balancing bypass damper or a combination of bypass and return air dampers. As the supply air volume is varied by the return air or dis- charge damper, the balancing damper is adjusted to maintain the re- quired air velocity over the burner. Adjustment of the bypass damper is controlled by the bypass damper pressure switches. One pressure switch is normally closed with a setting of .5" w.c.; the other is nor- mally open with a setting of .65" w.c. balancing bypass damper.

Sensing Pressure Check: (requires a slope gauge, several feet of

1/4" O.D. tubing and two 1/4" O.D. barbed tees.)

Page 6

10. Ignition System

Location: Ignition Controller Module in the Control Compartment Electrical Box (See Figures 7 and 13.); Ignitor and Flame Sensor on the Burner (See Figure 14.)

Figure 13 - Ignition Control	Figure 14 - Ignitor, P/N
Module in the Electrical	121865, and Flame
Compartment, P/N 157953	Sensor, P/N 134706, on
	the Burner
	Sensor

Ignitor

Hot Surface Ignition System with Prepurge Time Delay and Flame Sensor with 100% Lockout

Function: The ignition system including the controller, the hot surface ignitor, and the flame sensor function to ignite and prove the pilot flame. When there is a call for heat, the modular ignition controller is energized. When the controller reads 1.4 amps going to the hot surface ignitor, it opens the pilot valve for a 15-second trial for ignition. After the pilot flame rod senses pilot flame, the main gas valve is energized. If the pilot flame rod does not sense a pilot flame, the controller shuts down the pilot valve for a 10-second interpurge and then opens it again for a second ignition trial. If pilot flame is not proven on the second trial, the ignition controller locks out and must be manually reset by an interruption of the main circuit (disconnect switch).

Service: The modular ignition controller does an internal self-check each time that it is energized and will lockout if not found to be func-