The limit switch operates when a high temperature condition, caused by inadequate supply air flow, occurs shutting down the ignition control and closing the main gas valve.
TABLE 16 - LIMIT CONTROL SETTING
Units | Capacity, MBH | Limit Control | |
(Tons) | Input | Output | Opens, °F |
3 | 50 | 40 | 165 |
4 | 75 | 60 | 165 |
3 | 100 | 79 | 165 |
4 | 125 | 99 | 165 |
5 | 100 | 79 | 165 |
5 | 125 | 99 | 165 |
6 | 100 | 79 | 165 |
6 | 125 | 99 | 165 |
2.Centrifugal Switch
3.Redundant Gas Valve. This valve is an integral part of the main gas valve and is located up stream of the main gas valve. Should the main gas valve fail in the open position the redundant valve serves as a back up and shuts off the flow of gas.
4.Flame Sensor Rod. This sensor rod is located on the far side of the carryover tube. If the ignition control does not receive a signal from the flame sensor indicating that the pilot flame has ignited properly, the main gas valve will not open. If the flame sensor fails to detect the pilot flame during
operation of the main burners, a signal is sent to the ignition control to close the main gas valve.
5.Rollout Switch. This switch is located in the burner vestibule. In the event of a sustained main burner flame rollout, it shuts off the ignition control and closes the main gas valve.
6.Auxiliary Limit Switch. This control is located inside the heat exchanger compartment and is set to open at 180°F. It is a manual reset switch. If the auxiliary limit switch trips, then the primary limit switch has not functioned properly. Replace the primary limit switch.
HEAT ANTICIPATOR SETPOINTS
The anticipator must be set correctly. Too high of a setting will result in longer heat cycles and a greater temperature swing in the conditioned space.
Gas Valve | Anticipator Setpoint |
Honeywell VR8204M | 0.60 amp |
0.54 amp |
Reducing the value below the correct setpoint will give shorter “ON” cycles and may result in the lowering of the temperature within the conditioned space.
PRE-START CHECK LIST
Complete the following checks before starting the unit.
1.Check the type of gas being supplied. Be sure that it is the same as listed on the unit nameplate.
2.Make sure that the vent and combustion air hoods have been properly installed.
START-UP
OPERATING INSTRUCTIONS
CAUTION: This furnace is equipped with an intermittent pilot and automatic
TO LIGHT PILOT AND MAIN BURNERS:
1.Turn “off” electric power to unit.
2.Turn room thermostat to lowest setting.
3.Turn gas valve knob to “on” position.
4.Turn “on” electric power to unit.
5.Set room thermostat to desired temperature.
(If thermostat “set” temperature is above room temperature, pilot burner ignition will occur and, after an interval to prove pilot flame, main burners will ignite).
TO SHUT DOWN:
1.Turn “off” electric power to unit.
2.Depress knob of gas valve while turning to “off” position.
POST-START CHECK LIST (GAS)
After the entire control circuit has been energized and the heating section is operating, make the following checks:
1.Check for gas leaks in the unit piping as well as the supply piping.
2.Check for correct manifold gas pressures. See “Checking Gas Input”.
3.Check the supply gas pressure. It must be within the limits shown on rating nameplate. Supply pressure should be checked with all gas appliances in the building at full fire. At no time should the standby gas line pressure exceed 13", nor the operating pressure drop below 4.5" for natural gas units. If gas pressure is outside these limits, contact the local gas utility for corrective action.
MANIFOLD GAS PRESSURE ADJUSTMENT
Small adjustments to the
Adjust as follows:
PILOT ADJ. | |
(UNDER | CONTROL |
SCREW) | HIGH FIRE ADJ. |
| (UNDER SCREW) |
Honeywell
VR8204M
PILOT ADJ. (UNDER SCREW)
| |
PRESSURE REGULATOR | 36E36 |
| |
MANIFOLD ADJUST. SCREW | |
|
FIG. 11 - GAS VALVES
1.Remove the cap on the regulator. (High Fire adjusting screw on valve).
2.To decrease the gas pressure, turn the adjusting screw counterclockwise.
3.To increase the gas pressure, turn the adjusting screw clockwise.
NOTE: Check the unit data plate for the proper manifold pres- sure and orfice size of the furnace.
16 | Unitary Products Group |
