Energy Tech Laboratories DHG240, DHG180 Heating System, Gas Valve Piping, Safety Controls

035-17233-000-C-0702

3.A Low Pressure Switch/Loss Of Charge to protect against loss of refrigerant charge. (Opens at 7 psig + 3 and resets at 22 psig + 5)

If either one of the above safety controls opens, that individ- ual refrigerant system will be locked out. The other refrigerant system will continue in operation unless it too is effected by the same fault. The lock out of either system can be reset by opening the 24V circuit either at the room thermostat or at the unit disconnect.

HEATING SYSTEM

The following sequence describes the operation of the gas heat section.

CONTINUOUS BLOWER

With the room thermostat switch set to ON, the supply air blower will operate continuously. The normally closed contact K5-1 provides 24 volt power to the M3 contactor. The M3-1, 2 & 3 power contacts close and the blower motor operates.

INTERMITTENT BLOWER

With the room thermostat system switch set to the AUTO or HEAT position and the fan switch set to AUTO, the supply air blower will operate after the room thermostat calls for heat and the time delay relay closes.

The TH1 closes, the heat relay RW1 is energized. The RW1- 1 power contact closes energizing the line voltage draft motor. The RW1-2 contact is also closed. As the speed of the draft motor reaches approximately 2500 RPM, the centrifugal switch contact located on the end of the draft motor shaft closes to power the first stage ignition module IC1.

REDUNDANT		MAIN VALVE
VALVE
GAS MAIN
GAS		TO MAIN
VALVE	TO PILOT BURNER	BURNER

FIGURE 14 : GAS VALVE PIPING

Ignition module IC1 will immediately start the first stage igni- tor sparking and will open the redundant valve located inside the first stage main gas valve GV1 to allow a flow of gas to only the first stage carryover tube. See Figure 14. Only after the pilot flame has been ignited and the presence of pilot flame detected at the IC1 by a signal sent back through the flame sensor is sparking terminated and the first stage main gas valve opened.

Gas flows into each of the main burners and is ignited from the carryover tube flame.

If IC1 fails to detect a pilot flame, it will continue to try for a maximum of 85 seconds to ignite the pilot tube. If the pilot flame is not detected, then IC1 will lock out first stage furnace operation until 24V power is removed from the module either at the unit or by resetting the room thermostat.

At the same time power was supplied to the RW1, a parallel circuit activates ETD which closes the ETD contact after approximately 35 seconds and energizes K5 which closes K5-2 and starts the blower by energizing M3.

When TH2 closes, heat relay RW2 is energized. The RW2-1 contact is closed energizing the second stage ignition module IC2. IC2 will immediately start the second stage ignitor spark- ing and will open the redundant valve located inside the sec- ond stage main gas valve GV2 to allow a flow of gas to the second stage carryover tube. See Figure 11. Only after the pilot flame has been ignited and the presence of pilot flame detected at IC2 by a signal sent back through the flame sen- sor is sparking terminated and the main gas valve opened.

Gas flows into each of the second stage main burners and is ignited from the carryover tube flame.

If IC2 fails to detect a pilot flame, it will continue to try for a maximum of 85 seconds to ignite the pilot tube. If the pilot flame is not detected, then IC2 will lock out first stage furnace operation until 24V power is removed from the module either at the unit or by resetting the room thermostat. Note that the second stage furnace can operate even if first stage has locked out.

When the heating cycle is complete, TH2 opens de-energiz- ing the RW2 then TH1 opens de-energizing RW1 and ETD, thus closing all gas valves. The blower motor will continue to run (approximately 45 seconds after the furnace is shut down) until ETD opens, de-energizing the K5 relay and M3 contactor.

SAFETY CONTROLS

The control circuit includes the following safety controls:

1.Limit Control (LS). This control is located inside the heat exchanger compartment and is set to open at the tem- perature indicated in Table 16. It resets automatically. The limit switch operates when a high temperature con- dition, caused by inadequate supply air flow occurs, thus shutting down the ignition control and closing the main gas valves and energizing the blower.

2.Centrifugal Switch (CS). If the draft motor should fail, the centrifugal switch attached to the shaft of the motor pre- vents the ignition controls and gas valves from being energized.

20 Unitary Products