Energy Tech Laboratories DHG240 - Manual (Page 20)

035-17233-000-C-0702
20 Unitary Products
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.
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.
FIGURE 14 : GAS VALVE PIPING
GAS
VALVE
GAS MAIN
MAIN VALVE
TO MAIN
BURNER
REDUNDANT
VALVE
TO PILOT BURNER