Catalogue

42 UH-PRC002-EN

Sequence of Operations

Two-Stage Control, Direct Spark

With power applied to the unit:

1. The thermostat calls for heat.

2. The power venter is energized and the pressure switch measures the flow through the vent

system and energizes the direct spark ignition system beginning the pre-purge timing when the

flow is correct.

Important: CAUTION: THE PRESSURE SWITCH MUST NOT BE BYPASSED. THE UNIT MUST

NOT BE FIRED UNLESS THE FLUE VENT FAN IS OPERATING. If this procedure is not

followed, there may be a gas buildup that could cause an explosion.

3. The direct spark ignition lights the first stage main gas. The sensing device has seven seconds

to prove flame. Main burners are lit at 50 percent of unit's rated input.

4. The fan time delay in the board allows the heater to come up to operating temperature. At this

time, the fan time delay relay closes and activates the fan motor.

5. If additional heat is required, the second stage of the thermostat calls for heat.

6. The main gas valve opens to full fire. The main burners are now at full fire. The unit continues

at full fire until the second stage of the thermostat is satisfied and no longer calls for heat.

7. The main valve closes to low fire. The main burners are now at low fire. The unit continues at

low fire until the first stage of the thermostat is satisfied and no longer calls for heat.

8. The gas valve closes.

9. The fan time delay relay remains closed, keeping the fan operating to dissipate residual heat

from the heat exchanger. At this time, the fan time delay relay opens and deactivates the fan

motor.

See “Fan Control,” p. 30.

• Caution - Disconnect power before

servicing.

• Unit must be grounded.

• Use copper conductors only.

• If any of the original wire as supplied

with the appliance must be replaced,

it must be replaced with wiring

material having a temperature rating

of at least 125°C.

• High-limit wires minimum 200°C.

• Refer to installation instructions for

venting, gas piping and start-up

procedures.

• - - - Indicates Field Wiring.

• ___ Indicates Factory Wiring.