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Control Operation

Electric Control Operation


This controller has an adaptive algorithm that reduces the ignitor temperature to slightly greater than the minimum temperature required for ignition. The control measures the line voltage an determines an initial ignitor temperature. After each successful ignition, the control lowers the ignitor temperature. After each successful ignition, the control lowers the ignitor temperature for the next ignition attempt. When ignition does not occur, and the control goes into retry mode, the control increases the ignitor temperature to the value it was on the third previous successful ignition. After ignition is successful, the control repeats the adaptative algorith. The control is constantly making adjustments to the ignitor temperature to commpensate for changes in the line voltage.

Unit Functional Operation Modes

This section describes the functional operation modes of a PM Bowle Natural Gas External Heating Unit.

1 Heat Mode


On a call for heat, the thermostat contacts close signaling the control module.


The microprocessor runs its self-check routine.


The circulator is energized.


The control module checks the high limit circuit and air flow switch to ensures for normally close.


The system will energize the premix fan (40 seconds).


The ignitor warm up period begins after the inducer prepurge time has expired (20 seconds).


After completion of the ignitor warm up period :

-Trial for ignition begins;

-The gas valve is energized for 7 seconds;

-Flame failure response is set for 8 seconds.


The ignitor is de-energized when flame is sensed or at the conclusion of the ignition activation period.


When the thermostat is satisfied (i.e., when set temperature is reached), the gas valve is de-energized.*


The premix fan is de-energized following postpurge (5 seconds).


The circulator relay is de-energized after a 30 seconds delay.

*Note : The operation disk (C0028) can also stop the combustion cycle be setting the GAS valve to Off in order to keep the glycol temperature between 160 oF and 181 oF, even is the thermostat is not satisfied. The circulator is kept energized.

2 Retry


If the first ignition attempt fails during a normal heating cycle sequence, the control will :

  • De-energize the main gas valve;
  • The premix fan remains energized and the system is purged for inter purge time (30 seconds);
  • Normal ignition sequence is started.


Three ignition retries are attempted before System lockout.


After system lockout, the automatic reset feature is activated providing and additional trial for ignition sequence every one hour, provided a call for heat exists.

3 Recycle


If during the trial for ignition period the flame is established and then lost :

  • The main gas valve is de-energized;
  • The normal ignition sequence is started.


The number of recycles is 5 before system lockout.


After system lockout, the automatic reset feature is activated providing an additional trial for ignition sequence every one hour, provided a call for heat exists.

4 Normal operation


If flame exists 10 seconds after the completion of the ignition activation period :

  • The retry counter is set to zero;
  • The control enters its normal operating loop where all inputs are continously checked.

5 Premix fan operation


If during the start-up of a normal cycle, the pressure switch is sensed closed prior to energizeing the premix fan, the control module will await the opening of the pressure switch before continuing.


If after the premix fan is energized, the pressure switch does not close during the heating cycle, the control module will keep the inducer energized and await the closing of the pressure switch.


If the premix fan has been on for 60 seconds and the presure switch fails to close, the inducer will be turned off for 5 minutes.

6 System Lockout


When system lockout occurs :

  • The gas valve and the ignitor are de-energized;
  • The circulation is energized and will remain on as long as the following error conditions are detected;
  • a) High limit switches are opened;
  • b) Flame is present. (see Section 7.2, Safety Features);
  • c) When the error is no longer detected, the premix fan will run through the postpurge time and the circulator will be de-energized after a 30 second delay;
  • d) If the high limit switches close after being detected open, the control will reset once the circulator is de-energized;
  • System lockout will never override any of the safety features mentioned in Section 7, Safety Features


To reset the control, refer to section 8, Resetting the control.

7 Safety Features


If at any time the high temperature limit, air flow switch open, the gas valve and ignitor will be de-energized.


If at any time a flame is sensed when the gas valve is de-energized, the circulator and the premix fan are energized. The ignition sequence will go into system lockout.


If at any time during a call for heat the high limit circuit open, the circulator and premix fan are energized. When the circuit close, the circulator will run through the heat off delay and the premix fan through postpurge. Afterwards, the control will resume normal operation.

8 Resetting the Control

The control can be reset by 3 means :


System reset

Removal of the 24 V.A.C. power to the control for a period of time than 1 second will reset the control;


Thermostat reset

Interruption of the call for heat from the thermostat for a period of time greater than 1 second but less than 20 seconds will reset the control. If flame is sensed with the gas valve de-energized, interruptiong the callf or heat at the thermostat will no reset the control. .


Automatic reset

After one hour in lockout mode, the control will automatically reset itself.

9 Servicing

It is recommended that the appliance be serviced by a certified technician at least once a year. Combustion air supply and venting should be inspected seasonally for obstruction and debris:


Combustion air filter should be cleaned with soapy warm water regulary.


Venting system, should be tested for proper operation regularly.


Stack’s end should be regularly inspected for obstruction. In winter, perform regular inspection of the venting system to insure that the flue gas outlets are not blocked by snow.


Appliance area should be kept clear and free from combustible materials, gasoline and other flammable liquids


Air inlet should be inspect regularly for obstructions and debris

10 Operation indicator and Self-Check

Control panel and diagnostic indicator (see figure below)

carte electo

11 Control Diagram (Processor)

schema elect

12 Processor Control Electrical Characteristics

Input: 25 VAC, 60 Hz

XFMR Sec. Current: 405 m.a. + MV

Gas valve output (MV): 1,5 amp. @ 24 VAC

Ignitor output: 2.0 amp @120 VAC

Inducer blower output: 2,2 FLA, 3,5 LRA, 120 VAC

Circulator output: 10 FLA, 20 LRA, 120 VAC

Prepurge period: 40 sec. Interpurge period: 30 sec. Postpurge period: 5 sec.

Ignitor warmup period: 20 sec.

Ignition activation period: 7 sec.

Trial for ignition: 8 sec.

Automatic restart: 60 min.

Retries: 3 times.

Recycles: 5 times.