Saturday, November 14, 2009

Hi Efficiency Heating and Cooling Unit Operating Characteristics

by Roger Desrosiers

Blower Operation

1. The blower will operate continuously whenever the mode selector of the electronic thermostat is set in the cool mode and the fan selector is set to on.

2. The blower will operate intermittently whenever the mode selector is set to cool and the fan selector is set to auto. A delay is programmed into the microprocessor to allow the blower fan to continue to operate for one minute after the condensing unit has been commanded off by the thermostat.

3. The blower will operate intermittently whenever the mode selector of thermostat is set to heat. The fan is commanded on 35 seconds after the main burner lights and remains on for a predetermined time set by dip switches after the main burner is commanded off.

Heating Cycle

1. When the thermostat is set for heat mode and the room temperature drops one degree below the set point the circuit between the “R” and “W” terminals on the sub base is closed. The microprocessor on the electronic circuit board (ECB) sees a command for heat on the input terminal to the “W” terminal of the thermostat and commands the relay of the induced draft fan to close starting the induced draft fan motor.

2. If the passages through the combustion air piping, vent piping, and heat exchanger passages are unrestricted, the operation of the induced draft fan generates enough air pressure to cause the pressure switch to close within about two seconds. The microprocessor on the ECB reads the input signal from the pressure switch terminals and commands the hot surface igniter relay contacts to close allowing the ignition device to begin heating.

3. After the igniter has been hearing for 15 seconds, the microprocessor on the ECB commands the gas valve relay contacts closed for 7 seconds, opening the valve. As the fuel-air mixture passes across the red hot surface of the igniter, combustion occurs and the entire burner lights.

4. The presence of burner flame allows current to flow (1.7ua) from the tip of the flame sensing rod, through the ionized gases of the flame, and into the grounded surface of the burner. The current can only flow out the top of the rod and into the flame.

5. It cannot flow from the flame into the top of the rod. Consequently, the ac voltage across the flame rod and burner is rectified into a pulsating DC current. As long as a rectified DC current is present at the flame rod terminals, the microprocessor knows that a flame exists and allows the gas valve to remain open.

6. After the flame rod safety circuit has proved that the main burner has been lit for 30 seconds, the ECB microprocessor will command the blower motor relay contacts close, turning on the fan. Since the unit is operating in the heating mode, the fan speed relay coil remains de-energized.

7. When the signal from the “W” terminal of the thermostat opens, indicating that the room is satisfied, the ECB microprocessor will command the gas valve relay contacts open and the gas valve will close.

8. The ECB microprocessor allows the induced draft fan motor to operate for 15 seconds after the main burner is commanded off to purge the combustion gases from the burner box and heat exchangers. The blower motor will remain operating for some set time after the main burner is commanded off.

Cooling Cycle

1. When the thermostat mode is set to cool and the control point rises one degree above the set point, the input signal to the Y terminal on the ECB comes on. The ECB’s microprocessor reads the change in the “Y” terminal’s signal and commands the condensing unit relay contacts to close. This action energizes the condensing unit’s contactor, starting the compressor and condenser fan.

2. The blower fan unit is commanded on when a signal is present on the “G” input terminal from the thermostat. The fan operates for 60 seconds after the signal on the “G” terminal goes open, Indicating the thermostat is satisfied.The condensing unit is commanded off when the signal on the “Y” terminal of the thermostat and the ECB goes open.

The legend for the Electronic Control Board
· HSI = Hot surface igniter
· IDM = Induced draft motor
· GV = Gas valve
· BLM = Blower motor
· EAC = Electronic air cleaner
· ACC = Air conditioning control
· FPE = Flame proving electrode
· FRS = Flame rollout switch
· HLS = High limit switch
· PS = Pressure switch



1. The optional 24V AC humidifier is commanded on by the humidifier control relay whenever the furnace is in the heating mode and its blower motor is commanded on.

2. The humidifier is commanded off 15 seconds after the furnace is commanded off at the end of the post purge cycle.

Safety Strategies

1. During the hearing mode if the flame rod does not detect a flame within the first 7 seconds that the gas valve is commanded open, the ECB microprocessor commands the gas valve closed. A 15 second waiting period occurs before an ignition cycle will be attempted again. The induced draft fan motor remains operating during this period to purge unburned fuel from the heat exchangers.

2. After a flame failure, a second attempt is made to start the furnace. The igniter is started and allowed to operate for 25 seconds before the gas valve is commanded open. If the burner fails to light, another attempt will be made before the ECB microprocessor locks out the ignition cycle. The unit can be reset by a service technician by opening the disconnect switch to the furnace for 60 seconds.

3. If there is a, momentary loss of fuel or flame or a short or an open circuit occurs in the flame rod circuit, the microprocessor will close the gas valve. After 15 seconds the ECB will restart the ignition process. If for some reason the unsafe condition does not clear, the unit will lock out, requiring a manual reset.

Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.More About Roger