A/ Heating Operation - On a call for heat from the thermostat, the oil furnace will receive a 24 V.A.C. signal.  This signal will cause the primary control for the oil burner to become energized.  Consequently, the pump pressurizes the oil to the nozzle and causes a continuous high-voltage spark for ignition at the tip of the burner.  When the spark ignites the atomized oil, it will create a high-temperature flame in the heat exchanger.  A photocell pointed at the fire location sees the flame and allows the burner to continue to operate.  If no flame is seen for about 45 seconds after burner start, the burner will shutdown.  The increase of temperature in the heat exchanger causes the fan switch to close and energizes the furnace fan.  Once the thermostat is satisfied, it will open its contacts and de-energize the furnace primary control.  If the furnace develops an over temperature problem, it will cause the high limit control to open and de-energize the primary control.

B/ Cooling Operation - When the thermostat calls for cooling, the contacts in the thermostat will close and energize the coil of the compressor contactor and cause its contacts to close.  When these contacts close, the compressor motor will run and pump refrigerant through the system.  The condenser fan is also connected to the contactor and it runs anytime the compressor is activated.  When refrigerant is pumped by the compressor to the evaporator coil inside the furnace, heat is absorbed from the inside air as it is passes over the coil via the furnace or evaporator fan (which is controlled by a fan relay and comes on with the compressor).  The refrigerant is then pumped back to the outside, where it passes through the compressor and then the condenser coil.  The condenser fan moves air over the condenser coil causing the refrigerant to give up the heat absorbed from the house and the compressor.  This cycle continues until the thermostat is satisfied.

C/ Continuous Fan Operation - The thermostat or the manual fan switch located on the furnace can be set to run the furnace fan continuously (the "on" position), or set to run only when the compressor is energized (the "auto" position).  In order to run the continuously, the manual fan switch or the thermostat switch must be put to the "on" position.  When this occurs, a circuit is provided between the R terminal and the G terminal.  This will energize the fan relay coil of the evaporator or furnace fan, and cause the fan relay to close.  Anytime the fan switch is in the "on" position, the fan will be energized 1 00 percent of the time until it is taken off the "on" position.

Copyright © 2000 HVAC Mechanic.com . All rights reserved.
Revised:  .September 22, 2002