Sequence Of Operation On Your Furnace.

Posted by Darin DeVries on

When your furnace isn't running properly or you start hearing the furnace make different noises how can you determine what is the problem is or narrow down the part that is failing, I get call after call from customers that have problems but don't know what part has failed so here is a little summery to explain the operation of your equipment. We call it sequence of operation! Every technician that goes out to work on a furnace knows the operation steps and by that he can tell what is failing or what to replace.

Sequence Of Operation on an Electric Furnace

  1. The thermostat calls for heat.
  2. You should hear the furnace relay "Click".
  3. The Power now energizes the sequencer and the number one heating element has power (element starts getting warm) and the blower fan comes on,
  4. After about 18 to 20 Seconds the second heat element will come on. This takes place in the sequencer (Low voltage turns on High Voltage)
  5. After about 15 to 30 Seconds the third element will come on (If you have a 15KW furnace.
  6. After about 15 to 30 seconds the fourth element will come on (If you have a 20KW Furnace. 
At the end of the heat call the thermostat is satisfied all elements go off at the same time but the blower will continue to run for about a minute to cool down the furnace and to get the heat out of the duct work, Then the final step the blower shuts off. 
Sequence Of Operation of Your Gas Furnace.
  1. The thermostat calls for a heat call.
  2. The Enducer Blower will start up. (If you have one)
  3. The Pressure switch tells the board that the enducer blower is moving exhaust fumes out of the heat exchanger.
  4. The electronic ignition Spark or Hot surface igniter will come on and glow red for about 1 minute.
  5. The board will tell the gas valve to come on, ( you can hear the click of the gas valve. At this time you should have a burner flame inside the fire box or burner tubes to the heat exchanger. 
  6. The flame sensor rod by sending a small electric currant through the flame it can tell or (prove) that there is ignition and not raw gas going in the furnace. Now the flame is heating up the fire box.
  7. The Fan thermostat in the burner box will heat up and turn on the blower fan to push the heat to the duct work. 
  8.  At the end of the heat call the thermostat in the house is satisfied it will send a signal to the furnace for shut down it closes the gas valve and the flame goes out. 
  9. The enducer blower and the main fan blower will continue to run for about a minute to cool down the burner box and duct work. 
  10. Once the furnace is cool the fan thermostat in the furnace will turn off the blower until the next heat call.