It always seems that the coldest night of the year also is the night your furnace will decide to quit working. Today’s furnaces are much more efficient than your parent’s or grandparent’s home heating systems. In order to obtain higher efficiencies, furnaces have become much more complex. This complexity means that there are more mechanical parts in the operation chain, which have the ability to fail.
The heating cycle begins when the room temperature drops below the heating set point on the thermostat. The thermostat senses this drop in temperature and then closes a switch that sends 24 volts to the heat terminal on your furnace control board. When the furnace receives the voltage for heat, it begins the complex (delete complex) firing cycle.
First the furnace control board sends 110 volts to the combustion inducer fan. This fan draws air through the heat exchanger to produce a more efficient burn from the burner jets. A vacuum switch senses the negative pressure created from the inducer fan and closes a set of contacts to let the control board know the inducer fan is operating correctly. The control board then sends 110 volts to the hot surface or spark ignitor. The furnace control board measures the resistance occurring in the ignitor to determine if it is operating correctly.
If it determines the igniter is working correctly then it will open the gas valve to allow natural or propane gas to flow into the heat exchanger. The furnace senses that the flame is lit and turns off the voltage to the igniter. If at anytime a failure during any point of this ignition chain is detected, the inducer fan will continue to run for 2-3 minutes to clear any possible unlit gas and then the unit will begin the ignition process again. The furnace will attempt ignition 3-4 times depending on the brand of furnace and will then go into lock-out where it stops the unit from operating until the problem is corrected.
If everything works well in the ignition chain and the furnace begins to burn, then it continually senses the temperature from two internal thermometers. One of these thermometers is known as the over-limit switch. This thermometer will open a set of contacts if the furnace gets too hot or goes “over limit”. When the contacts open the gas valve will close immediately and the furnace will shut down.
The second thermometer is called the “roll out switch”. If for some reason the draft of exhaust air out from your furnace and out your flue becomes blocked then the flames that normally burn in your heat exchanger will “roll out” the front of the furnace. The roll out switch will detect this and it will shut down the gas valve and the furnace. Potential roll out is also why you should never store any combustibles within 5 feet of your furnace in any direction.
As you can see when you have a heating failure, it can sometimes make your furnace act funny. But knowing what is occurring behind the scenes gives you a better understanding of why the furnace is misbehaving.
The best way to avoid these heating failures is to have your furnace inspected and cleaned. Empire HVAC’s heating tune-up includes cleaning of your furnace mechanical parts, lubricating all motors and bearings and checking the electrical switches and controls to determine if any are close to failure. Replacing failing parts before your furnace quits working, avoids losing heat on the coldest days of the year.
Empire HVAC’s main office is located in Decatur, Georgia, but we service the metro Atlanta Georgia area including DeKalb, Fulton, Forsyth, and Gwinnett counties. Our heating experts will help keep your home warm this winter. Call us today at 404-294-0900 to schedule your heating tune-up appointment or click on the link below.