Dan has been in the HVAC industry for 23 years with experience ranging from installation and service to sales and distribution.
How a Furnace Works and How It Turns Itself On and Off
Often the best way to figure out what is wrong with something is to understand how it is supposed to work. If you know the sequence of operations, you can pinpoint where the sequence is being disrupted. Your furnace is no exception to the rule.
When your furnace is called into action by the thermostat, there is a rhyme and reason to the procedure it follows in safely turning itself on. Next time your furnace doesn't respond to the call, you will be able to see where the problem lies, and you can either decide that it is within your skills and resources to repair it or that you need a professional HVAC technician to handle the issue for you.
The following discussion can save you time and money, whether you fix the problem yourself or end up showing the problem to your service technician.
Firing Order: Steps in the Operation of a Modern Forced-Air Furnace
When the thermostat asks for heat, the furnace jumps into action, going through the steps and components below. You can see many of these components in the video.
- The thermostat tells the furnace to come on.
- The inducer motor starts up.
- The pressure switch confirms proper venting of the chimney.
- The hot-surface ignition module (if you have one, and not a spark ignitor or a pilot) begins to glow.
- The gas valve opens and the gas is ignited by the ignition source.
- The flame sensor verifies that the gas has been lit.
- The high-limit switch reaches its set temperature.
- The blower motor comes on.
- The furnace runs until the thermostat is satisfied and tells it to shut down.
- The gas valve shuts.
- The high-limit switch reaches its low-temperature setting.
- The fan shuts down.
A newer furnace may have even more bells and whistles; an old furnace may have just the minimum: a gas valve and a thermocouple (like an old-fashioned flame sensor). The thermocouple or flame sensor tests whether the pilot is lit, and it stops the main gas valve from opening if it is not. Of course, a broken thermocouple can also stop the gas valve from opening.
The thermostat is where it all begins. The thermostat is really just a set of switches that open and close depending on the temperature, allowing power to flow to certain circuits in the heating and cooling system. Think of them like drawbridges that swing up and down. They are normally drawn up, but can be lowered to close the bridge and allow power to pass.
When the temperature in your home drops, the thermostat drops its bridge, sending power to the furnace to let it know that heat is needed. As the room temperature rises again, the thermostat raises its bridge and shuts off the power. For air conditioning, the thermostat works the same way but in a mirror image, closing its bridge when the house gets too warm.
If the temperature in the house is lower than the temperature to which you set your thermostat, your thermostat may not be functioning as it should. You can test this by jumping (touching together) the red and white wires to your thermostat. If the furnace comes on, your thermostat is quite likely the problem.
What Do the Wires in the Thermostat Do?
|Wire Color||What it Controls|
Red is for power. This carries 24-volt power, supplied by the furnace. This power waits at the “bridge” until it is told where to go.
White is for heat. When the bridge (switch) between the red and white wires closes, the thermostat is calling for heat.
Yellow is for cooling. When the bridge (switch) between the red and yellow wires closes, the thermostat is calling for cooling.
Green is for the fan. When the bridge between red and green closes, only the fan runs. No heat or cooling is called for.
Blue is a rogue or wild card. It can be used to power a display or for advanced features. Usually, though, it is wrapped back into the wall and not used at all.
Inducer Motor and Fan
The inducer motor and fan push exhaust up the chimney, getting rid of carbon monoxide (CO). Older systems relied solely on "natural draft" to vent these gases, but inducers have been a great addition to the furnace system. They extend the life of chimneys, and when used with pressure switches, they help prevent CO poisoning.
Read More From Dengarden
The Pressure Switch
The function of the pressure switch is to verify that the inducer motor is actually pushing air up the chimney. If the chimney is blocked, for example by a bird's nest, the pressure switch shuts down the system in order to eliminate the risk of CO entering the home. Of course, the pressure switch may also shut down if the motor is old and running too slow to satisfy the switch, or the hose from the fan to the switch is pinched or broken, or the switch itself is bad.
Hot Surface Ignitor
Again, a rather simple device in a complex system. The hot surface ignitor uses electrical power to heat a very fragile ceramic element, charcoal-like in appearance, to such a high temperature that natural gas or propane bursts into flame as it flows by. The hot surface ignitor is located at the orifice at the first burner port, in order to light the gas immediately as it's introduced to the port. This prevents a buildup of unburned gas in the unit.
Some furnaces employ what is called spark ignition. Instead of a red-hot surface, a spark ignitor creates a series of sparks to ignite the gas.
It's not important whether you have hot-surface or electronic ignition in your furnace; the important thing is that you are avoiding using a pilot light. Pilot lights waste gas; they burn gas constantly during the heating season, and some people let them burn year-round.
Want to save a few dollars? Make sure you shut off the pilot on your furnace when it's not being used for the season. Just be sure you know how to relight it next year before you do.
The Gas Valve
The gas valve is basically an electronically controlled gateway. When the unit calls for heat, and the circuit board confirms that the right conditions have been met, the unit passes energy to the valve, causing it to open and release gas to be ignited by the ignitor or pilot. When the heat cycle has run its course, the energy to the valve is cut, and the valve once again shuts, cutting off the gas supply to the burners.
The Flame Sensor
The flame sensor is very simple, however, it causes a lot of problems for homeowners. Its only job is to verify that the gas has been lit, by sensing the heat. If there's no heat, it shuts off the gas, to avoid any dangerous buildup of gas in the unit. The flame sensor is simple to fix when it goes bad or just needs some TLC.
I wrote a simple step-by-step guide for people needing to clean their furnace flame sensor. Have a look—a lot of issues can be fixed just by a simple cleaning of a flame sensor.
Video: Cleaning a Flame Sensor
The High-Limit Switch
The high-limit switch serves several purposes. First, it keeps the furnace fan from turning on until it detects that a set temperature has been reached. Otherwise, the fan would come on prematurely, and blow cold air into the house each time the thermostat asks for heat.
Second, it also tells the blower how long it should continue to run before shutting off, once the thermostat has been satisfied. Otherwise, hot air remaining in the ductwork and heat exchanger, after the furnace has cut off, would just go to waste.