Hi, I’m Vance and welcome back to Repair and Replace. Furnace is something you never really think about until it stops working, but with a little bit of knowledge, it’s much easier to troubleshoot problems. So today we ‘re gonna learn how a gas furnace works. First we’ll cover the basics, then we’ll dive a little deeper into the sequence of operation. So you can understand what each component does. Let’s begin ! Although there are some differences, all gas furnaces work in the same basic way. Natural gas is burned to warm up the heat exchanger.
The exhaust fumes are released through the flue gas vent. The blower fan then pushes air over the heat exchanger and circulates it around your home. The furnace runs until the thermostat detects the right temperature and shuts the furnace off. Modern furnaces use a draft inducer fan to help exhaust the emissions, this force draft also increases the efficiency. Condenser furnaces use a dual heat exchanger to condense water vapor , this extracts even more heat out of the combustion. Furnaces are rated by AFUE or (Annual Fuel Utilization Efficiency). This number is a ratio of the fuel burned versus the fuel converted into heat. Older furnaces will have an efficiency rating of 70 AFEU or less, meaning that only 70 percent of the fuel is turned into usable heat. Mid efficiency furnaces are around 80 and High-efficiency condenser furnaces are generally 90 and above. Furnaces can be single stage or dual stage. Single stage furnaces run at one speed, which means it’s either full heat, or no heat. Two stage furnaces can run at full speed or a reduced speed when less heat is needed.
This allows for the furnace to run quieter and may contain a more consistent temperature. Modern furnaces have a set of safety switches that shut down the furnace, when the system is compromised. if a switch is tripped, it will cause the furnace to lockout. The furnace will attempt ignition, several times before going into a hard lockout for up to an hour. Every time a lockout happens the furnace will use flashing lights, as codes to identify the source of the shutdown These codes are unique to each model so check your furnace manual to see what each code means.
When the thermostat calls for heat, voltage is sent to the control board. Next the draft inducer fan pulls fresh air into the burner, which allows for clean and efficient combustion. When the draft motor reaches full speed the pressure switch will close, allowing for the sequence to continue. The pressure switch ensures that there is enough airflow inside the heat exchanger to safely operate the furnace. If the pressure is low, the switch will stay open and will prevent the furnace from igniting. If the pressure is good, then voltage is sent to the hot surface igniter. Some furnaces might use a spark igniter instead. After the igniter heats up the gas valve opens and the burners ignite. When the flame will continue as long as the safety switches remained closed. If the flame isn’t detected after 7 seconds the flame sensor will shut off the gas. When the burner is lit, the blower fan is too late from running to allow the heat exchanger to warm up.
This prevents the furnace from circulating cold air. The blower fan then pushes the air over the heat exchanger and circulates it around your home. The limit switch monitors the temperature of the air passing over the heat exchanger and will shut off the furnace if the system overheats. Any restriction of the airflow might cause the heat exchanger to overheat. That’s why it’s essential to replace the furnace filter every three to six months. Once the thermostat signals that the temperature of the home is normalized, the gas is shut off and combustion stops. The blower motor will continue to run for several minutes until the furnace is cooled down. Finally the unit is placed in standby until the thermostat calls for heat again. Hopefully, this has given you a better understanding of how a furnace works.
Now if your furnace is having trouble running, then watch our troubleshooting guide for a step-by-step breakdown and there’s a link to this in the description below. If you found this video useful then subscribe to our Channel. We’re constantly filming new content every week, so let us know what repairs you’d like to see next and if you need a part for that repair then visit our website. We stock thousands of hard-to-find parts, and we’ll ship it out to you the same day. Thanks for watching .