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Understanding your heating system’s basics is essential to help you know when you need furnace repair services or if the issue is typical. When you have a basic understanding of the various parts of the furnace and how they work together, you can more easily isolate the problem and find a solution. 

The heating system is one of the most important aspects of your home. It’s what keeps you and your family comfortable in the cold weather and ensures that the pipes don’t freeze.

Without a reliable heating system, you may find yourself needing emergency furnace repair at an inopportune time. But a heating system can be complicated, especially for someone without the proper expertise.

Here are some frequently asked questions about heating systems to help you understand the basics of how this vital system works. Think of these questions as Heating System 101.


How Does a Furnace Work Overall?

The most common type of furnace is a gas furnace, which connects to a natural gas line. Other types of furnaces are electric or oil, although those are less common and rarely found in northern Colorado.

If you are unsure what kind of furnace is in your home, check the front of the heating system for a gas heat exchanger and a glowing blue flame. If you see that, you have a gas furnace. If not, your furnace is likely electric.  

A gas furnace connects to a thermostat, which is set to a specific temperature. When the thermostat realizes the house is below that temperature or is set to run, it sends a low-voltage electrical signal to the furnace. That signal opens a valve and delivers natural gas to the burners. Natural gas ignites the pilot light or electronic ignition lights inside the combustion chamber, creating heat in the heat exchanger.

Pilot lights tend to only exist in older homes and furnaces. At the same time, newer houses have a hot surface ignitor that does the same job but is more safe and reliable. The surface ignitor turns on the gas jets, which light the heat exchanger. The heat exchanger is a set of coils that warms the air as it passes by.

The heat exchanger releases exhaust through the flue to the outside of the house, ensuring that the dangerous combustion gases don’t spread through the house. 

As the air is warmed, it is pushed by blowers into the ductwork, which is a network of ducts that moves warm air throughout the house. Ductwork sends air to these ducts, which are typically in the wall or ceiling of a home.

The ducts finally push warm air into the room. The warm air replaces the cooler air in the room, and that cool air moves to the return ducts and returns to the furnace, where it is warmed, and the cycle continues.

The furnace continues to run until a signal from the thermostat tells it to stop. The thermostat can send that signal when the house reaches the proper temperature, or the furnace has been running for a set amount of time. 


How Does a Pressure Switch Work?

There are many furnace parts, and each is important for the system to work. These components work together to warm a house. If one part is out of place or isn’t working correctly, it can affect the entire heating system.

One of the most important pieces is the pressure switch. The pressure switch is a relatively small piece, but it plays a crucial role in keeping your system running safely. 

The pressure switch performs a few different functions. It acts as an automatic safety feature. It shuts the furnace down if it senses specific functions or statuses of the draft inducer motor. These may include a negative pressure coming from the draft inducer motor or the fan no longer moving or moving too slowly.

These processes protect against exhaust fumes being reintroduced to the system and spreading throughout the house. The draft inducer fan is what vents combustion gases out of the house. The pressure switch also prevents the furnace from cycling if there is a mechanical failure, which stops an issue from worsening and causing damage to other areas of the house. 

The pressure switch is activated by suction. The suction of the pressure switch is created by the inducer fan or draft inducer. When the inducer fan turns on, it creates suction that pulls the pressure switch through a rubber tube, closing the circuit that allows power to pass to the next part of the furnace system: the ignitor. 

The pressure switch closes once the draft inducer fan motor reaches maximum speed. When the pressure switch is closed, it ensures that gas won’t enter the furnace unless the toxic fumes in the burner can be exhausted. 


How Does a Flame Sensor Work?

A flame sensor plays an important safety role by detecting a flame burning inside the furnace. It ensures that gas from the furnace is being used to heat the house and not escaping into the vent system. If the flame sensor doesn’t detect a flame, it shuts off the furnace to avoid a gas leak. The flame sensor is crucial to preventing carbon monoxide poisoning within the house.

The flame sensor is a short metallic rod with a porcelain base located inside the furnace’s burner assembly. When the furnace turns on and the gas valve opens, the flame sensors send out a small electrical current to detect heat from a flame. If the sensor detects the flame, the gas ignites, and the furnace continues to run as usual.

However, if the sensor doesn’t sense a flame within the first 10 seconds of the furnace turning on, it will shut down. In most furnaces, the flame sensor will run through the cycle one or two more times to detect a flame before locking the heating system down.

If this happens, most heating systems are reset by turning off the power or turning off the furnace at the thermostat and then turning it on after a few seconds.  

The flame sensor can get dirty and dusty over time, impacting its ability to send the current and detect if there is a flame. If that’s the case or the base is chipped or broken, you may need a replacement flame sensor. 


How Does a Gas Valve Work?

The furnace gas valve is what opens to ignite the burner of the combustion chamber. When the thermostat signals to the furnace to run, the gas valve opens and sends gas to the rest of the system.

Together with the thermostat, the gas valve regulates the amount of gas that flows into the furnace. Once the home has reached its desired temperature, the gas valve closes to regulate the pressure of the gas flowing into the furnace. 

If the gas valve isn’t working, you may need emergency furnace repair to ensure enough voltage and a connection between the gas valve and the rest of the furnace circuit. 


How Does a Heat Exchanger Work?

Most appliances that use heat have heat exchangers. A furnace heat exchanger is what separates the breathing air from the combustion process. It allows gas to pass to the breathing air and heat it without the two gases coming into direct contact. Combustion gases are dangerous for human consumption and can cause significant health and safety issues.  

The heat exchanger is typically a series of looped tubes. It transfers the heat created in the combustion chamber outside the furnace, where it blows through the ductwork to the rest of the house. When the furnace turns on, hot combustion gases enter the heat exchanger and heat the metal walls.

The furnace then blows cold air from the rest of the house through the return air ducts along the outside of the heat exchanger, which warms the air without mingling it with the dangerous combustion gases. The warm air is then sent back through the ductwork to rooms in the home. Throughout this process, combustion gases and the air you breathe are kept separate.  

The gases created during the combustion process then blow out of the heat exchanger through a vent on the home’s exterior. High-efficiency condensing furnaces don’t send the gases outside but instead move the combustion gases to a second heat exchanger to re-use the heat and continue the heating process. 

Understanding how your heating system works can help detect any problems and allow you to get professional assistance before they grow into larger issues. The heating system is one of the most important parts of the house, so make sure you understand the basics to keep it working at the best level.

To ensure your furnace is working as it should, schedule a furnace inspection with an HVAC professional.