If you want to feel comfortable and safe during the cold season, you should have a well-functioning gas furnace. One of the ways of ensuring it’s functioning properly is being in touch with how it works. That way, when it malfunctions, you’ll know when to worry and when not to. Unfortunately, like most people who are not very tech-savvy, it’s easy to think that your unit is too complex to make out its tiny details and features, even when it may not be.

Let’s begin by looking at the steps it takes to heat your home and the different components that are involved.

Step 1: Your Thermostat Kicks In

Whether it’s using natural gas or propane, your gas furnace is meant to convert cold air to warm air and distribute it throughout your house. The heating cycle is started by a component known as a thermostat that maintains a uniform temperature in your home.

It essentially switches your heating system on when the temperature drops below a certain setting and switches it off when the temperature reaches the setting. As soon as the temperature drops below your desired setting, this device relays a signal to your furnace to heat the cool air.

Step 2: Your Furnace Runs a Safety Check

When your furnace gets the signal to heat your home, it needs to make sure all the components in the heating system are working properly. It does so by running a safety check that involves diagnosing the health of the unit to prevent an explosion, fire, or carbon monoxide leak. Your furnace also has different sensors that interact with the circuit board.

They help in ensuring all the safety switches are in the right position before going ahead with the next step. If one of these switches is tripped, it immediately activates an emergency shut-off.

Step 3: Your Inducer Motor Begins Running

When the sensors fail to detect a malfunction in your system, a component known as the inducer starts. It removes any gas or debris that was left in the heat exchanger in the previous cycle. Essentially, this motor removes the air in the heat exchanger through the combustion flue, thereby creating the necessary draft to burn the flame. This keeps it running during the whole heating cycle.

Step 4: The Burners are Ignited

Once the inducer motor runs for a couple of seconds, the gas valve opens up and the gas moves past the pilot light for the burners to ignite. The burners are found in your unit’s combustion chamber. They burn different gases sucked by the motor, and the fuel coming from the oil or gas valve.

Step 5: Your Heat Exchanger Starts Warming Up

As the burners fire up, the heat exchanger starts warming up so that cool air is heated. The heat exchanger is a collection of coils looped together, and they usually contain a lot of heat. This heat is safely transferred to their outside environment.

Step 6: The Flue Releases the Dangerous Gases

As the heat is created within your heat exchanger from gas and flame, it flows through the loop. However, this leads to a toxic concoction. Luckily, your furnace is designed to release the gas through the flue once it goes through the loops.

Step 7: Your Blower Starts Running

Once the burners warm up your heat exchanger for about 30 minutes, the unit’s blower motor starts. Its job is to transfer the warm air to your rooms. Firstly, this part blows air over the outer surface of the heat exchanger coils. This is the point at which the air is heated. After that, the air is sent through your home’s ductwork where it is distributed to the different rooms.

How Your Furnace Operates Safely

A long time ago, heating systems often caused numerous disasters in homes; they didn’t have the necessary safety measures. People had to deal with CO poisoning incidences, fires, explosions, and extreme heating inefficiencies. Today, we are lucky to have furnaces that are very safe. They have several switches and detectors whose job is to prevent the problems above and keep our families safe.

Limit Switch

This component is located above your furnace’s heat exchanger. The limit switch helps in measuring the temperature outside another component known as the combustion chamber. If there are flames, or for some reason, the temperatures are too high, it means that there is a possibility of experiencing a fire.

This switch is meant to detect such issues. When it does, it shuts off the gas supply to your furnace immediately to keep the flame from spreading outside the combustion chamber. Similarly, it keeps the carbon monoxide from spreading into your home.


If you’re using an older furnace that has the traditional pilot light, chances are you have this safety device. The thermocouple ensures that your unit’s pilot light is burning correctly. This component is connected to the system’s gas valve leading to the pilot light. Using its sensor, it detects the heat from the flame. If the flame is weak or goes out, the thermocouple cuts off the supply of gas to the pilot light to prevent gas from getting into other sensitive areas of your furnace.

Air Pressure Switch

If you’re using a newer furnace, then it definitely has a fan known as a daft inducer that runs after every heating cycle. This component sends the remaining gas out of the combustion chamber so that it clears out right before the subsequent cycle.

In the draft inducer is a safety device known as the air pressure switch. This device measures the amount of air being driven out by the inducer. If the pressure falls below the required level, it cuts off the gas being supplied to the system so that it doesn’t accumulate in the combustion chamber.

Carbon Monoxide Detector

Even though gas furnaces don’t usually have a carbon monoxide detector as part of the unit, this component helps in reducing the risk of carbon monoxide poisoning. When it’s well installed, it would effectively warn you when there is any carbon monoxide leak in your system. A professional HVAC technician can install for you low-level CO detectors to detect extremely low levels of carbon monoxide in your home and warn you accordingly.


If all your unit’s safeties are fine, the burner kicks on and your furnace starts heating up. When the temperature reaches the set temperature, the blower is engaged. Almost simultaneously, the fan starts and sends the hot air to your duct system.

Everything runs smoothly until your thermostat’s temperature setting is attained. When that happens, the main burners switch off, but the unit’s blower keeps on running. This component keeps running to cool down the furnace and use the heat that is still inside your furnace and duct system.

If your thermostat is set up properly, the heating anticipator will shut off the signals calling for heating right before the desired temperature is attained. This happens because the device takes into account the fact that the blower is still running to cool your furnace down.

In case you’re wondering, the heat anticipator is the electrical resistor device on your thermostat that helps in fine-tuning the level or point at which your thermostat switches off your furnace’s burners. This device helps in preventing the thermostat’s setpoint from overshooting or the furnace itself from overheating. This may happen as the blower releases the extra heat in the furnace once the main burners switch off.

When the extra heat is removed, the blower is shut down by the fan limit control and the system gets into standby mode, awaiting the thermostat to call for heating once again.

Not All Gas Furnaces are Similar

To have a complete understanding of how gas furnaces work, you have to be familiar with the different options available.

Single-Stage Systems

If you have a single-stage furnace, it means that your unit has a single stage of heat output, and it always runs at full capacity regardless of how cold it is outside. Such a furnace is not efficient because it works at full output each time it switches on so that it produces optimal heat. More precisely, it operates at about 80% efficiency. This furnace has nothing between the on and off switch.

The single-stage furnace generally costs less, and its repair costs are usually low since it doesn’t break down quickly.

Two-Stage Systems

If your furnace operates at about 90 percent efficiency, then you likely have a two-stage furnace. This kind of furnace can change the gas flowing through the valve from low to high, and it works with the thermostat to gauge the heating in your home.

The first phase of the furnace operates most of the time and runs at between 65% and 70% of your furnace’s full capacity. When the temperature outside your home drop extremely low and the first stage fails to keep up with your heating needs, the second stage swings into action to boost the healing process. This makes this type of furnace better at providing the right amount of heat and comfort to homes during winter.

When you use a two-stage furnace, you’ll hardly notice noise coming from the unit. This is because the system doesn’t operate at full capacity all the time it’s on. As you can expect, it also emits less carbon dioxide.

Modulating Furnaces

This kind of furnace interestingly expands the mechanism of a two-stage furnace to have many heat output settings. Consider a situation where it’s cold outside and your household needs a bit more heat. In this case, a standard modulating furnace automatically increases its heat output in tiny increments. These can be as low as 1%.

As your unit regulates the amount of fuel being burned constantly based on the setting on your thermostat, it can maintain the temperature to roughly ½ degree of the setpoint (on your thermostat) across your home.

What’s more, the unit’s gas burner and the fan never run at full capacity. This means that the modulating furnace doesn’t require the typical on and off cycling in furnaces. This makes this type of furnace more efficient than the other two. If you’re living in a cold climate, or are in the middle of a very cold season, this would be the best furnace to invest in.

Additional Furnace Options

PSC Blower/Standard/Multi-speed

If you have any of these units, then it’s likely you’re using a split-capacitor electric motor. This device doesn’t change its speed. It may either cycle on or off at a single selected speed- be it low, medium, or high- based on whether the thermostat is signaling the unit to heat or cool. These furnaces are more cost-effective when compared to the variable speed option below.

ECM Blower/Variable Speed

The difference between variable speed and the multi-speed above is that the former can run anywhere between zero RMP and the maximum possible speed designed for the motor. The variable-speed blower can circulate air at lower volumes constantly without many off cycles.

When the time to cool your home comes, this prevents hot air from accumulating at the ceiling. The variable-speed blower can assist lower cold spots in your home during the heating season. This is because it runs longer at lower speeds, thereby helping in the circulation and mixing of indoor air. As you can expect, having this type of furnace also means that you’ll enjoy high fuel efficiency. Other benefits include less noise, cleaner air, and heating consistency, although this also depends on your ductwork.

Heating SLC Services

Have you recently experienced a problem with your gas furnace?

Do you want to replace your furnace?

If you want a professional to install, maintain, or repair your gas furnace, Whipple Service Champions is the team to call. Call us today to learn more about our heating SLC services.