How Does A Furnace Work? (The ULTIMATE Guide)

Having a furnace in your home means having a cozy place on winter days. How does a furnace work, though?

You must already know that a furnace produces heat. This heat transforms a cold house into a warm place.


Furnaces have undergone a series of developments over years. That’s why a furnace today doesn’t function exactly the same way as it did before.

Let’s take a look at the science behind how different types of furnaces work.

The Science Behind a Working Furnace

To simplify the science behind a working furnace, let’s take a gas furnace as an example.

A gas furnace has six parts that complete its system: burners, a gas valve, blowers, a heat exchanger, an air duct, and a thermostat. Each of these parts works in tandem to provide heat for your home.

First, the furnace’s burner sets the natural gas on fire to warm up the heat exchanger. This process releases exhaust fumes that are taken up through the gas vent.

Then, the blower works to distribute the heat throughout your home. This operation continues until the air reaches the temperature on your thermostat. Once it reaches the right temperature, the thermostat turns the furnace off.

For modern furnaces, there’s a draft inducer fan to help in distributing the heat better. What’s more, this component increases your furnace’s efficiency.

Type of Furnaces Based on Air System

Knowing the type of air system your furnace operates can help you determine how air is circulated in your home.

Natural Draft

Natural draft furnaces are one of the earlier types of furnaces but now considered to be outdated.

With an estimated energy efficiency of below 70% on the AFUE rating, natural draft furnaces have low efficiency in converting your fuel to heat. This means that when the fuel burns, byproducts such as carbon monoxide and nitrogen dioxide get released into the air.

Most natural draft furnaces still in use today have undergone retrofitting to increase their efficiency.

Forced Air

A forced-air system circulates both cool and warm air in your house. The air handler unit forces the air through the ducts, hence its name “forced air.”

In this system, you have the option to control the amount of warm or cool air circulating in your room.

Induced Draft

In an induced draft furnace, a draft inducer fan controls the general airflow.

Having the draft inducer brings in outside air to help with combustion. An induced draft also minimizes the production of carbon monoxide byproducts.


A condensing furnace, or condensers, also uses a forced-air system but with an additional system.

In a condenser, you’ll see a primary and secondary heat exchanger to condense water vapor. This dual system extracts more heat from the combustion.

The secondary heat exchanger makes the carbon monoxide byproduct cooler, which lets it condense as it escapes.

Types of Furnaces Based On Fan Operation

Fan operation can also differentiate furnaces. You can choose between a single-stage, dual-stage, or a modulating type.

Single-Stage Furnace

If you have a single-stage furnace, you’ll know that you can only operate these at full heat or none at all.

As suggested by its name, a single-stage furnace only runs at one speed and at maximum capacity.

Dual-Stage Furnace

A dual-stage furnace offers more than a single-stage furnace when it comes to speed and efficiency. As a result, dual-stage furnaces are the most common type of heating system in many homes.

If you want to warm up your home a little, you can use this furnace’s partially opened setting. At this stage, the furnace works only at 65% capacity, which makes it suitable for chilly days.

When winter is at its peak, you can turn this furnace on full blast and make it work at its highest capacity.

Modulating Furnace

For a modulating furnace, its continuous control over the generated heat and air movement are its most attractive features.

A modulating furnace can use 40% up to 100% of its capacity depending on the conditions of the day. Because of this, it helps in ensuring that you have a constant temperature around your home. What’s more, it minimizes temperature spikes.

If you like keeping your home at a certain temperature throughout the year, a modulating furnace can help. Since it only works when it needs to, it saves money and energy.

Types of Furnaces Based on Energy Source

You can find different types of furnaces based on the most accessible energy source at your disposal.

Electric Furnace

Electric furnaces are one of the most highly efficient types of furnaces today. They operate with an AFUE of 95 to 100%. Since electricity is readily available, you can easily operate an electric furnace in your home.

Electric furnaces are suitable for homeowners that don’t have easy access to gas. It’s also relatively safer because it doesn’t use fire.

Furnaces running on electricity are expected to last between 20 to 30 years. The only downside is that it can be expensive to operate.

Gas Furnace

Gas furnaces that use natural gas are the most commonly used type in American homes. When they’re new, these furnaces can operate with 89 to 98% efficiency. They can also last up to 25 years if maintained properly.

Take note of the possible dangers of using a gas furnace, though. Because it uses non-renewable energy, it produces carbon monoxide as a byproduct. This is susceptible to carbon monoxide leaks. It is important to have CO meters for this reason.

It’s essential to install carbon monoxide detectors around your home if you’re using gas furnaces.

Oil Furnace

Using oil furnaces is another option for homeowners who can’t easily access natural gas. New oil furnaces have an AFUE rating of 80 to 90% and last up to 30 years.

Oil furnaces are in some instances more affordable than gas furnaces. Even so, you’ll need to take into account the cost of using imported oil to power your furnace. It is also unclear how long these will be allowed. Heat pumps might be more future-proof.

When operating an oil furnace, you’ll need to regularly check its heat exchanger. It’s susceptible to soot and carbon formation, which need to be cleaned. If it’s left uncleaned, your oil furnace may suffer from reduced efficiency.

Key Takeaway

“How does a furnace work?” isn’t a question you’d find yourself asking a lot. However, if you plan on installing a furnace in your home, it’s important to know the nitty-gritty of your appliances.

Now that you know how a furnace works and its different types, you can choose the heating unit that works best for your home.

We have discussed cold air return furnaces here.