Updated August 3, 2023

Two-Stage Furnaces: What Are They and How Do They Work?

There are a few important factors to consider when replacing or installing a new furnace. This includes the size of the unit and its AFUE (annual fuel utilization efficiency), both of which will impact how effectively the furnace heats and how much energy it uses. Another important consideration is whether the furnace is single-stage or two-stage. Although they are more expensive, two-stage furnaces are a fantastic option that can help reduce your heating costs and improve your comfort. To understand how, let’s look at exactly what a two-stage furnace is and how it works.

Single-Stage vs. Two-Stage Furnaces

Most furnaces are single-stage, which means that they only have two settings—on and off. A single-stage furnace always operates at 100% power whenever it is heating. This is great for extremely cold days, but often you don’t actually need that much heat. On milder days, there is really no reason that the furnace needs to run at full power, and the same is usually true on colder days once the building is sufficiently heated.

Two-stage furnaces are different in that they can run at both full and low power. On most units, the lower setting is usually around 60 or 70% power. To achieve this, the furnace uses a three-way gas valve. Single-stage furnaces have a two-way gas valve that is always either fully open or fully closed. A three-way valve has an additional setting where the valve is only partially open. This reduces the amount of gas that can flow into the unit and the amount of heat the furnace produces.

When the furnace runs, it will always start up in the low setting. If the indoor air temperature is only a few degrees below what the thermostat is set to, the furnace will continue to run in the lower setting until the temperature is sufficiently raised. On extremely cold days or if the temperature is far below the thermostat setting, the furnace will switch to run at full power until the home is fully warm. After that, it will typically only ever run at the lower setting for the rest of the day.

When running at the lower setting, the furnace will burn much less gas. This can be hugely beneficial on milder days when your heating may only need to run a few times. In these situations, a two-stage furnace will typically use around 30 to 40% less energy than a single-stage unit would.

Another advantage of two-stage furnaces is that they tend to be much quieter. Most of the noise you hear when your furnace is heating comes from the gas being combusted. The less gas the unit burns at one time, the less noise it will make.

Two-Stage vs. Modulating Furnaces

There is another type of unit known as a modulating furnace that is even more energy efficient than a two-stage unit. Modulating furnaces require a special blower motor that has the ability to run at different speeds, and the furnace itself also has multiple power settings. Whenever the heating system runs, both the modulating furnace and the fan are constantly adjusting these settings to heat as consistently and work as efficiently as possible.

How Two-Stage Furnaces Can Reduce Heating Costs and Improve Comfort

By reducing the gas flow and slowing the rate of heating, a two-stage furnace will both reduce energy costs and provide more consistent indoor temperatures. Instead of only ever running for a short time at full power, the heating system will always run for longer while using less energy. This can help to overcome issues with cold spots in the home and ensure that the temperature remains consistent throughout the entire building.

Single-stage furnaces often cause major temperature fluctuations throughout the day. If the unit only ever runs for a short period of time, it often won’t fully heat some parts of the home before it shuts off. The only way to overcome this is to turn up the thermostat, which can result in other parts of the home quickly getting too hot.

A two-stage furnace can overcome these problems by running for longer times instead of constantly turning on and off. This also reduces the strain on the furnace and is why two-stage furnaces typically have a longer lifespan. The furnace and the blower fan both work much harder when first starting than they do when running, and turning on and off frequently leads to more wear and tear and can cause the furnace to break down and need to be repaired more often.

With a two-stage furnace, you can often set your thermostat a few degrees lower than you otherwise since the temperature won’t fluctuate nearly as much. By doing so, you can reduce your energy costs even more.

How a Two-Stage Furnace Can Contribute to Better Indoor Air Quality

Another major advantage of two-stage furnaces is that they make your heating system much more effective at filtering out the air inside your home. Whenever your furnace and fan are running, air is constantly drawn into the system through the HVAC air filter. The main purpose of filtering the air is to trap any dust and debris so that it can’t get inside and damage the furnace and blower. However, the filter also works to reduce the amount of pollen, pet dander, mold spores, dust mites, and other airborne allergens and pollutants.

If the furnace only ever runs for a few minutes at a time, it won’t have that much of an effect on your air quality. Since a two-stage furnace will run for longer periods, more air will be drawn through the filter, and your indoor air quality will be greatly improved. This can be especially beneficial for anyone with asthma, severe allergies, COPD, or other respiratory ailments and breathing issues.

Of course, this does mean that your air filter will get dirty more quickly, and you’ll need to replace it more often. Still, that is a small price to pay considering all of the other important benefits that a two-stage furnace will provide. In fact, the cost of a few additional air filters a year is nothing compared to the potential energy savings the furnace will provide.

Commerce City, Home Heating Experts

At Smith & Willis, we specialize in furnace installation and can help you understand all of your options so you can choose what is best for your home. We install a wide selection of energy-efficient furnaces from only the top brands, and we also install air conditioners, boilers, and ductless AC units. If you’re having issues with your existing heating or cooling system, our team repairs and services HVAC equipment from all manufacturers. To learn more about the benefits of upgrading to a two-stage furnace, give the team at Smith & willis a call today.

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