Single Stage vs. Two Stage Furnace: What is the Difference?

What Are Heating Stages?

Before we dive into comparing single-stage and two-stage furnaces, let’s define what heating stages are first. 

In regards to furnaces, stages refer to the level of heating the furnace can provide. A single-stage furnace can only provide one level of heating, 100% on. In contrast, a two-stage unit can provide two stages of “on” – usually 70% and 100% on. 

To elaborate, a single-stage furnace can only provide heat at full blast. On the other hand, a two-stage unit can blow heat into your home at 70% of its maximum capacity at the first stage and 100% of its maximum capacity, i.e., full blast at the second stage.

There are also variable-speed or modulating furnaces that can vary their heating output anywhere between 40-100% at any time, providing even finer heat control. 

Top 5 Differences Between Single-Stage vs Two-Stage Furnace

Single-stand and two-stage furnaces are two of the most common furnaces available on the market. They’re often paired with air conditioning systems in homes to provide heating and cooling. 

While they both provide heat to homes through the combustion of natural gas, they do have a few differences in the way they deliver the heat. 

Here are the top 5 differences between them: 

Amount of Heat Control

Since two-stage furnaces basically have a low and high setting, they can provide a more stable temperature in the home by running on the low stage for a more extended period of time. 

They will run at 65 to 70% of full capacity, depending on brand and model, and will keep your living space free of cold spots. When they need to heat quickly, they can run at 100% or full capacity. 

On the other hand, single-stage furnaces can only run at the high setting or 100% capacity. When they are on, their blowers are running loudly at full blast. 

Additionally, since they do not run often, the home can have cold spots and have a temperature swing of up to 5°F before kicking back on!

In terms of comfort, the two-stage unit provides the best results. 

Efficiency

The efficiency of all furnaces is measured by the Annualized Fuel Utilization Efficiency or AFUE. AFUE is a measure of thermal efficiency of the furnace throughout the entire year, instead of at just the peak conversion efficiency. In basic terms, it is a measure of how much gas or electricity is fully transferred into usable heat during the heating season. 

These days, furnaces have a minimum of 80% AFUE, with some high-efficiency ones holding a 90%+ AFUE. A two-stage gas furnace is capable of achieving a higher efficiency than a single-stage unit. Here’s how they compare: 

One of the reasons that single-stage furnaces are less efficient is because they use 100% of their heating capacity every time they turn on. Full capacity is similar to you sprinting everywhere instead of walking; as you can imagine, this uses a lot more energy than a fast walk.

Two-stage furnaces are on for longer periods but instead run mainly at the low settings (65% or 70%). If we use the same analogy, two-stage furnaces walk fast most of the time and only sprint when they need to catch up. Therefore, they use less overall energy to heat the same amount of space. 

If you’re seeking out furnaces with even greater efficiencies than a two-stage unit, look into furnaces with modulating gas valves (also known as variable-capacity or variable-speed units). Lennox, Carrier, Trane, and other top furnace manufacturers make variable-speed furnaces with efficiencies ranging from 95% to almost 99%. 

Noise

The blowers in all types of furnaces create noise. When the blower motor of the single-stage furnace is running (at full power), it is noticeably louder than the slower speed setting on the two-stage furnace. 

The low setting of the two-stage unit is quieter, but it does run for longer. With that being said, furnace fans are not obstructively loud unless you’re right next to them. 

Air Filtration

Since two-stage furnaces run for a longer time, they pull more air through the ductwork. Therefore, more air also passes through the filter. Thus, the indoor air quality of a home with a two-stage furnace generally is usually better. 

A potential downside is that the air filter will have to be cleaned or replaced more often than a single-stage furnace. 

Humidity Control

A two-stage furnace provides more even and consistent temperature and humidity because they run more often at the lower setting. The single-stage unit runs at full blast each time it is on and tends to dry out the air faster. 

For optimal home humidity control, using a whole-house humidifier attached to your furnace can provide better home comfort in the winter. While whole-house humidifiers work well with both single-stage and two-stage furnaces, they offer better humidity control when paired with two-stage units since they run longer. 

What Are The Advantages of Single-Stage Furnaces?

Single-stage furnaces are less efficient than two-stage furnaces but don’t let that dissuade you from thinking they are completely inferior. They have outstanding performance and can achieve excellent AFUE ratings without the need for two stages. 

Here are some of the top advantages of single-stage furnaces: 

• Lower upfront cost
• Excellent AFUE ratings ranging from 80% to 95%
• Shorter heating cycle
• Less frequent fan noises since it doesn’t run as often as a two-stage unit 
• Simple thermostat operation since it is only on/off control

What Are The Advantages of Two-Stage Furnaces?

Two-stage furnaces primarily run at a lower than maximum capacity, 65% to 70%, depending on the manufacturer. Since it doesn’t run at full power each time it turns on, it uses less energy and will cost you less money than a single-stage system. 

Here are the advantages of a dual-stage furnace: 

• Better AFUE energy efficiency ratings, up to 97%
• More energy savings and lower energy costs
• Longer heat cycle with a more consistent, comfortable temperature
• Less cold spots and temperature fluctuations in the home
• Quieter when running at 65-70% capacity stage

Which is Cheapest: Single-Stage vs Two-Stage?

Single-stage furnaces are simplest in their mechanical design and are usually lower cost than two-stage units of the same heating capacity. 

However, two-stage units are only slightly more expensive, typically no more than a 10% price premium. For example, a 120,000 BTU 96% AFUE two-stage furnace costs on average $1,500 to $1,800 and a 120,000 BTU single-stage unit with an 80% AFUE costs between $1,000 to $1,200. 

In terms of average repair costs, the two-stage furnace has additional components to operate at two stages, resulting in additional repairs and costs if those components fail.

Since two-stage furnaces are more energy-efficient, the monthly heating costs tend to be lower than their single-stage counterparts. However, there are single-stage units that are rated up to 95% AFUE, which is on par with some lower-end two-stage units. 

Which Option is Best for My Climate?

In general, one-stage and two-stage units are great options for any type of climate with frigid winters. 

Neither one is “better” for this climate. Instead, single and two-stage units should be compared mostly by their efficiency and the type of heating the homeowner desires. For climates with mild winters, heat pumps are a better option, plus they can provide cooling too! 

If you prefer longer heating cycles with a more consistent temperature and slightly lower heating bills, choose a two-stage furnace. Alternatively, if you prefer a lower upfront cost, simpler heating control, and only a marginal increase in energy bills, go with a single-stage unit. 

Again, both single-stage and two-stage furnaces work well for climates with cold winters that are frequently below 30°F. However, unlike heat pumps, they don’t have the ability to provide cooling. As such, both types of furnaces have to be used with air conditioners. 

Any Furnace Type Should Be Professionally Installed

Furnace installation is no easy project. If you’re considering a single-stage or two-stage unit for your home, don’t plan the install as a “weekend DIY project.” Instead, hire an HVAC professional to ensure it is done right. 

No matter what type of furnace you select, installing it is a complex process. It requires the extensive knowledge, skills, and training of an experienced professional. 

Whether you’re just looking for a new furnace, or need help with an installation, give our team of skilled HVAC technicians a call today!