What is a Two-Stage Furnace and How Does It Work?

How Does a Two-Stage Furnace Operate?

A Carrier two-stage furnace uses a two-stage gas valve to provide a lower and higher level of heat output, allowing the furnace to adjust its operation to the heating demand. Carrier two-stage models may also use a variable-speed blower motor to deliver controlled airflow. For example, the Carrier Infinity® 96 Gas Furnace (59TN6) combines two-stage heating with a variable-speed blower and operates at up to 96.7% AFUE.

When you set your thermostat,  the two-stage furnace responds to the home's heating demand and can begin operation in its lower stage. On a typical winter day, the system will start in its first stage. Because it runs at a lower capacity, it operates for longer, slower cycles. This steady flow of warm air eliminates the harsh temperature fluctuations commonly associated with older heating systems.

If the outdoor temperature suddenly plummets or you manually increase the thermostat setting by several degrees, the furnace can transition to its higher stage when additional heating capacity is needed. It can then provide more heat to bring the home to the desired temperature. Once the target temperature is reached, the system can return to its lower stage or shut off, depending on the home's heating demand.

Carrier's Infinity® 97 Gas Furnace (59TN7) is another example of a two-stage Carrier furnace. It offers up to 97% AFUE, a variable-speed blower motor and two-stage heating, with low-stage operation up to 90% of the time.

What Are the Main Benefits of a Two-Stage Furnace?

The primary benefits of Carrier two-stage furnaces can include more consistent indoor comfort, quieter operation and improved efficiency compared with single-stage operation. By operating at a lower stage when full heating capacity isn't required, a two-stage furnace can provide longer heating cycles and more gradual temperature changes. Carrier's current furnace lineup describes two-stage models as providing enhanced temperature control and quieter operation.

• Consistent Indoor Comfort: One of the most noticeable advantages is the potential for fewer temperature swings throughout the house. Because Carrier two-stage furnaces can operate at a lower heating output when demand is lower, they can provide longer heating cycles and more gradual temperature changes.
• Quieter Operation: Traditional furnaces can be louder when they start at maximum capacity. Carrier two-stage furnaces are designed to operate more quietly, particularly when operating at their lower stage. Models such as the Carrier Infinity 96 Gas Furnace include a fully insulated cabinet and variable-speed blower for quiet performance. 
• Enhanced Air Filtration: Longer furnace run times can provide more opportunities for air to pass through the home's furnace filter. However, actual filtration performance depends on the filter, blower operation and overall HVAC system. Carrier's Infinity® 96 and Infinity® 97 furnaces also offer Ideal Humidity System™ technology, which can help manage indoor humidity and enhance home comfort.

Brad Gooch, HVAC manager of Lawson Air Conditioning and Plumbing in Gainesville, Georgia, explains when a two-stage furnace may make sense for a homeowner.

“Here in Northeast Georgia, we have a lot of days when it's 45 or 50 degrees outside, along with plenty of days when temperatures drop to around 20 degrees. A two-stage furnace gives homeowners more flexibility to match the system's output to the heating demand.

“When the home doesn't need the furnace's full heating capacity, a two-stage furnace can operate at a lower stage instead of running at full capacity. That can help provide longer, more consistent heating cycles, reduce temperature swings and potentially use less fuel.

“This can be particularly valuable in rural areas where homeowners rely on propane instead of natural gas. When you're paying for propane, being able to operate the furnace at a lower capacity when heating demand is lower can potentially help reduce fuel consumption.”

Single-Stage vs. Two-Stage Furnace: Which is Better?

Carrier two-stage furnaces provide an additional level of heating control compared with single-stage furnaces, which operate at one heating output. A single-stage furnace operates at full capacity when it is heating, while a two-stage furnace can use a lower or higher stage depending on heating demand. Carrier notes that two-stage furnaces can provide better comfort and efficiency in climates with variable winter temperatures.

To make an informed decision, it is essential to understand the trade-offs:

• Single-Stage Furnaces are better suited for homeowners with a strict upfront budget, or for those living in very mild climates where the heating system is rarely used. They are simpler in design but provide less flexibility in how much heat the furnace delivers during a heating cycle.
• Two-Stage Furnaces can be a good fit for homeowners who prioritize more consistent comfort, quieter operation and the potential for improved efficiency. Carrier offers two-stage furnaces across its Infinity® and Performance™ Series, including the Infinity 96 Gas Furnace (59TN6), Infinity 97 Gas Furnace (59TN7), Performance 96 Gas Furnace (59TP6) and Performance 97 Gas Furnace (59TP7).

Carrier's Performance™ 96 Gas Furnace (59TP6) offers up to 96.7% AFUE with two-stage heating and a variable-speed blower motor, while the Performance 97 Gas Furnace (59TP7) offers up to 97% AFUE with two-stage heating and a variable 25-speed blower.

“A single-stage furnace can still be a good option for many homeowners, particularly when upfront cost is a priority,” Gooch said. “But if you're looking for improved comfort, quieter operation and the potential for greater fuel efficiency, a two-stage furnace is worth considering.”

When is a Two-Stage Furnace the Best Choice for Your Home?

A Carrier two-stage furnace may be a good choice if your home experiences variable heating demands, uneven temperatures or if you want the added comfort and control of two heating stages. Carrier specifically describes two-stage furnaces as a good fit for climates with variable winter temperatures.

If your home features multiple levels, you may struggle with the upper floors being too warm while downstairs and the basement remain colder. Longer, lower-output heating cycles can help distribute heat more gradually, although ductwork, insulation, system sizing and zoning also affect temperature differences throughout a home.

Furthermore, if you live in a region where winter temperatures fluctuate from day to day, a two-stage furnace can adjust its heating output to better match changing demand.

How Do Two-Stage Furnaces Improve Indoor Air Quality?

Carrier two-stage furnaces can support indoor air quality by providing longer heating cycles and, in some models, advanced humidity-control technology. Because the system can operate at a lower stage when heating demand is lower, air may circulate through the furnace filter for longer periods.

Carrier Infinity 96 and Infinity 97 furnaces feature Ideal Humidity System™ technology, which can help manage indoor humidity and improve overall home comfort.

When a furnace runs for longer periods, more air can pass through the home's filter. The amount of filtration depends on the filter type, blower operation and how the complete HVAC system is configured. For households concerned about airborne particles, pairing a compatible furnace with an appropriately selected and maintained filter can help support indoor air quality.

Are Two-Stage Furnaces Energy Efficient and Cost-Effective?

Carrier two-stage furnaces are designed to improve heating efficiency by operating at a lower stage when full heating capacity isn't required. Actual energy savings depend on the furnace model, AFUE rating, climate, home characteristics, installation and how the system is operated.

For example, the Carrier Infinity 96 Gas Furnace offers up to 96.7% AFUE, while the Carrier Infinity 97 Gas Furnace offers up to 97% AFUE. Both are two-stage models that can operate in low stage up to 90% of the time.

While it may seem counterintuitive that a system running for longer periods can save energy, operating at a lower heating output when full capacity isn't needed can help provide more efficient, consistent heating. Carrier describes its two-stage furnaces as operating on the lower stage most of the time and adjusting to high stage when necessary.

Rather than assuming a fixed percentage of fuel savings, homeowners should compare the AFUE rating, furnace size, installation requirements and expected operating conditions of the models being considered.

Talk to your local Carrier dealer today to see if a Carrier two--stage furnace is right for you. They can recommend the best unit for furnace installation for your home.