A variable-speed gas furnace is one of the best choices for homeowners who want quieter heating, more consistent indoor temperatures and better airflow control. Unlike a conventional furnace with a basic fixed-speed blower, a variable-speed furnace can continuously adjust blower airflow to match the system’s operating requirements.
It is important to understand one distinction before comparing models: variable-speed blower and variable-capacity heating are not the same thing. A furnace can have a variable-speed blower with single-stage or two-stage heating, while premium models such as the Lennox SLP99V and Carrier Infinity 98 combine variable-speed blowers with modulating heat output.
For homeowners seeking the best overall combination of comfort, efficiency and advanced operation, our leading choices are the Carrier Infinity 98 and Lennox SLP99V. The Carrier reaches up to 98.5% AFUE, while the Lennox reaches up to 99% AFUE. Both combine variable-speed blower operation with modulating heating.
Best Variable-Speed Gas Furnaces: Our Top Picks
| Furnace | Heating Type | Blower | Maximum AFUE | Best For |
|---|---|---|---|---|
| Carrier Infinity 98 59MN7 | Modulating | Variable-speed | 98.5% | Best overall |
| Lennox SLP99V | Modulating | Variable-speed | 99% | Maximum efficiency |
| Carrier Infinity 96 59TN6 | Two-stage | Variable-speed | 96.7% | Best two-stage option |
| Lennox EL297V | Two-stage | Variable-speed | Up to 97% | High-efficiency value |
| Lennox EL296V | Two-stage | Variable-speed | Up to 96% | Best value |
Important: Maximum AFUE can vary by furnace size, configuration and installation. Always verify the exact model number and matched system before purchasing.
What Is a Variable-Speed Gas Furnace?
A variable-speed gas furnace uses a blower motor capable of operating across a range of airflow rates rather than simply running at one predetermined speed.
The blower can adjust airflow according to the furnace’s heating stage, cooling demand, thermostat requirements and other system conditions.
This can produce a noticeably different comfort experience from a basic fixed-speed furnace.
Variable-speed blower vs. variable-capacity furnace
These terms are often incorrectly used interchangeably.
| Technology | What It Controls | Example |
|---|---|---|
| Fixed-speed blower | Airflow at limited preset speed(s) | Basic furnace |
| Constant-torque blower | More controlled airflow | Mid-range furnace |
| Variable-speed blower | Continuously adjustable airflow | Premium furnace |
| Two-stage gas valve | Two heating outputs | Mid/premium furnace |
| Modulating gas valve | Many heating outputs | Premium furnace |
A furnace can therefore be two-stage + variable-speed or modulating + variable-speed.
Why Variable-Speed Blowers Matter
The blower is responsible for moving heated air through the home’s ductwork. If the blower only operates at one high speed, the furnace may deliver a large burst of airflow whenever it runs.
A variable-speed blower can instead ramp airflow up and down gradually.
Potential benefits include:
- Quieter startup
- More consistent airflow
- Improved temperature distribution
- Longer, gentler heating cycles
- Better continuous-fan operation
- Improved compatibility with advanced HVAC controls
- Potentially better summer dehumidification when matched with compatible air-conditioning equipment
Carrier specifically describes the variable-speed blower in its Infinity 98 as part of its comfort system and says it can also enhance air-conditioning and summer dehumidification.
1. Carrier Infinity 98 59MN7 — Best Overall Variable-Speed Furnace
The Carrier Infinity 98 59MN7 is our top overall pick for homeowners who want premium variable-speed operation.
Carrier rates the furnace at up to 98.5% AFUE. More importantly, it combines a variable-speed blower with a modulating gas valve. Carrier says the gas valve can adjust output in 1% increments, allowing the furnace to match heating output more closely to the home’s demand.
This is a major step above a furnace that simply has a variable-speed blower but a single-stage burner.
Key features
- Up to 98.5% AFUE
- Modulating gas valve
- Variable-speed blower
- Infinity System Control compatibility
- Premium temperature control
- Humidity-control capabilities
- Insulated cabinet for quieter operation
- ENERGY STAR certified configurations
Carrier describes the system as capable of operating longer at lower capacities, which can help reduce temperature swings and improve comfort.
Pros
- Excellent efficiency
- Very precise heating control
- Variable-speed airflow
- Excellent comfort potential
- Strong humidity-control capabilities
- Premium controls
Cons
- Premium purchase price
- More sophisticated electronics
- Best performance may require compatible Carrier controls
- Installation and commissioning are particularly important
Best for: Homeowners who want premium comfort and sophisticated heating control without necessarily choosing the absolute highest AFUE available.
2. Lennox SLP99V — Best for Maximum Efficiency
The Lennox SLP99V is arguably the most compelling choice for homeowners whose priority is maximum furnace efficiency.
Lennox lists the SLP99V at up to 99% AFUE. It combines a variable-speed motor with modulating heating and is ENERGY STAR qualified.
Unlike a conventional two-stage furnace, the SLP99V can continuously vary its heating output. Lennox also emphasizes its low-noise operation and precise temperature control.
Key features
- Up to 99% AFUE
- Modulating heating
- Variable-speed blower
- ENERGY STAR qualified
- Enhanced humidity control
- Insulated blower compartment
- Ultra-low continuous fan capability
- Dual-fuel capability
Lennox’s current specifications identify the SLP99V as a variable-capacity, modulating furnace with a variable-speed motor and enhanced humidity control.
Pros
- Extremely high efficiency
- Excellent modulation
- Very quiet operation
- Excellent temperature consistency
- Variable-speed airflow
- Strong premium-system integration
Cons
- Expensive
- Complex electronics and controls
- May have limited availability depending on market
- Requires a qualified installer
Best for: Cold-climate homeowners with high annual heating demand who want maximum efficiency and premium comfort.
3. Carrier Infinity 96 59TN6 — Best Two-Stage Variable-Speed Furnace
If you want variable-speed airflow but do not need a fully modulating burner, the Carrier Infinity 96 59TN6 is a strong alternative.
Carrier lists the furnace at up to 96.7% AFUE with two-stage heating and a variable-speed blower.
This combination gives homeowners many of the comfort advantages associated with premium variable-speed systems while avoiding the additional complexity of a fully modulating furnace.
Why choose it?
- Two-stage heating
- Variable-speed blower
- Up to 96.7% AFUE
- Premium Carrier control compatibility
- Strong temperature-control potential
- Excellent choice for cold and moderate climates
Best for: Homeowners who want a premium variable-speed system but prefer two-stage heating over fully modulating operation.
4. Lennox EL297V — Best High-Efficiency Value
The Lennox EL297V is another strong option for homeowners who want variable-speed airflow without moving into the company’s most expensive furnace.
The furnace combines two-stage heating with a variable-speed motor and reaches up to approximately 97% AFUE depending on configuration.
That makes it a particularly attractive middle ground between a basic furnace and a premium modulating system.
Best features
- Two-stage heating
- Variable-speed blower
- Up to 97% AFUE
- High-efficiency condensing design
- Strong comfort potential
Best for: Homeowners who want high efficiency and variable-speed airflow without paying for a flagship modulating furnace.
5. Lennox EL296V — Best Value Variable-Speed Furnace
The Lennox EL296V is particularly interesting for homeowners who want variable-speed airflow while keeping equipment cost below the premium Signature Collection level.
Lennox rates the EL296V at up to 96% AFUE and pairs two-stage heating with variable-speed blower operation.
For many homes, that is an excellent specification combination. You get two-stage heating, variable-speed airflow and high efficiency without paying for a 98%–99% modulating furnace.
Best for
- Moderate and cold climates
- Homeowners focused on value
- Homes where comfort matters more than absolute maximum AFUE
- Replacement projects where a premium modulating furnace is unnecessary
Variable-Speed vs. Two-Stage Furnace
This is one of the most common points of confusion.
Variable-speed normally describes the blower motor. Two-stage describes the gas burner.
A furnace can have both.
| Configuration | Comfort | Efficiency Potential | Cost |
|---|---|---|---|
| Single-stage + fixed-speed | Basic | Basic to good | $ |
| Two-stage + constant-torque | Good | Good | $$ |
| Two-stage + variable-speed | Very good | Very good | $$–$$$ |
| Modulating + variable-speed | Excellent | Excellent | $$$ |
Variable-Speed vs. Modulating Gas Furnace
A variable-speed blower does not necessarily mean the furnace itself is modulating.
For example, a two-stage furnace can have a variable-speed blower. The blower can adjust airflow continuously while the burner still has only two primary firing levels.
A modulating furnace goes further by continuously adjusting the burner output.
The Carrier Infinity 98 and Lennox SLP99V combine both technologies. Carrier’s 59MN7 modulates its heating output in 1% increments, while Lennox identifies the SLP99V as a modulating, variable-speed furnace.
Benefits of Variable-Speed Gas Furnaces
1. Better temperature consistency
Variable-speed airflow allows the furnace to move air more gradually instead of delivering the same large airflow burst every time.
2. Quieter operation
The blower can ramp up and down rather than abruptly switching between off and high airflow.
Lennox specifically markets the SLP99V’s variable-speed operation as a major contributor to quiet operation.
3. Better air mixing
Longer, gentler airflow can help distribute heat throughout the home more evenly.
4. Improved continuous circulation
Variable-speed blowers can operate at very low airflow levels when the homeowner wants continuous air circulation.
5. Better cooling performance
The furnace blower also moves air when the air conditioner operates. Variable-speed technology can therefore influence the cooling side of the HVAC system.
6. Better humidity management
When properly matched with compatible cooling equipment and controls, longer airflow cycles can help improve summer humidity management.
Are Variable-Speed Furnaces More Efficient?
They can improve overall system operation, but it is important not to confuse blower efficiency with furnace AFUE.
AFUE measures the furnace’s annual fuel utilization efficiency. Variable-speed blower technology primarily affects airflow and comfort.
A variable-speed blower by itself does not turn an 80% furnace into a 96% furnace.
For the best overall performance, look for the combination of:
- High AFUE
- Two-stage or modulating heat
- Variable-speed blower
- Correct sizing
- Proper ductwork
- Compatible controls
Best Variable-Speed Furnace for Cold Climates
Homeowners in northern states typically have the strongest economic reason to consider premium variable-speed furnaces because the equipment may operate for many hours during the heating season.
A 97%–99% AFUE variable-speed furnace can be attractive when annual gas consumption is high.
Good candidates include:
- Carrier Infinity 98
- Lennox SLP99V
- Carrier Infinity 96
- Lennox EL297V
The final decision should be based on the home’s heating load, local gas prices, installation price and expected annual heating use.
Best Variable-Speed Furnace for the Southern USA
In warmer parts of the United States, homeowners may not recover the premium cost of a 98%–99% furnace as quickly because the furnace operates for fewer hours annually.
A 95%–97% variable-speed furnace may provide a better balance between upfront cost and comfort.
In these climates, the variable-speed blower can still be valuable because it can improve airflow and cooling comfort even though annual heating demand is comparatively low.
Best Variable-Speed Furnace for a Two-Story Home
Variable-speed furnaces can be particularly useful in multi-story homes because they can provide more controlled airflow.
However, a variable-speed furnace cannot completely solve poor duct design.
If one floor is consistently too hot or cold, investigate:
- Duct balancing
- Return-air capacity
- Insulation
- Air leakage
- Zoning
- Thermostat placement
- Furnace sizing
A high-end furnace installed on an inadequate duct system may still provide disappointing comfort.
Does a Variable-Speed Furnace Need a Special Thermostat?
It depends on the furnace.
Some variable-speed furnaces can operate effectively with conventional controls, while premium communicating systems may achieve their full capabilities only with the manufacturer’s compatible thermostat or system controller.
For example, Carrier’s Infinity 98 uses its Infinity System Control to manage its modulating heating capability, while Lennox integrates the SLP99V into its communicating comfort ecosystem.
Before purchasing, ask the contractor:
- Which thermostat is included?
- Is the thermostat communicating?
- Will the furnace modulate automatically?
- Can the blower operate at low continuous speed?
- Will the thermostat control both heating and cooling?
Variable-Speed Furnace and Air Conditioning
One of the major advantages of variable-speed furnace technology is that the same blower is generally used for heating and cooling.
This means that upgrading the furnace can also influence summer comfort.
Potential advantages include:
- More controlled airflow
- Quieter cooling operation
- Better air mixing
- Improved dehumidification potential
- Better integration with communicating air conditioners
Carrier specifically notes that the variable-speed blower in the Infinity 98 can improve air-conditioning efficiency and summer dehumidification.
Do Variable-Speed Furnaces Save Money?
They can, but the financial calculation depends on the entire system.
There are two separate questions:
- Does variable-speed airflow improve comfort and system operation?
- Does the additional equipment cost pay for itself through lower energy consumption?
The answer to the first question is often yes when the system is properly designed.
The second question depends on climate, fuel prices, heating demand, equipment efficiency, electricity consumption, installation cost and how long the homeowner plans to keep the furnace.
Do not assume that buying the most expensive variable-speed furnace will automatically produce the lowest lifetime cost.
How Much Does a Variable-Speed Gas Furnace Cost?
Installed prices vary considerably across the United States, so a single national price is often misleading.
The total project can include:
- Furnace
- Thermostat
- Installation labor
- Venting
- Condensate drainage
- Gas-line modifications
- Electrical work
- Duct modifications
- Permits
- Removal of old equipment
- Startup and commissioning
Premium variable-speed and modulating furnaces generally cost more than single-stage systems because of their more sophisticated blower motors, controls and gas valves.
Variable-Speed Furnace Installation
Installation quality is critical.
A variable-speed furnace needs to be configured correctly for the home’s duct system and matched equipment.
A professional installation should address:
- Heating-load calculation
- Furnace capacity
- Static pressure
- Airflow
- Gas pressure
- Combustion
- Vent configuration
- Condensate drainage
- Thermostat setup
- Blower programming
- System commissioning
Do not accept a quote that simply says “install furnace” without specifying what is included.
Why Furnace Sizing Is So Important
A variable-speed blower does not eliminate the consequences of an oversized furnace.
Although premium modulating systems can reduce the impact of oversizing by operating at low output, proper sizing remains essential.
An HVAC contractor should calculate the home’s heating load rather than simply choosing a furnace based on square footage.
Problems caused by incorrect sizing
- Short cycling
- Uneven temperatures
- Excessive energy consumption
- Reduced comfort
- Higher component cycling
- Difficulty maintaining stable indoor temperatures
Variable-Speed Furnace Maintenance
Variable-speed systems should receive regular professional maintenance just like conventional gas furnaces.
- Replace filters regularly.
- Keep return-air grilles clear.
- Keep supply registers unobstructed.
- Inspect the blower assembly.
- Check electrical connections.
- Inspect the heat exchanger.
- Check combustion and venting.
- Clean condensate drains on condensing systems.
- Verify airflow and static pressure.
- Check thermostat and control operation.
Variable-speed blower motors are designed for long-term operation, but they still depend on clean filters and proper airflow. A severely restricted filter can increase static pressure and put unnecessary strain on the HVAC system.
Are Variable-Speed Furnaces Reliable?
Modern variable-speed furnaces can be highly reliable when properly installed and maintained. However, they contain more electronics and control components than basic single-stage systems.
This creates a trade-off:
| Characteristic | Basic Furnace | Variable-Speed Premium Furnace |
|---|---|---|
| Controls | Simple | Advanced |
| Blower | Fixed-speed | Variable-speed |
| Comfort | Good | Excellent potential |
| Complexity | Low | High |
| Repair diagnosis | Generally simpler | Potentially more complex |
| Upfront cost | Lower | Higher |
That is why local contractor expertise matters. A premium furnace installed by a contractor who regularly services that particular brand can be a better long-term choice than a technically impressive system that local technicians rarely encounter.
Variable-Speed Furnace Warranties
Always examine the warranty for the exact model rather than relying on a brand-wide warranty statement.
Pay particular attention to:
- Heat exchanger warranty
- Parts warranty
- Labor warranty
- Registration requirements
- Transferability
- Warranty exclusions
- Requirements for professional installation
For example, Lennox currently states that eligible Dave Lennox Signature Collection models can receive 10-year covered-component parts protection after registration, with heat exchanger coverage depending on the specific terms.
Carrier similarly provides specific warranty terms for eligible equipment and requires timely registration for certain coverage.
Common Variable-Speed Furnace Buying Mistakes
1. Assuming variable-speed means variable-capacity
A variable-speed blower does not necessarily mean the burner can modulate.
2. Buying the highest AFUE
A 99% furnace may not provide the best financial return in a mild climate.
3. Ignoring the duct system
Variable-speed technology cannot compensate for badly designed or severely restricted ducts.
4. Choosing an oversized furnace
Correct sizing remains essential.
5. Ignoring thermostat compatibility
Some premium systems require communicating controls to unlock their full capabilities.
6. Comparing equipment prices instead of installed prices
One contractor’s quote may include extensive modifications while another’s may not.
7. Choosing a brand without checking local service
Local parts availability and technician experience can be more important than small differences between major brands.
How to Choose the Best Variable-Speed Gas Furnace
- Calculate your home’s heating load.
- Determine the appropriate furnace capacity.
- Choose the desired AFUE range.
- Decide between two-stage and modulating heating.
- Look for a genuine variable-speed blower.
- Check thermostat compatibility.
- Evaluate the duct system.
- Compare complete installed prices.
- Review warranty terms.
- Choose an experienced local installer.
Who Should Buy a Variable-Speed Gas Furnace?
A variable-speed furnace is particularly attractive if you:
- Want quieter heating.
- Experience uneven temperatures.
- Want better airflow control.
- Have a long heating season.
- Want premium comfort.
- Want better integration with a modern air conditioner.
- Plan to stay in the home for many years.
- Are replacing both the furnace and air conditioner.
Who Should Not Pay Extra for One?
A variable-speed furnace may not be the best value if:
- You are primarily concerned with the lowest upfront price.
- Your heating season is extremely short.
- You plan to sell the home soon.
- Your existing duct system is poorly designed and will not be corrected.
- There is no local contractor capable of servicing the selected system.
Best Variable-Speed Gas Furnace by Category
| Category | Best Pick | Why |
|---|---|---|
| Best Overall | Carrier Infinity 98 59MN7 | Excellent combination of modulating heat and variable-speed airflow |
| Highest Efficiency | Lennox SLP99V | Up to 99% AFUE with modulating and variable-speed operation |
| Best Two-Stage | Carrier Infinity 96 59TN6 | Two-stage heating with variable-speed blower |
| Best High-Efficiency Value | Lennox EL297V | Two-stage and variable-speed operation with up to 97% AFUE |
| Best Value | Lennox EL296V | Two-stage variable-speed system at a lower tier |
Frequently Asked Questions
What is the best variable-speed gas furnace?
The Carrier Infinity 98 and Lennox SLP99V are among the strongest premium choices. Both combine variable-speed blower operation with modulating heat. Carrier lists the Infinity 98 at up to 98.5% AFUE, while Lennox lists the SLP99V at up to 99% AFUE.
Is a variable-speed furnace better than a two-stage furnace?
They are not mutually exclusive. A two-stage furnace can have a variable-speed blower. In fact, two-stage heating plus variable-speed airflow is an excellent combination for many homes.
Does variable-speed mean the furnace is modulating?
No. Variable-speed generally describes the blower motor. Modulating describes the gas valve and heating output.
Is a 99% furnace worth the money?
It can be, particularly in cold climates with high annual heating demand. In a mild climate, the additional purchase cost may take much longer to recover.
Are variable-speed furnaces quieter?
They can be. Variable-speed blowers can ramp airflow more gradually, reducing abrupt changes in airflow. Premium manufacturers specifically design their variable-speed systems for quieter operation.
Do variable-speed furnaces save electricity?
The answer depends on the motor, operating pattern and system design. Variable-speed motors can provide efficient airflow control, but total household energy savings depend on the entire HVAC system.
Can a variable-speed furnace improve air conditioning?
Yes. The furnace blower typically moves the air during cooling as well. A properly matched variable-speed blower can improve airflow control and may assist with dehumidification. Carrier specifically highlights this capability for the Infinity 98.
Are variable-speed furnaces more expensive to repair?
They can be more expensive to diagnose or repair because they use more sophisticated electronics, controls and blower technology. However, this does not mean they are inherently unreliable.
What AFUE should I choose?
For many U.S. homeowners, 95%–97% is an excellent balance. Cold-climate homeowners with substantial heating demand may consider 97%–99% models.
What is better: two-stage variable-speed or modulating variable-speed?
Modulating variable-speed systems offer more precise control, but two-stage variable-speed furnaces can provide an excellent comfort-to-cost ratio. For many homeowners, two-stage plus variable-speed is the sweet spot.
Does a variable-speed furnace require a variable-speed air conditioner?
No. However, the best comfort and efficiency results generally come from properly matched HVAC components. Ask the contractor to provide the exact matched furnace, coil, outdoor unit and control combination.
Final Verdict
Variable-speed gas furnaces are among the most advanced residential forced-air heating systems available in the U.S. They can provide quieter operation, better airflow control and more consistent comfort than basic fixed-speed systems.
For the best overall premium system, the Carrier Infinity 98 59MN7 is an excellent choice. Carrier lists it at up to 98.5% AFUE, with a modulating gas valve and variable-speed blower.
For maximum furnace efficiency, the Lennox SLP99V stands out with up to 99% AFUE, modulating heat and variable-speed operation.
If you want a two-stage system rather than full modulation, the Carrier Infinity 96 and Lennox EL297V are compelling options.
For value, a 95%–96% two-stage furnace with a variable-speed blower can often deliver most of the practical comfort benefits without the cost of a flagship 98%–99% modulating system.
Above all, don’t choose a variable-speed furnace solely because the specification sounds impressive. Correct sizing, ductwork, thermostat compatibility, commissioning and installer quality determine how well the system performs in your actual home.
Manufacturer Resources
Carrier Infinity 98 59MN7 — current manufacturer specifications.
Lennox SLP99V — current manufacturer specifications.
Carrier Furnace Buyer’s Guide — Carrier’s current furnace lineup and buying information.
About the Author
ManBearPig is a part-man, part-bear, part-pig creature and a full-time expert on global warming—mostly because he has personally experienced the consequences of being extremely, extremely, super cereal.
When he isn’t roaming the wilderness, terrorizing unsuspecting humans, or demanding that people take climate change seriously, ManBearPig studies rising temperatures, melting ice, extreme weather, carbon emissions, and other inconvenient signs that the planet may not be particularly happy with us.
His research philosophy is simple:
“I’m super cereal.”
ManBearPig believes climate change is too important to discuss without facts—and too ridiculous to discuss without a little humor. Through this website, he explores global warming, climate science, environmental changes, energy, and the strange ways humans respond when their own planet starts sending them warning signals.
He has no university degree, no laboratory, and absolutely no intention of becoming a respectable academic.
But he does have hooves, claws, a snout, and an unusually strong opinion about melting ice.