Understanding Variable Speed and Two-Stage Furnaces: Mastering Modern Heating

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Understanding Variable Speed and Two-Stage Furnaces: Mastering Modern Heating

SEPTEMBER 21, 2023

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Variable speed and two-stage furnaces are becoming the preferred choice for many homeowners. While these terms might sound complex, their concepts are simple yet effective. Dive in to explore the numerous benefits and the mechanics behind this modern HVAC solution.

Under The Hood: What’s a Variable Speed Furnace and Air Handler?

“Variable Speed” primarily addresses the blower motor inside the furnace or air handler. Central to its operation is the Electronically commutated motor (ECM). Unlike traditional AC motors, ECMs are DC motors with built-in inverters and a magnet rotor. This design facilitates exceptional efficiency.

The advantages of an ECM don’t stop at efficiency alone. They’re also known for their low maintenance requirements and energy-saving capabilities. Despite the initial investment being higher than standard furnaces, homeowners can typically expect a return on investment within five years.

Previous decades of furnaces were built with 1-speed, so it’s either on 100% or OFF. There are no variable speeds or lower RPMs, just ON or OFF.

How Does a Variable Speed Furnace Work?

Standard fan motors function at a fixed speed, but a variable-speed blower motor offers flexibility. It can operate at different speeds, ensuring optimal air control, whether heated or cooled. By leveraging advanced technology, the system perpetually assesses incoming data from your HVAC system, making automated adjustments to fulfill your comfort requirements.

One of its remarkable features is its ability to tweak the circulated air volume, especially when it encounters obstacles like blocked vents or dirty filters. It guarantees the proper air for your desired comfort level.

Additionally, these systems have a gentler start, gradually ramping up to their full speed. This eradicates the abrupt and noisy air bursts often associated with conventional furnace motors, ensuring your home remains a haven of peace. Not to forget, the reduced strain on the motor minimizes wear and tear.

Comparable to driving your car through town or on a highway. Is it efficient to just have 1st gear and speed light to light, starting and stopping? Or, is it more efficient to have multiple speeds to vary the RPMs and run longer at lower RPMs as we do on the highway?

Key Benefits of Variable Speed Furnaces

Variable speed heating systems excel in delivering precise airflow. This equates to superior temperature control, humidity management, and air distribution, thus elevating the comfort levels within a home.

When pitched against traditional single-speed furnaces, variable speed variants perform better and slash electricity consumption by about two-thirds. During cooling phases, they can amplify the system’s efficiency, saving homeowners hundreds of dollars annually.

The gradual operational ramp-up ensures minimal noise. Furthermore, when set to continuous operation, these systems persistently circulate air, enabling filters to trap more contaminants, resulting in a cleaner indoor environment.

Again, running longer at lower RPMs will save energy AND gas when the furnace calls for heat, but also save energy when the AC calls for cooling. Remember that the furnace motor not only blows heat in the winter but cool and conditioned air in the summer.

Understanding Two-Stage Furnaces

Two-stage furnaces elevate efficiency by operating differently than their single-stage counterparts. They’re designed to meet your home’s heating needs efficiently without always running at full throttle. This conserves energy, minimizes operational noise, and reduces carbon emissions.

In typical scenarios, the furnace operates in its first stage, leveraging about 65% of its total capacity. However, during extreme cold, the second stage activates to provide the requisite warmth. This dual-stage operation ensures consistent warmth distribution and minimizes temperature fluctuations.

For clarity, while “Variable Speed” indicates the blower motor’s air circulation through the ducts, “Two-Stage” pertains to the heat output regulated by the gas valve and burners.


With their myriad benefits spanning comfort, efficiency, noise reduction, and air quality enhancement, variable speed furnaces and air handlers are a worthy addition to modern homes to make them “Smarter, Healthier, and More Efficient.” For those contemplating a new furnace or heat pump, variable speed and two-stage furnaces are the gold standard in HVAC solutions.

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