Single Stage vs. Two Stage Furnaces

HVAC Technician Looking at Gas Furnace
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An energy efficient furnace keeps you comfortable without running up high heating bills. Because the number of stages your furnace can operate in affects both its efficiency and its performance, it’s an important factor to consider when upgrading your heating equipment.

A basic single-stage furnace is a reasonable choice if you’re on a budget, especially if you live in a mild climate. If you’re in a cold-winter climate and want to keep your heating costs down, a two-stage furnace might be a better way to go.

Single-Stage Furnaces: Low-Cost Equipment for Basic Home Heating

In furnace terminology, “stages” refers to the levels of heat output a furnace can provide. A single-stage furnace, the most basic design available, has a fixed gas valve and a single-speed blower fan motor. This furnace has only two settings: off and high. As soon as the furnace turns on, it starts running at full blast. Whether it’s a mildly chilly fall evening or a bitter cold winter night, the furnace runs at the same setting.

If your furnace was manufactured before 1992, there’s a good chance it’s a single-stage model. While these furnaces perform the basic job of heating your house, they leave something to be desired.

Pros

Low purchase price – Single-stage furnaces are the least expensive models available. If your existing furnace fails suddenly and you can’t wait to save up for a more advanced model, a single-stage model is an acceptable replacement.

Improvements over old technology – Single-stage furnaces are among the least energy efficient models manufactured today, but a modern single-stage furnace is still more efficient than one built 20 years ago. If your furnace is decades old, but a high-efficiency upgrade isn’t in your budget, you can still lower your energy bills by investing in a new single-stage furnace.

Cons

Wasted energy – If you live in a mild climate, for most of the year, you don’t need to run your furnace at full capacity. In fact, the gas valve and blower fan’s maximum capacities are designed to make sure they can keep you warm in your area’s coldest winter temperatures.

Throughout much of the late fall and early spring, a lower, energy-saving setting would be more than enough to maintain a comfortable temperature in your home.

Because a single-stage furnace has no other option, though, it spends a lot of time burning energy unnecessarily. These furnaces also cycle frequently, so the motor also has less time to reach its optimally efficient speed, similar to a car in stop-and-go traffic.

Inconsistent temperatures – In mildly cold weather, a single-stage furnace will kick on, quickly push out a large amount of warm air, then shut off. These sudden blasts contribute to fluctuating temperatures as well as hot and cold spots in your home.

Two-Stage Furnaces: Heating That Adjusts to Your Needs

Two-stage furnaces, also known as dual-stage furnaces, can run at two different settings. These furnaces have a two-stage gas valve and a variable-speed blower motor. Instead of being either open or closed, the gas valve can also be partly open. The blower fan can adjust its speed to meet your heating needs efficiency.

These two factors allow your furnace to operate at full capacity when you need a lot of heat, and at around 60 to 65 percent capacity when you need just a little heat.

Pros

Energy savings – In most climates, a two-stage furnace runs on the lower setting around 75 percent of the time. During those times, it uses less energy compared to a furnace that doesn’t have the option of a lower setting. Keep in mind, though, that this doesn’t change the furnace’s annual fuel utilization efficiency (AFUE), which tells you how fuel efficient the furnace is over the year.

A 95 percent AFUE furnace is 95 percent efficient throughout the year regardless of what setting it runs at. Likewise, a 95 percent AFUE single-stage furnace is more efficient than a 90 percent AFUE two-stage furnace.

More stable temperatures – When running at its lower setting, a two-stage furnace provides a lower volume of warm air over a longer period. You’ll get a gentle, steady flow instead of intermittent blasts. This keeps your indoor temperature more stable and allows the warm furnace air to mix with the cold room air for more even heating. It’s especially beneficial in small houses where temperature fluctuations are more noticeable.

Improved indoor air quality – Because these furnaces run for longer periods than single-stage models, they circulate your home’s air through the air filter more often. This helps the filter remove more airborne contaminants, such as pet hair, pollen, and mold spores, from your air.

Reduced noise – When the blower fan runs at a lower speed, it makes less noise. These furnaces don’t do much to cut down on noise from air moving through the ducts, though.

Cons

Higher upfront costs – Two-stage furnaces are more expensive to buy than single-stage models. You’ll save on monthly heating bills, but unless you live in a cold climate, you might not recoup your investment as fast as you hope.

Possible maintenance concerns – Two-stage furnaces have a reputation for being more prone to breakdowns, but evidence is inconclusive. While they run for longer periods than single-stage models, which puts a certain amount of extra wear and tear on the motor, they also cycle less frequently, which protects the motor from the wear caused by frequent starting and stopping. When parts do fail, replacements are often harder to find and install, so repairs are more expensive.

Whether a single-stage or two-stage furnace is right for you depends on your climate, your home’s size and floorplan, your personal preferences, and your budget.

Before you decide, talk with an HVAC professional who can give you a personalized recommendation. Get the right furnace model, have it sized and installed correctly, and you’ll enjoy reliable comfort for a reasonable price.

Before you decide, talk with an HVAC professional who can give you a personalized recommendation. Get the right furnace model, have it sized and installed correctly, and you’ll enjoy reliable comfort for a reasonable price.

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