Single Stage vs. Two Stage Furnaces

An energy-efficient furnace keeps you warm and cozy without high heating costs. It maintains a comfortable temperature in your home without burdening you with excessive energy bills.

The number of stages your furnace can operate in — either a single-stage furnace or a two-stage furnace — affects both its efficiency and performance. When upgrading your HVAC system, consider this important factor.

For homeowners on a budget, a basic single-stage furnace is a sensible choice, particularly in mild climates. However, if you live in a cold winter climate and want to reduce heating costs, investing in a two-stage furnace is worth it.

In this article I’ll explain the differences between both furnace types and the pros and cons of each so you can make an informed decision.

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

In furnace terminology, “stages” refers to the levels of heat output a type of 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 type of 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 bitterly cold winter night, the single-stage furnace runs at the same setting.

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

Pros

Low purchase price — Single-stage furnaces are the least expensive type of furnace 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 types made today. However, 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 single-stage furnace at full capacity. In fact, the gas valve and blower fan’s maximum capacities are designed to keep you warm in your area’s coldest winter temperatures.

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

A single-stage furnace is less energy efficient because it only has one setting and can’t adjust to different heating needs. These single-stage furnaces also cycle often, so the motor has less time to reach its optimal energy-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 efficiently.

These two factors enable your two-stage furnace to operate at full capacity when you need a lot of heat and at around 60 to 65% 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% of the time. During those times, a two-stage furnace uses less energy compared to a furnace type 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% AFUE furnace is 95% efficient throughout the year regardless of what setting it runs at. Likewise, a 95% AFUE single-stage furnace is more efficient than a 90% 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 desired 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. However, these two-stage furnaces don’t significantly reduce the noise from air moving through the ducts.

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 hoped.

Possible maintenance concerns — Two-stage furnaces have a reputation for being more prone to breakdowns, but the evidence is inconclusive. While they run for longer periods than single-stage models, they also cycle less often. This means they put extra wear and tear on the motor due to longer operation but protect 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 or things like furnace blower motor replacement costs are more expensive.

So, What Is the Best Stage Furnace for You?

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

If you live in a mild climate and have budget concerns, a single-stage furnace is a practical and cost-effective choice. However, if you experience bitterly cold winters and want to maximize energy efficiency and your home’s comfort, invest in a two-stage furnace.

Consult with an HVAC professional to get a personalized recommendation specific to your home. With the right furnace model, properly sized and installed, you can enjoy reliable comfort and efficient heating for years to come.