Single stage, two stage or modulating furnace?

When it’s time to replace your old furnace, there are several key factors to consider. One of those factors is how many “stages” the new furnace will have. Basically, staging refers to how many levels of heat output a furnace can produce. Why does this matter? Because the more control your furnace has over heat output, the better it can maintain a steady temperature, reduce energy use, and improve the overall comfort of your home.

Single-Stage Furnaces

A single-stage furnace is the simplest type of furnace with just two settings: on or off. It either runs at full capacity or not at all.

Advantages of a Single-Stage Furnace

The main advantage a single-stage furnace offers is its price. In terms of upfront cost, it will be the less expensive option. If your home is a smaller, one-story home, a single-stage furnace might be plenty adequate to meet your heating needs.

With fewer components and less complex functionality, these furnaces are easier and cheaper to maintain.

If you plan on moving soon there isn’t much reason to invest in a better model. You won’t be there to benefit from the energy savings or improved comfort a more expensive option might provide.

Drawbacks of a Single-Stage Furnace

If you have a larger home, a single-stage furnace may struggle to warm it evenly and efficiently, particularly if your home is two or more stories. These furnaces only run at full power; they heat up certain areas of your home quickly and then shut off before the rest of your home reaches the same temperature. This creates hot and cold spots throughout your home.

To reduce the cold spots, many homeowners set their thermostats higher so that their furnace will run longer. This results in a higher energy bill.

As the fan only runs at full speed, a single-stage furnace is the noisiest furnace option.

Two-Stage Furnaces

Two-stage furnaces have three settings: low, high, and off. Most of the time, the furnace uses the low stage (60–70% output), only kicking into high (100%) when the temperature drops significantly or it needs to reduce a large gap from the current temperature to the thermostat set point.

Advantages of a Two-Stage Furnace

Because a two-stage furnace can run at a lower power setting, it allows your home to achieve a more even temperature before shutting off. This reduces hot and cold spots as well as temperature fluctuations in your home.

By using a low setting for moderate heating needs, it consumes less energy overall than a single stage furnace, particularly in temperate climates.

Running at lower capacity most of the time means less noise most of the time, compared to a single-stage furnace.

Running at a lower speed results in less stress on the internal components. For this reason, two-stage furnaces will typically outlive single-stage units.

Drawbacks of a Two-Stage Furnace

A two-stage furnace can provide long-term energy savings and more comfort compared to a single-stage furnace. However, the initial cost of this furnace is higher than a single-stage unit. The energy savings may pay for the difference in upfront cost if you will be living in your home longer than a few years. 

With more parts and settings than a single-stage furnace, these systems can require more maintenance, this can result in higher costs in the years ahead.

You might also need a new thermostat to properly control the low-to-high fan options if you are upgrading from a single-stage furnace.

Modulating Furnaces

If you want the most energy savings and the most temperature control a furnace can offer, a modulating furnace is the choice to make. Beyond a two-stage setting, a modulating furnace can adjust heat output in tiny increments (1–2%), anywhere from about 40% to 100% output. The modulating gas valve, paired with a variable-speed blower, provides outstanding efficiency and economical heating performance compared to a traditional single or two-stage gas furnace.

Advantages of a Modulating Furnace

Modulating furnaces can keep your home within one or two degrees of your desired temperature. Because of their adjustable heat settings and the variable-speed blower they are also able to minimize hot and cold spots significantly.

They generally have the highest energy efficiency ratings. Top-quality modulating furnaces can achieve an incredible AFUE rating of up to 98%.

As their fans rarely run at the highest speed, modulating furnaces provide the quietest operation.

Drawbacks of a Modulating Furnace

A modulating unit will usually be the most expensive. If you own a smaller, one-story home, it may not make sense to pay the upfront cost for this furnace type, particularly if your current furnace was able to meet your heating needs during its prime.

This is not a purchase to make if you’ll be moving soon. If you don’t plan to live in your current home for several years the upfront cost will outweigh long-term savings.

Due to its more complex parts and operation, it can be more expensive to repair than the other options.

The recap

Single-stage furnaces are a good choice for budget-conscious buyers heating a small to medium-sized, single-story home. These furnaces are simple and highly reliable, heating your space with less of an upfront cost.

Two-stage furnaces are a step up. They will provide increased comfort and greater energy efficiency. They are an excellent midrange option for larger or multistory homes.

Modulating furnaces are the top of the line. They will provide the greatest comfort with the highest efficiency. Their upfront investment may be worth it if you are planning to live in your home for some time to come.

Need help choosing the right furnace? Call Elbert Heating & Air. We’ll come look at your current home and system and provide you with some options and recommendations.