The Difference Between Types of Furnaces by J&A South Park

Here’s the Lowdown on Single Stage, Two-Stage
or Variable Speed Furnaces

Let’s face it; the furnace is one of those things in your home that you can’t afford to live without, especially if you reside in a colder climate such as we do in the ‘burgh. Throughout Pittsburgh’s sometimes frigid and often snowy winters, you and your family’s comfort at home depends largely on your furnace’s performance. While all furnaces heat the air, not all do so in a way that maximizes your comfort and meet your energy efficiency needs at the same time.

That’s why we’re going to review the three main types of furnaces so you can find the perfect system that meets the specific comfort needs of your family while saving you money on your monthly heating bill.

There are a number of variables that come into play when talking about the ideal furnace for your home. Things like size and efficiency might be the most obvious outside of what brand you prefer. While these are crucial considerations, you’ll also want to look at the differences among a single-stage, two-stage, and variable speed furnaces.

The system that will work best for you is based on a number of factors, including heating requirements, existing ductwork, and insulation, size and levels of your home, budget, and more.

Before proceeding, however, we’re going to break down and simplify how a typical gas furnace works.

  1. A furnace is basically made of six parts: A gas supply line, an ignitor, a heat exchanger, a blower, an exhaust vent, and electronics to control it all.
  2. A gas line carries gas into the ignitor chamber where the gas is ignited and pushed through a metal tube called a heat exchanger.
  3. The heat exchanger warms up from the hot gas, and a blower fan pushes air over the exterior of this heat exchanger to warm the air that travels through your home via the ductwork.
  4. Those toxic gases from the heat exchanger are exhausted from the home via a PVC pipe or chimney.
  5. All of this is controlled via a control board (electronics) that dictate the timing of these steps, so nothing goes wrong, and is communicated with your thermostat to give you control over your home’s temperature.

Simple enough? Well, now it gets a bit more complicated.

Over the past 30 years or so, technology has impacted heating products immensely. Today, different models of furnaces can save you money and make you more comfortable. Long gone are the days of simple furnaces that are oversized and inefficient. There are three types of furnaces available: Single-stage, two-stage and variable speed (also referred to as modulating).

The specific stage is all about the gas valve and the burner. The different stages work similarly to how the heat is adjusted on your gas grill or gas stovetop.

One-stage furnace

Chances are, you’ve had a traditional single-stage furnace in your home at some point.

A single-stage, or one-stage, furnace means exactly what the name implies – it only has one setting–high. Whenever it’s turned on, this type of furnace is running at full capacity regardless of how cold it is on the outside or in certain areas of your home.

Basically, the single-stage furnace is a design of the past. It has a single, fixed gas valve accompanied by a single-speed blower motor. When the thermostat calls for heat, the furnace comes on at full power until the set temperature is reached. The single-stage furnace only has an off and on setting with no in-between. Imagine yourself going from a walk to a full out sprint every time you wanted to work out. This is how a single-stage furnace operates.

Because of this, the furnace must cycle on and off frequently to maintain the thermostat temperature. Even with this frequent cycling, however, you’ll probably find that your rooms overheat and then rapidly cool off, rarely staying at the temperature you prefer. Typically, if you set the thermostat at 72 degrees, the actual temperature might fluctuate between 68 and 76, often leaving your house either too hot or too cold.

This full-time operation can also result in higher energy bills because the furnace must cycle on and off frequently to adequately warm your living space during a cold snap, but can also waste energy by overheating your home when a Chinook blows in.

If, however, you live in a bungalow or other type of small, one level house, a one-stage furnace may be enough to heat the space evenly. Moreover, a one-stage furnace costs about $500 less than a comparable two-stage model, so they can make sense if you’re on a tight budget.

Two-stage furnace

Flexibility is the main difference between one-stage and two-stage furnaces. As you probably already guessed, a two-stage furnace has two levels of output – high and low. Most of the time, the furnace runs on the low setting and only kicks into high gear in really cold weather when an extra boost is needed to bring your home up to the desired temperature.

From a homeowner’s perspective, it works just like any other furnace. All you need do is set your thermostat and your furnace automatically chooses the ideal stage. When the thermostat calls for heating, the furnace initially kicks on in the low setting. Throughout the heating cycle, the furnace typically functions at around 60 to 68 percent capacity on this lower setting.

If the thermostat’s temperature isn’t achieved within a few minutes, the furnace will kick into the second or higher setting. On this setting, the furnace generates more warm air to heat your home faster. With this two-stage unit, your home will always be at a comfortable temperature no matter the weather outside. Plus, a more even distribution of warm air also means a reduction in air fluctuations.

You’ll also find that two-stage furnaces will also help increase energy efficiency on moderate-temperature days since it will, in most instances, remain on the low stage. Most homeowners will also find they don’t need to adjust the thermostat as often as with a single-stage furnace.

Two-stage pros:

  • Improved comfort – There are fewer temperature fluctuations with two-stage furnaces. The temperature remains quite consistent so you’re not too hot and not too cold.
  • Higher efficiency – Because the temperature is more consistent, two-stage furnaces are more energy efficient. They use less fuel because they’re not pumping out excessive heat or constantly cycling off and on to maintain a comfortable temperature.
  • Quieter – Two-stage furnaces run much quieter than one-stage furnaces.
  • Environmentally friendly – Two-stage furnaces produce less carbon dioxide emissions.
  • Fewer stops and starts – reduces the wear on components, increasing their lifespan.

There are, however, several drawbacks to a two-stage furnace. First and foremost, a two-stage unit costs more than a one-stage furnace because the technology is more advanced. Plus, they’re not necessarily ideal for large homes. Two-stage furnaces can be problematic in large homes unless the home has zone heating. The first stage of heat might not have adequate blower speed to push the air to the entire house.

That’s why you might want to consider a variable speed furnace.

Variable speed furnace

Unlike a single-stage and two-stage unit, a variable speed furnace doesn’t actually refer to the number of stages your furnace goes through to provide heat. Rather, it refers to the fan motor in the furnace and how much air it moves through the ductwork.

In variable speed furnaces, the fan motor can move at different speeds to control the amount of heated air dispersed throughout your home. Using advanced technology, these units monitor the data coming from your heating system and continuously make modifications to meet your comfort needs. It does so by varying the amount of circulated air, compensating for factors like dirty filters or blocked vents by increasing the fan speed. Better airflow results in a more comfortable environment regarding both temperature and humidity.

With these continuous adjustments, the blower also helps keep your furnace running as efficiently as possible; they use approximately six times less electricity than a standard blower.

Even when the furnace is not “on,” the motor in a variable speed furnace can still operate to continually circulate air throughout the home. This feature allows for healthier indoor air quality since your air is continuously passing through the filter in your HVAC system and being cleansed of such things as bacteria, viruses, and mold spores. Contrast this to a single-stage or two-stage furnace that only circulates air when it’s running, and you’ll observe that a variable speed unit truly does deliver better air quality.

In addition, this advanced control over airflow will rid of the need for the furnace to operate as often since air is continually being distributed throughout the home, leading to less energy consumption and lower utility bills each month.

A variable speed furnace is also definitely quieter than a single-stage or two-stage furnace.

In fact, you won’t even hear it running. That’s because the furnace runs on a lower setting the majority of the time, so the amount of heat that’s required is low, as opposed to being on “full blast,” so there’s no massive blast of air when it starts up. In addition, some furnace models use sound-absorbing materials in its construction to further dampen any sound.

Variable speed furnaces are also great for zoning which allows the homeowner to position the thermostat on different settings for various parts of the house.

Is a variable speed furnace worth the extra investment?

It’s worth considering how long you will be in your home. You will initially spend more money on your furnace, and you want to make sure that you will be there long enough to recoup some of those costs through lower energy bills. Because variable speed furnaces tend to be more energy-efficient and may be Energy Star® certified, you may also be eligible for rebates to help offset some of the cost. Be sure to ask your professionals at J&A South Park for details.

Generally speaking, if you’re planning on staying in your home for at least five years, it’s worth considering a significant investment in your home heating that, with proper installation and maintenance, will increase your comfort, reduce your energy bills and last for the long haul.

A Pittsburgh resident turns down her furnace thermostat in her South Park home.

Conclusion and final comparison – Which furnace is best suited for my home?

To recap, here are some general recommendations on the different furnace types:

  • One-stage: Buy this if you really only want or can only afford the most basic and cheapest furnace, otherwise get at least a two-stage model.
  • Two-stage: A good step up and worth the investment for homeowners looking for better performance and efficiency.
  • Variable speed: If you have a bigger budget and want improved energy efficiency, better home comfort, with quieter operation, and want to potentially qualify for rebates with an Energy Star certified heating system, a variable speed furnace is definitely worth your consideration. Consider this: While the typical payback on a variable speed furnace is 4 to 5 years, they will last 15 to 20+ years if they are well maintained with annual maintenance.

We invite you to contact your furnace experts at J&A South Park to discuss your specific heating needs, and how we can help fit those needs to your budget.

You might be pleasantly surprised to find that you can afford a variable speed unit.