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Single-Stage or Two-Stage Furnace: Which Is Right For You?

If you’re planning a furnace swap soon, it’s important to know that different devices offer different efficiencies. As you collect free estimates, consider the information below about single and two stage furnaces and use it as a resource. As always, it’s best to get a free estimate to determine which furnace will best suit your home, budget and comfort level.

What is a Single Stage Furnace?

This type of technology is about 30 to 40 years old. In operation, a single stage furnace has only one stage or heating mode. Single stage furnaces operate at maximum capacity and require full current draw as soon as the gas valve opens. The thermostat calls for heat, the furnace runs at full speed until the thermostat reaches the desired temperature, then the unit shuts off. A single-stage furnace works at full capacity without being able to reduce energy consumption. The gas valve is always 100% open as it is a fixed gas valve, which means there are no fluctuations in the amount of energy it is allowed to use.

When the thermostat reaches the desired temperature, the single stage turns off and the air in the house is allowed to stabilize. When warm air rises and cool air settles, there is nothing that allows the air to mix or circulate. This can lead to temperature fluctuations in the house. When the thermostat calls for heat, the single furnace cycle repeats. Single stage furnaces short out more often, which can wear out components. Upstairs also tends to be noisier and less energy efficient, resulting in higher energy bills.

What is a Two-Stage Furnace?

Today’s two-stage or modulating furnace is a gas furnace that offers longer and shorter run times. The two-stage furnace is often quieter due to its variable speed fan motor. This part of the furnace is very important as it controls the amount of airflow delivered. These furnaces are equipped with a two-stage gas valve. You can control the opening of the valve with two different capacities: a high level and a low level.

A two-stage system varies between 70% and 100% capacity. The two heat settings and variable speed fan motor reduce energy consumption and running costs by operating at a lower speed while still providing adequate heat.

A two stage furnace starts at 70% capacity while operating at the first stage for 10 minutes. If your thermostat is happy with that, you’ve used 30% less energy at much higher efficiency than your old, outdated single-stage system.

If, due to an extreme temperature load (colder temps or warmer outside temps), the two-stage furnace isn’t satisfying your thermostat, it will operate in single-stage (but still at higher efficiency) to keep your thermostat happy.

On milder winter days, a two-stage furnace on the first stage does a great job of heating the house. On colder days, the second stage triggers according to the thermostat.

Longer run times mean fewer starts and stops, so components experience less wear. Each electrical component uses eight times more energy to start and stop. What does that mean? Here’s an example: When does a lightbulb usually break? When you turn it on. Now that you know this, you will understand that longer runtimes result in fewer performance spikes when using your system. Fewer spikes create a longer lasting, more reliable system with minimal wear and tear.

The Pros and Cons of a Single-Stage Furnace


  • When considering a single stage furnace, you may want to limit your initial investment. A one-step application is great for rental housing with other opportunities to generate ROI.
  • Sometimes the additional costs can be lower. A single-stage model can be a good choice if you’re not concerned about maximizing your fuel economy or don’t mind higher utility bills.
  • Two stage systems and modulating furnaces should be vented from the side of the house. In some situations it is not possible to install new or additional chimneys. In this case, only one stage furnace can be used.


  • Single stage furnaces are not the most efficient way to heat your home when using a gas furnace. Operating a single stage system as opposed to using a two stage or modulating kiln can result in high energy costs.
  • Single stage systems can often be noisy, and depending on where the furnace is located in the house, this noise can be annoying.
  • Single stage furnaces are not designed for continuous air circulation. Homes that use single-stage furnaces often suffer from hot and cold spots throughout the home and temperature fluctuations from room to room.

Now let’s see how a two stage furnace can benefit your home.

The Pros and Cons of a Two-Stage Furnace


  • A two-stage furnace uses less energy than a one-stage furnace. This is important when considering your energy bills.
  • Using a two-stage furnace improves air circulation. Improved airflow prevents air from settling, reducing temperature fluctuations. This allows homeowners to set their thermostats lower in the winter because the air in the house is warmer and they don’t have to deal with cold spots. Some of our customers even say they can lower their heat setting by a degree or two in the winter because the house just feels better with a two-stage furnace compared to the old single-stage furnace. .
  • Although maintenance is always recommended, a two stage furnace should have a longer service life due to less wear and tear on the unit due to fewer starts and stops.
  • With improved airflow, the air filter captures pollutants and allergens more effectively, resulting in cleaner indoor air quality.
  • A two-stage system also generates less noise than a single-stage system.


  • New PVC piping needs to be run around the side of the house instead of using existing chimneys.
  • In the case of two-stage systems, there must also be access to the internal drainage, i.e. a washing machine or a floor drain must be available.
  • Two stage furnaces are generally not suitable for attics and cannot be used in unconditioned crawlspaces.

Frequently Asked Questions

Q: How do I know which type of furnace is right for my home?

A: The best way to determine which type of furnace is right for your home is to consider your heating needs, the size of your home, and your budget. Single-stage furnaces are a good option for homeowners on a budget or those with smaller homes, while two-stage furnaces are more energy-efficient and provide better temperature control. If you’re unsure which type of furnace is right for your home, consult with a professional HVAC technician.

Q: Will a two-stage furnace save me money on my energy bills?

A: Yes, a two-stage furnace is generally more energy-efficient than a single-stage furnace, which means it can save you money on your energy bills in the long run. However, the initial cost of a two-stage furnace is typically higher than a single-stage furnace, so it may take some time to recoup the additional cost through energy savings.

Q: Do two-stage furnaces require more maintenance than single-stage furnaces?

A: Two-stage furnaces are generally more complex than single-stage furnaces, so they may require more maintenance. However, regular maintenance is important for both types of furnaces to ensure they operate efficiently and to prevent breakdowns.

Which One is Best for you?

Choosing the right type of furnace for your home is an important decision that requires careful consideration. Single-stage and two-stage furnaces are two of the most common types available, and each has its own set of pros and cons. Single-stage furnaces are cost-effective and easy to use, while two-stage furnaces are more energy-efficient and provide better temperature control. When making your decision, consider your heating needs, the size of your home, and your budget. If you’re unsure which type of furnace is right for your home, consult with a professional HVAC technician who can help you make an informed decision.

When you complete your furnace replacement search, our team at Carrano Air HVAC Contractor Inc. will provide you with a FREE at-home estimate. Our comfort consultants have the knowledge and experience to ensure you get the right system that meets your comfort needs and fits your home and budget.

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