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  • Writer's pictureRob Mitchell

Furnace Types Explained

Updated: Jan 10

Which Type of Furnace Should I Get? Single-Stage? Two-Stage? Modulating?

Knowing more about the differences should help make your decision easier.

The classic furnace has two settings: ON or OFF. This is a single-stage furnace, because it has only one heat setting. The gas valve of a single-stage furnace opens at 100% capacity when in operation.

Single-stage furnaces are great for smaller homes as they can quickly heat areas. They are less expensive to purchase and install than a two-stage furnace, making them perfect for families on a budget. They only have one set function and less moving parts, and the chances of them breaking down are pretty low.

However, the single power mode can be problematic when it comes to heating a large or multi-story home. Because the furnace will run at full power whenever it is on, the areas near the vents will warm up quickly. While the rest of your home’s temperature plays catch-up, your thermostat might “think” that your whole house has reached your desired temperature and shut the furnace off prematurely. This results in warm and cold spots around your home.

To prevent cold spots, many homeowners will raise the desired temperature on their thermostat so that the furnace won’t shut off so early. While this does solve one problem, it creates a different one: higher energy bills.

Furnace with Humidifier

Two-stage furnaces use different programming than single-stage furnaces, which allow its gas valves to allow gas into the furnace at two separate levels. The first level is at about 60-70% capacity, which is where two-stage furnaces tend to run throughout the day. Then, if the indoor temperature shifts noticeably, typically over 2 degrees, the gas valve will open to 100% capacity. This will allow the furnace to start burning at full power.

This means, if your two-stage furnace’s programming decides it is only moderately cold, then it will only operate at a low power level.

This is different than a one-stage furnace, which will only operate at full power until it has heated your room to the desired temperature set on your thermostat.

Will one additional setting really make big difference? Yes! In many ways.

A two-stage furnace starts and stops less frequently. This reduces wear and tear on various parts. Over many years, this can add up.

Do you always need 100% heating capacity? For most of us, the answer is no. A typical autumn evening in the Denver area could dip down to 40 degrees, and you might want to heat your home. However, the amount of heat needed to stay comfortable will be a lot less than during a below zero February night.

These are the perfect times for that lower stage. The other upside is that fewer BTUs means less money spent on your monthly heating costs. Of course, a two-stage furnace will cost more to purchase initially.

Two-stage furnaces are running more constantly, but at a lower speed. They are circulating more of the air throughout your home. This runs the air through the air filter more often, removing more airborne contaminants and leading to cleaner air.

One-stage furnaces only operate at 100% power, when on, they simply make too much noise. As two-stage furnaces often run on less power, they tend to make less noise. However, both still make the same amount of noise at full power.

You are most likely to benefit from owning a two-stage furnace if you own a two or multi-story home in which you intend to spend the next several years or more. Otherwise, you won’t get to reap the long-term benefits of lower energy bills. You might also want to consider a two-stage furnace if your home’s single-stage furnace is unable to heat your home evenly.

Modulating furnaces provide homes with even more precise temperature control and less dramatic temperature fluctuations than single-stage and even double-stage furnaces. This type of furnace will aim to keep your home within one to two degrees of your desired temperature by adjusting its flame in slight increments to the changes it detects. Yes, a modulating furnace costs even a bit more.

A modulating furnace will be even more energy-efficient if it’s paired with a variable-speed (or multi-speed) blower motor. By using more than one fan speed, the blower motor can help warm your home more evenly while using less energy.

If your home currently costs a fortune to heat, or if it seems impossible for your home to reach a consistent, comfortable temperature, you’ll enjoy owning a modulating furnace. It will keep your entire home as close to your ideal temperature as possible while costing you significantly less in energy bills than you’re probably used to--especially if you currently own a single-stage furnace. You’ll get the most out of a modulating furnace if you plan on staying in your current home long enough to benefit from the energy savings.

Single-stage or double stage is far from the only decision you’ll have to make when purchasing a furnace. You’ll have to decide on fuel type - natural gas, propane, oil or electric - as well as size (heat output) and the energy efficiency rating.

Fuel type is often determined by the type of furnace you have now. If you have an electric furnace and there is no gas line running to your home, chances are it won’t be cost effective to switch. However, natural gas is typically less expensive than oil, and both tend to be less expensive than electricity.

Energy efficiency is measured by annual fuel utilization efficiency (AFUE). If a furnace has an AFUE of 80, that means 80 percent of the fuel is converted to heat while the other 20 percent is lost. For gas- and oil-fueled furnaces, the lowest efficiency rating allowed by federal rules is 78 percent (except in mobile homes, where the minimum is 75 percent). High efficiency units have an AFUE of 85 percent or greater. Electric furnaces are always highly efficient; it’s just that electricity is a more expensive heating method than gas or oil.

Heat output is measured in British thermal units (BTUs). Common heat outputs include 40,000, 60,000, 80,000, 100,000 and 120,000 BTUs. The appropriate heat output is directly related to the size of your home - there’s only one number that works for each home. A furnace that is too large will not be energy efficient; one that is too small won’t provide enough heat. Consult a professional to find out which heat output is appropriate for your home.

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