The heart of nearly every forced air heating system is the furnace. Gas furnaces come in a variety of sizes, types, and efficiencies to satisfy almost any application.

Our furnace buyer's guide is designed to assist you as you start the journey of finding the best furnace for your home!

Basic Design

A furnace consists of two or three main components depending on the fuel source.

Oil, propane, and natural gas furnaces use a burner to combine fuel and air and then burn the mixture to produce heat. In all furnaces, this heat is then transferred to the air via a heat exchanger. In electric furnaces, the heat exchanger also converts the electricity into heat. Finally, a blower distributes this warmed air throughout the house.

Be sure to read our guide on how furnaces work for more details.

Burner Options

Furnace burners are designed to operate in one of three ways: single-stage, two-stage, and modulating.

	Single-Stage Furnace	2-Stage Furnace	Modulating Furnace
Burner Modes	Off, High-Fire	Off, Low-Fire, High-Fire	Variable
Comfort
Cost	$$	$$$	$$$$$
Who It's For	Those on a budget Rental properties  Consistently cold climates	Cold climates  Zoned heating systems	Mild climates  Energy-efficient buildings Zoned heating systems Those needing highly consistent airflow
Who It's Not For	Mild climates  Energy-efficient buildings Zoned heating systems Those needing highly consistent airflow	Mild climates  Energy-efficient buildings	Those on a budget

 

Single-Stage

The first and most common is a single-stage burner. A furnace using a single-stage burner has only two operating modes, on or off. Single-stage furnaces will always operate at the rated capacity regardless of conditions inside or outside of the home. This operation often leads to inefficiency when temperatures are mild.

Two-Stage

To address the inefficiency with traditional burners, two-stage burners were introduced. A furnace using a two-stage burner has a total of three operating modes, off, low-fire, and high-fire.

Like in a single-stage furnace, high-fire means the furnace is operating at its full rated capacity.

Low-fire is an intermediate step that allows the furnace to operate at some factory set level less than the maximum capacity. This level is typically 40%-60% of the rated capacity and will allow the furnace to better match a home’s heating requirements in mild weather. This leads to a more comfortable home, and a lower fuel bill.

Modulating

Expanding on the benefits of two-stage operation, modulating burners can operate at almost any output. Modulating furnaces have a minimum and maximum heating capacity but can operate at almost any point within this range.

By constantly monitoring conditions and adjusting output to account for any changes, modulating furnaces are able to eliminate large temperature swings sometimes present with other types of furnaces. They also maximize efficiency by using the minimum amount of fuel to satisfy heating demands.

Two-Stage vs Modulating Performance

The charts below show a simulation of average temperature and fuel usage for single-stage, two-stage, and modulating furnaces. As you can see, the two-stage and modulating furnaces maintain a more stable temperature than does the single-stage unit.

The two-stage and modulating options also offer savings of approximately 8% and 10%, respectively, when compared to a single-stage model, even when all three have the same rated efficiency.

Blower Options

Furnace blowers have different speeds that impact how quickly they blow air through your space. They come in three main varieties: single-speed, multi-speed, and variable-speed.

	Single Speed Blower	Multi-Speed Blower	Variable Speed Blower
Comfort
Efficiency
Cost	$$	$$$	$$$$$
Who It's For	Those on a budget  Rental properties Cabins & seasonal dwellings	Basic zoning applications  Cabins & seasonal dwellings	Energy-efficient buildings Zoned heating systems  Those needing highly consistent airflow
Who It's Not For	Energy-efficient buildings  Zoned heating systems  Those needing highly consistent airflow	Zoned heating systems	Those on a budget

 

As its name suggests, a single-speed blower will blow air at only one speed and cannot adjust to conditions. That means, no matter how quickly you need your space to warm up, the blower will perform at the same speed. It's the least efficient, but also the least expensive, making it a great budget option.

A multi-speed blower allows the installer to select a speed for each type of operation at the time of installation. For example, you might want medium speed for heating but, if your furnace is part of a split system that includes air conditioning, you might want high speed for cooling.

The blower then operates at the designated speed when the associated operation is required. The blower does not respond to changes in operation, such as dampers opening in a zoned system or a filter gradually becoming clogged.

Like with a multi-speed blower, an installer sets the operating conditions for a variable speed blower at the time of installation. However, instead of picking a speed that provides the approximate air flow rate desired, the speeds on variable speed often correspond to a specific air flow rate.

The furnace then monitors flow rates, temperatures, and other information and adjusts the blower operation to maintain ideal operating conditions. This allows variable speed blowers to respond to changes in conditions and provide optimal comfort.

Orientation Options

Furnaces come in a variety of orientations to satisfy all varieties of installations. The orientations for furnaces include upflow, downflow, horizontal, and low-boy.

There are also furnaces that can be installed in a number of different orientations. These are often called multi-position or use a combination of the possible orientations, e.g. upflow-horizontal.

	Upflow	Downflow	Horizontal	Low-Boy
Description	Air Enters the Bottom, Flows Out the Top	Air Enters the Top, Flows Out the Bottom	Air Enters One Side, Flows Out Other Side	Air Enters and Flows Out the Top
Cost	$$	$$$	$$	$$$$
Who It's For	Anywhere ductwork is above the furnace, i,e, basements, closets	Anywhere ductwork is below the furnace, i.e. crawlspaces, Mobile Home	Anywhere with limited vertical space, i.e. attics, crawl spaces, closets	Older or historic homes

 

Upflow Orientation

In an upflow furnace, the air enters the furnace near the bottom and exits after being heated at the top. The return air connection can be located on either the bottom of the furnace or on either the left or right side of the blower compartment.

Upflow systems are commonly used when the furnace is installed in the basement or in a closet, with the ductwork above the level of installation.

Downflow Orientation

Downflow furnaces have a return connection near the top of the furnace and supply heated air out the bottom of the furnace. Return air is typically routed directly to the top of the furnace.

Downflow furnaces are used in applications where ductwork is installed below the furnace. This is common in homes with ductwork installed in a crawlspace or in commercial buildings where ductwork was installed within the slab.

Horizontal Orientation

Horizontal furnaces typically have a return connection at one end and a supply connection at the opposite end, with both connecting horizontally.

Horizontally-oriented furnaces are typically used in attics, crawlspaces, and other areas with limited height.

Low-Boy Orientation

Low-boy furnaces have both supply and return connections at the top of the furnace. This design results in a shorter but wider unit.

Low-boy furnaces were common in older homes but are being gradually replaced with other types of units. An upflow furnace is often a suitable replacement for a lowboy model, but duct reconfiguration will be required.

Venting Options

Furnaces have two primary venting configurations: chimney and direct venting.

	Chimney/Draft Induced Venting	Direct Venting
Operation	Intake air pulled from inside, exhaust vented through chimney	Intake air pulled from outside, exhaust vented through wall
Efficiency	Average	High
Cost	$$	$$$$
Who It's For	Homes with chimneys Historic homes Those on a Budget	Homes without a chimney Applications where intake air is dirty, like a basement workshop New construction Energy efficient homes
Who It's Not For	Homes without a chimney  Applications where intake air is dirty, like a basement workshops  Energy efficient homes	Those on a Budget

Chimney-vented furnaces have large vent connections and rely on the natural buoyancy of hot air to exhaust combustion gasses and draw fresh air into the furnace. To make sure this process is working correctly, they require large chimneys. Many modern chimney venting systems include a draft inducer blower that forces exhaust out the chimney, preventing the chance of backflow.

Direct-vented furnaces are more efficient and exhaust either vertically through a ceiling or horizontally through a side wall using a draft inducer blower. This eliminates backflow and makes it safer for building occupants. They also use smaller venting than chimney-vented furnaces and pull fresh air directly from outside rather than the indoor space.

Pulling outdoor air is more efficient because the furnace isn't removing already-heated air from your home. Furthermore, outdoor air can be cleaner and better for your furnace than indoor air, especially if your furnace is in an area where you work with wood or chemicals.

Efficiency (AFUE)

Furnace efficiency is measured by the annual fuel utilization efficiency (AFUE). AFUE can range between 80% (the federally mandated minimum) to 97%.

Most manufacturers continue to offer basic single-stage, multi-speed, 80% AFUE models for the budget-minded consumer. However, models with AFUE rates of 90% or higher are becoming more and more common. In some instances, government mandates require higher AFUE ratings in cold climates.

Most models with AFUE rates of 90% or higher use condensing technology to capture as much of the heat available from combustion as possible. A condensing furnace will condense water vapor from the combustion process back into a liquid. This process releases a substantial amount of energy that can then be used to heat your home.

The AFUE rating is a great way to compare energy efficiency between models, but may not accurately reflect the efficiency of the unit once it is installed.

Duct sealing, insulation, and routing, as well as filters and humidifiers, can all impact operational efficiency. It is important to address any existing issues in these components to ensure your system can operate at peak efficiency and keep your home comfortable.

 

Stay Warm

Now that you know how furnaces work and the different kinds available, you're ready to check some out for yourself. Remember, you can contact the team at eComfort with any further questions.

 

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