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How Does a High-Efficiency Furnace Work?

Most people know vaguely what a furnace does and how it works. The furnace heats your home using some kind of fuel, usually gas. What you may not know is that, over the years, as energy-efficiency technology has improved, furnaces have changed. At Entek, we want homeowners to understand all of their options  which is why we’ve created the explanation below.

When the average person thinks of a furnace, they’re usually imagining a “conventional” furnace. These vary in efficiency based on their age and certain features but have operated in much the same way over the last few decades. These conventional furnaces suck in cold air from the house, run it through an air filter, heat it via a burner with an electronic or pilot ignition switch, and blow it back out through the vents in your home. Voila! Your house is heated.

Homeowners now have the option, however, of newer, high-efficiency “condensing” furnaces that both look and work a bit differently. A warm home is a crucial comfort and knowing how your home is heated—or whether you might want to upgrade—is an important part of homeownership.

Heat Exchange

You may wonder how exactly a furnace’s efficiency is measured. When it comes to heating, furnaces are typically rated using the annual fuel utilization efficiency rating (AFUE). Newer, conventional furnaces usually achieve around an 80 percent AFUE, meaning that around 20 percent of the natural gas used to heat your home’s air is expelled through the exhaust. High-efficiency condensing furnaces, on the other hand, have attained the highest recorded AFUEs on record—often around 90 percent, but sometimes up to 98 percent.

While the processes of a condensing furnace are similar to that of a conventional furnace (both use circular fans to warm the air and both typically use an electric ignition to ignite the gas used to create heat), the main difference between the two is how the machinery deals with exhaust. Both furnaces have what’s called a primary heat exchanger, but a high-efficiency condensing furnace also has a secondary heat exchanger.

Two Heat Exchangers

The primary heat exchanger works like this: there’s a combustion chamber in the furnace where the gas is ignited, and that heat is circulated with the cold air before being redistributed through the vents into the home. Any gas leftover from that exchange is pushed out through a metal duc into the outdoors. The gas and air mixture that is expelled is still very hot, and that heat is lost instead of being used to heat the home.

The secondary heat exchanger works just how you might imagine. Instead of expelling the leftover gas and hot air mixture into the air, a condensing furnace pushes it into a second chamber. Inside the second chamber, as much heat is extracted from the mixture as possible before the excess is vented, this time through a plastic duct. A metal duct isn’t necessary because, by the time the excess gas is pushed outside, it has become quite cool. So cool, in fact, that condensation forms in the chamber as well, giving the “condensing furnace” its name. The condensate drips into your home’s drain pipe.

The lack of heat waste also accounts for why conventional furnaces and condensing furnaces look different. A large metal duct is used as a flue on standard furnaces. High-efficiency furnaces use a small PVC or ABS plastic tube for exhaust, like something you might see under your kitchen sink. The small plastic tube versus the wide, metal duct is also part of what makes high-efficiency furnaces quieter than conventional ones.

Burners and Blowers

Simply put, the burner on a furnace heats the gas and the blower controls how much heat is pushed into the home. The blower compartments on conventional furnaces are not insulated. High-efficiency furnaces, however, have insulated blower compartments. This is not, however, where the differences end.

Experts break conventional and condensing furnaces down into even more complex categories based on how the furnace’s burners and blowers operate. While it may seem a bit complicated, these categories really just relate to the sophistication (and likely, therefore, the cost) of your furnace. The different types of burners and blowers break down like this:

Single-Stage Furnaces

The “stage” referred to in these names reference the technology that controls the burner and the blower. Single-stage furnaces have only one ON setting, meaning that the furnace is either ON or OFF. Modification beyond the thermostat setting in the home isn’t possible. As a result, these furnaces are typically the least expensive.

Dual-Stage Furnaces

These furnaces have HIGH and LOW settings in addition to ON and OFF functions. This allows the owner to control the intensity of the burner and the burner’s flame according to how much heat is needed in the home. These furnaces are more energy efficient than single-stage furnaces and, consequently, are, on average, more expensive.

Modulating Furnaces

Modulating furnaces have sophisticated electronic controls that allow for minute adjustments to the blower and burner. You may have noticed with older furnaces that if your thermostat is set at 68 degrees, it will feel very much warmer than that while the furnace is cycling. That is because the single-stage furnace pumps in more hot air than needed. When the furnace cycles off, the temperature will be closer to your setting. Modulating furnaces, on the other hand, make small changes in burner intensity and blower speed to keep your home as close to your thermostat’s setting as possible. These are the most energy-efficient furnaces available.

Is a High-Efficiency Furnace Right for You?

It’s important to note that condensing furnaces have different repair needs than conventional furnaces. Regular filter changes, cleaning, and repairs can help folks maintain the efficiency of their condensing furnaces. There are also changes you can make around your home to make your current heating system operate more efficiently. Keep an eye out for doors and windows that may leak cold air into your home, forcing your furnace to work harder to maintain the same temperature. You can contact Entek at any time for more information on energy-efficient furnaces as well as possible rebates and government incentives that may help with the cost of an upgrade.


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