How Does a High-Efficiency Furnace Work?

You know a furnace does not last forever, and you fear yours may be on its last leg. Before you find yourself without heat, it is time to consider what will take its place. You have heard about high-efficiency condensing furnaces and that they can save you money. But what is the difference, and is it worth the investment?

How a Normal Forced Air Furnace Works

To understand how the newer high-efficiency condensing furnaces work, you need to understand a bit about traditional furnaces. These traditional units are known as forced air systems simply because they force air through the system.

The traditional system draws air in through the return vents and pushes it through a heat exchanger to warm the air. It then continues to push it back out through the air ducts and into your home.

To warm the air, the furnace either has an electric heating element or burns natural gas, kerosene, or propane. This burns in the combustion chamber, and the exhaust escapes through a flue. The heat of the exhaust requires a metal flue pipe and cap to prevent damage. It also means that an extraordinary amount of heat is lost through the flue rather than being used in your home.

Making Use of Hot Exhaust Fumes

One of the biggest differences in how a condensing furnace works is that it makes use of the exhaust heat. Rather than venting straight up through a flue, these high-efficiency furnaces force the exhaust further into the system. The remaining heat warms up a secondary heat exchanger to preheat the circulating air before it reaches the primary exchanger.

This secondary exchanger removes most of the heat from the exhaust. That makes it safe to vent through a PVC or plastic exhaust pipe. It can then vent through a wall rather than out through the roof.

The exhaust is forced through the system by using a draft-inducing fan. Basically, this is a smaller version of your circulating fan to move air through the combustion chamber and exhaust out.

Built-in Heat Recovery

One of the optional components to your heating system is a heat recovery ventilator or HRV. This works very similarly to how the condensing furnace works in that it makes use of the exhaust fumes.

The HRV typically installs in the attic space of your home near the furnace flue. It draws air in from outside your home and preheats it on the way to your furnace. Once the exhaust passes through the HRV, it continues on to vent through the flue.

While the use of the exhaust seems similar in concept, a condensing furnace does not take the place of an HRV. If you have a newer home or have sealed your house well, then you need to draw more fresh air into your home. Normally this air is naturally drawn in through cracks and air leaks and is needed to keep your air fresh and balanced.

If you have a tightly sealed home, you will still need to draw in that extra fresh air. What little heat is left in the exhaust can be used to warm that fresh air during cold winter months. This makes it a little easier for your furnace to maintain your home’s heat efficiently.

Fuel Combustion and Efficiency

A traditional furnace has a single-stage gas valve on it, meaning it is either on or off. In the most efficient condensing furnaces, there is a variable-capacity, or modulating, valve. This, combined with an electronic control unit and ECM-type blower motor, allows for precise control over the system.

It is these more sophisticated components that give your condensing furnace such high efficiency. A standard furnace may run at about 80% gas usage efficiency. In other words, of the gas it burns, about 80% of the heat created is used to actually heat the air. Condensing furnaces are rated at over 90%.

The extra efficiency translates into shorter heating cycles. Additionally, you will experience a drop in your gas bill because the flow of the gas is more tightly controlled.

Difference in Insulation

High-efficiency condensing furnaces also have an insulated blower compartment. Many furnaces are kept in an area that is not normally heated. In some homes, this is the basement, while in other homes, it is in a utility room. Regardless of the location, you generally do not need a lot of heat in that space.

However, with traditional furnaces, you lose some heat from the blower compartment. The loss causes your system to run longer to achieve the desired temperature, driving up your operating costs. The high-efficiency units insulate the blower compartment, forcing more heat through the ducts.

Combustion Intake

There are two different types of combustion intakes for furnaces. The traditional intake draws air from around the furnace to provide oxygen for the fuel to burn. The second intake is a sealed system, which draws air from outside the home, and then exhausts it out.

High-efficiency condensing furnaces always use a sealed system for their intake and exhaust. It must do this to keep the right pressure through the system to carry the exhaust to heat the secondary exchanger.

Difference in Condensate

As a furnace runs, it creates condensation that drains through the condensate line. In a high-efficiency condensing furnace, the condensate is complicated by the secondary heat exchanger.

As the exhaust passes through the secondary heat exchanger, it is cooled enough for the gases to condense. The resulting liquid is called condensate and consists of water and carbon dioxide. This liquid then drains through the furnace and out through a floor drain.

These two liquids together form carbonic acid, which is corrosive to standard aluminum alloy heat exchangers. The secondary heat exchanger must therefore be made of stainless steel to resist corrosion.

Cooler Venting Needs

With a traditional furnace, the exhaust is quite hot and carries with it a lot of moisture. As you vent the exhaust, you want to ensure you are venting as much of that moisture as possible to prevent any biological growth. This is why the furnace flue is usually a straight shot up and must be made of metal to withstand the heat.

With the high-efficiency condensing furnaces, most of the heat is recovered to actually warm your air. As the exhaust cools, most of the moisture it contains condenses and drains. That means you do not have to vent through the roof but can take the vent through a wall. It also means that you can use a PVC vent rather than aluminum, which is generally more durable.

Buckeye Heating, Cooling & Plumbing has provided expert heating and cooling service to the Worthington, OH, area since 1948. Our technicians install, service, and maintain all types of furnaces, including high-efficiency condensing models. We also offer expert air conditioning installation, repair, and maintenance, so you only need one number for all of your HVAC needs. Call today to schedule your consultation to explore if a high-efficiency furnace is right for your home.

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