The History of Efficiency Standards for Draft Inducer Blowers

Modern home fossil fuel heating systems don’t look or operate anything like they did 40 years ago. Ask homeowners and they'll say their systems are more energy efficient (but a little noisier) than in the past. Technicians will say those changes are because of technical and design innovations. In recent decades, one innovation, the draft inducer blower, has become a practical necessity. Higher efficiency standards have led to the widespread adoption of this important component.

What is a Draft Inducer Blower?

You'll hear draft inducer blowers called many names, including:

• Induced draft blower;
• Combustion air blower;
• Draft inducer assembly;
• Exhaust blower; or
• Any combination of these words.

Draft inducer blowers move the products of combustion through the heat exchanger and out of the dwelling. Appliances rated 80 and 90 percent Annual Fuel Utilization Efficiency (AFUE) use these motor/blower assemblies. The assembly typically incorporates a pressure safety switch. The switch is designed to prevent the burners from operating if there is not adequate draft in the heat exchanger. For 90 percent appliances, the switch may also detect intake or exhaust restrictions.

Before draft blowers, fossil fuel furnace heat exchangers and exhaust systems used gravity. They depended on natural force to move the products of combustion through the heat exchanger and outside the dwelling. With very little restriction in the heat exchanger and an upwards pitched vent system, the products of combustion rise. These early furnaces (referred to as natural draft) were about 60-70 percent efficient. To put it another way, the system wastes 30-40 percent of heat generated from combustion by sending it up the chimney.

The 80 Percent Energy Efficiency Standard

In 1992 the Department of Energy established the current AFUE standard of 78 percent. These furnaces are commonly referred to as 80 percent efficient. But the actual efficiency rating could be between 78-83 percent. Before 1992, there were no federal regulations overseeing energy efficiency for these appliances.

Increasing efficiency involves transferring more heat from the combustion gases, through the heat exchanger, to the breathable air. This all has to happen before the combustion gases leave the exchanger. After passing through the exchanger, the combustion gas exits the dwelling through the vent system. Accomplishing this involved changing heat exchanger designs. Rather than having very little restriction, efficient heat exchangers have more restriction. This keeps combustion gases in the heat exchanger longer and allows more heat to be transferred.

The most common design for adding restriction is the serpentine heat exchanger. Serpentine exchangers route combustion gases horizontally and include paths with multiple turns. The combustion gases must take this path before exiting the heat exchanger.

These added restrictions make it impossible for the combustion gases to naturally leave the heat exchanger. Enter the draft inducer blower. The blower creates the draft, negative pressure, in the heat exchanger. Without it, the combustion gases would back up to the burner area and roll out. With it, the combustion gases exit safely through the heat exchanger outlet. The vent system must be pitched to allow gravity (natural draft) to move the combustion gases to the outside of the dwelling. Draft blowers are not only important from an efficiency standpoint, but a safety one as well.

90 Percent AFUE

Shortly after the 1992 standard was enacted, manufacturers upped the ante by creating 90 percent AFUE furnaces. While there was not then (nor now) any federal regulation requiring 90 percent AFUE, they did it anyway.

You may notice a gap between the 80 percent and the 90 percent furnaces. As mentioned earlier, increasing efficiency relates directly to removing more heat from the combustion gases before leaving the heat exchanger. But at some point, around 87 percent AFUE, the combustion gases are cool enough to potentially condense in the vents. So furnace manufacturers skipped from 83 percent AFUE to 90 percent AFUE. At the higher percentage, you can predict where the condensation of combustion gases will occur. The secondary heat exchanger was engineered for this purpose.

The secondary heat exchanger connects to the outlet of the primary heat exchanger. The secondary heat exchanger has smaller passages than the primary and is built of material that can withstand heat and the corrosive nature of the condensation. The combustion gases exit the secondary exchanger through the draft inducer blower and leave the dwelling through PVC pipe. The pipe is also rated to hold up to the temperature and corrosive condensation.

The 90 percent AFUE draft inducer blower has two functions. One is to create the proper negative draft in the heat exchangers. The other is to push the combustion gases outside the dwelling through the PVC pipe. Ninety percent furnaces are also built in a two-pipe design. In this setup, an intake pipe connects from outside the dwelling to the burner chamber. This pipe delivers fresh, clean outdoor air for combustion. Like the PVC pipe, components like blower housings and impellers are made of materials that can withstand the heat and corrosive nature of the condensation exiting the secondary heat exchanger.

The draft inducer blower is vital to the safe operation of the home furnace. There are several different equipment manufacturers, each with many different furnace designs. Considering this, it's easy to understand why we strongly recommend only using OEM replacements.

Find the Right Draft Blower For You

Fasco®, a Regal Rexnord brand, leads the industry in OEM and OEM-replacement induced draft blowers. Fasco draft inducer blowers are designed to meet OEM specifications and are tested to match OEM performance requirements. Customers get a factory match that restores their equipment to a full and safe operating condition.

To find the correct OEM-recommended replacement draft blower, go to the Regal Rexnord cross reference tool.