Furnace Draft – What It Is And Why It Causes So Many Problems

If you've had to replace or repair parts on your natural gas furnace in the past, you may have heard your technician discuss problems with the furnace's draft or draft inducer. If you're not an HVAC technician, these terms might sound confusing. What makes your furnace drafty, and why should a lack of draft prevent your furnace from operating?

All combustion furnaces must have a way to remove harmful combustion byproducts from your home, but modern, higher-efficiency furnaces take this process a step further. While older furnaces relied on gravity to carry the hot (and less dense) gases away, higher-efficiency models extract so much heat from the exhaust gases that they will no longer efficiently float away on their own.

How Does a Draft Improve Efficiency? 

When it comes to furnaces, the term "draft" refers to the negative pressure created by a small fan called a draft inducer. The draft inducer is one of the first things to turn on when your furnace starts up, and you'll hear it engage before the burners ignite. The draft inducer creates a negative pressure through the exhaust flue, pulling combustion gases away from the burner while drawing fresh oxygen in.

This draft helps to ensure that exhaust gases leave your home, even after your furnace's heat exchanger extracts much of their heat energy. However, the draft inducer also clears these gases from the burners. By removing combustion gases from the combustion chamber, the draft inducer ensures a cleaner and more efficient flame.

Why Do Draft Problems Occur?

Despite the simplicity of this system, it's common for furnaces to develop draft-related issues. Because of the importance of proper negative pressure for safety and efficiency, modern furnaces use a pressure sensor to prove proper draft before igniting. Before continuing the ignition process, the control board will wait for the draft inducer to engage and confirm the correct negative pressure.

As a result, draft problems will usually stop your furnace from igniting. However, there are numerous potential causes for draft-related issues. Anything from a clogged exhaust flue to a faulty draft inducer motor can stop the pressure switch from closing. Sometimes, the problem may be the pressure switch or the tube used to sense pressure in the exhaust flue.

How Do HVAC Technicians Diagnose Draft Issues?

The good news is that modern furnaces will usually display an error code indicating a draft fault. The bad news is that it still takes a significant amount of diagnostic work to uncover the underlying cause of the fault. While an error code can point your technician in the right direction, they will still need to test multiple components and confirm proper draft using a manometer before deciding on their next steps.

If you suspect your furnace has a draft problem, the safest, easiest, and cheapest option is usually to contact a professional. An HVAC technician can properly test for the underlying problem, ensuring your furnace is fixed correctly without needlessly replacing parts. 

Contact furnace repair services for more info.