A man looks at his furnace to answer the question: where is my furnace exhaust vent?

Where is My Furnace Exhaust Vent

A Custom Services, Chicago’s trusted furnace tune-up contractor, put together this guide to furnace tune-ups. You might be wondering, “where is my furnace exhaust vent?” You can find the vent near the top of the furnace. This small duct connects to the outside of your house to vent fumes out.

As winter approaches, many of us prepare our homes for the cold weather. A professional furnace tune-up is a great way to ensure that your family stays warm and cozy. A furnace tune-up includes vital checks such as operating condition, air filter quality, and smoke and carbon monoxide safety. It is important to examine your furnace’s air intake and exhaust vents for blockages or leaks.


Before you troubleshoot, identify the type of furnace you own. There are two types of gas furnaces: conventional and high-efficiency condensing furnaces.

Understanding How Efficiency is Measured

AFUE or Annual Fuel Utilization Efficiency is an energy rating that measures how efficiently the furnace converts fossil fuel into heat. For example, an AFUE rating of 80% means 80% of energy is converted into heat, while the remaining is unused.

A higher AFUE rating is better—it means you have a highly efficient furnace.

Characteristics of Conventional vs High Efficiency

Conventional furnaces are not very energy efficient—with an AFUE rating between 80-90%. This type of furnace usually vents exhaust gases through the roof, using old-fashioned metal or steel piping.

High-efficiency furnaces have an AFUE rating of 90% and higher. This type of furnace uses two heat exchangers to heat the air. Because the furnace produces cooler gases, it uses plastic piping such as PVC or CPVC to vent exhaust gases.

Now that you’ve identified your system, let’s start troubleshooting.


Why do exhaust vents get blocked, and what quick home remedies can you do to keep your furnace running smoothly?

Common Reasons for Blockage

  • Ash and soot buildup
  • Birds or small animals inside vents
  • Snow, ice, and frost blocking vents
  • Branches, twigs, or bushes
  • Damage, corrosion, or rust

Unblocking the Vent

If your vent gets blocked, you need to unblock it. Follow this guide to answer the question, “where is my furnace exhaust vent?” and head outside to find the vent.

1. Do a physical inspection

  • First, check if the vents are installed correctly. The intake (if you have one) should point downwards while the exhaust points upwards. This prevents harmful gases from entering your home.
  • Look inside the vents to make sure there are no birds, dead animals, bugs, debris, or anything else that could block airflow.
  • Lastly, make sure the space around the vents is clear. Keep all things or items at least five feet away. This allows for better airflow and unobstructed ventilation.

2. Clear the blockage

  • If you find animal remains, follow simple safety precautions to remove them. Wear a pair of gloves and place the remains in a disposable bag to throw away.
  • If you find soot buildup or debris, wear gloves and use a brush or your hand to remove any dirt. For those hard-to-reach places, try using a wire. You can use a handheld vacuum cleaner to pick up what you missed.
  • If you find snow, shovel the area around the furnace. If there is any snow blocking the vents, use your hands to clear it. Using a shovel or snow blower can damage the vents.
  • If you find any damage or corrosion, call a professional to fix it.

It is advisable to check your vents periodically and do a comprehensive professional tune-up annually.


A furnace generates a large amount of heat, toxic fumes, and carbon monoxide as a by-product of the combustion process. To keep you safe, your furnace has an exhaust flue that expels these harmful gases and prevents them from entering your home.

When the exhaust flue is blocked, it cannot remove these poisonous gases. Think of it as a blocked chimney. Since these fumes have nowhere to go, they remain in your home. As air recirculates, the carbon monoxide mixes with the air and continues to rise, greatly increasing health risks.


Here are some ways you can tell your exhaust vent is blocked.

Moisture Inside Your Home

If you start noticing condensation or moisture on your windows, your exhaust vent could be blocked. The trapped air in your home causes moisture to collect on the glass.

Inconsistently Heated Air

If your furnace doesn’t heat the air, your exhaust vent may be blocked. Your furnace requires airflow and proper venting to produce heat. A blocked exhaust flue prevents air from circulating through the combustion chamber. As a result, your furnace doesn’t ignite or kick in, and it won’t heat the air.

Sudden Starts and Stops

If your furnace turns on and off on its own, your exhaust vent is likely blocked. This happens because your furnace isn’t receiving enough airflow to light the flame. As the flame extinguishes, the hot gases roll back into the furnace. A rollback switch detects this heat and shuts down the furnace as a safety precaution.

Your furnace then tries to turn back on and repeat this process three to five times until it shuts down permanently. If this happens, call a professional HVAC contractor to clear the blocked vent and inspect the furnace.

Strange Smells

If you smell unpleasant or strange odors, your exhaust vent may be blocked. Some of the common smells include:

  • Gas
  • Moldy or damp smells
  • A burning smell

When these strange smells occur, you should safely turn off your furnace and check if the exhaust vent is blocked.


Now you won’t ever need to ask, “where is my furnace exhaust vent?” or worry about your furnace suddenly breaking down. A Custom Services is a family-owned Chicago HVAC company specializing in heating and air conditioning installation, repair, replacement, and maintenance. Learn more heating services.

When your furnace needs a professional tune-up, let A Custom Services in Chicago, IL, help. Call 773-831-4891 for heating and air conditioning service in the Greater Chicago area.

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