Is Furnace Exhaust Dangerous?

All gas-burning appliances generate combustion gases that can be potentially dangerous if your system is not able to get rid of them properly.

Is furnace exhaust dangerous? The exhaust is mainly water vapor and carbon dioxide, but the absolute majority of furnaces also form a wide range of byproducts (particulate matter, nitrogen dioxide, sulfur dioxide, hydrocarbons, carbon monoxide, etc.); carbon monoxide is especially dangerous as this invisible, odorless gas can lead to poisoning. 

Is the Exhaust from a High Efficiency Furnace Dangerous?

In theory, the combustion of gas should generate only water vapor and carbon dioxide. Unfortunately, even the furnaces that are highly efficient aren’t able to combust the gas entirely.

This partial combustion leads to the formation of various byproducts, starting from nitrogen dioxide and ending with carbon monoxide.

If the system has been installed correctly and is operating right, then these gases will be expelled out of your house through the ventilation system. 

But you should remember that the exhaust from a high-efficiency furnace can be dangerous in two ways.

  1. The byproducts contain carbon monoxide that can get leaked into your house if there is something wrong with the ventilation system.
  2. The acidic elements that are present in the exhaust can end up damaging the chimney if the chimney is not protected.

Read: Is It Worth Getting A High Efficiency Furnace? And Why?

Can My Furnace Make Me Sick?

Yes, a heating system that is not properly maintained can make you feel sick. 

In the worst-case scenario, the exhaust from your furnace can lead to carbon monoxide poisoning. However, there are a few other health-related issues that can be caused by your heating system:

  • Breathing problems

If the unit is not regularly cleaned, then it can become home to various bacteria and fungi (the HVAC system can also harbor mold). All these tiny particles will end up in the air that you breathe, as soon as the furnace gets switched on.

  • Headaches

The ‘sick building syndrome’ can’t be directly linked to your HVAC system, but people who spend a lot of time in buildings with central air tend to get sick and suffer from headaches more often. Experts recommend taking a break from time to time to have a walk outside.

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  • Itchy, watery eyes

Your furnace can trigger allergy-related symptoms. Especially, if there are gaps in the ductwork, through which unfiltered air can get sucked in. 

  • Skin irritation

If your HVAC system does not have a built-in humidifier, then you might experience skin-related problems. Spending too much time in a hot space with low humidity can lead to cracking skin, dry nose and eyes, and so on.

Read: Is Furnace Condensate Toxic?

Does Furnace Exhaust Contain Carbon Monoxide?

Yes, furnace exhaust does contain carbon monoxide as it is created during the process of burning gas.

This colorless and odorless gas is harmless when breathed in small amounts (that’s what we do all the time), but in enclosed spaces, a high concentration of CO can become extremely dangerous.

Can You Get Carbon Monoxide Poisoning from a Gas Furnace?

Unfortunately, yes.

The carbon monoxide in the furnace is mostly contained within the walls of the unit’s heat exchanger. The gas is then directed through the flue pipe and safely vented out of your house.

However, there are a few things that can potentially cause a carbon monoxide leak:

  • There is a leak in the venting system
  • There is a crack in the heat exchanger or the combustion chamber
  • A dirty air filter that can cause a backup of airflow in the system
  • A blocked or disconnected vent pipe
  • An older furnace model that is over 16 years old and is now producing too much combustions gases

Read: Why Furnace Is Making A Clicking Noise – Troubleshooting Guide

How Do You Know If Your Furnace Is Emitting Carbon Monoxide?

The main signs of a carbon monoxide leak include:

  • A yellow or flickering flame
  • Moisture around walls or windows and/or streaks of soot
  • Rust on the vent pipe
  • Physical symptoms (vomiting, nausea, frequent headaches, dizziness, brain damage, heart complications, memory loss)

How Do You Check for Carbon Monoxide in Your Home?

The most common locations for a carbon monoxide leak within the HVAC system are the exhaust flue and within the heat exchanger. 

You have to make sure that there is no rust developing on your flue pipe as it can lead to holes. The heat exchanger, in its turn, can also develop cracks due to age or exposure to moisture.

You would have to regularly check your unit to ensure that these components are not damaged. 

You can invite a professional to perform an indoor air quality test (this will help figure out if you have a leak or not). However, the easiest way to check for CO in your house is to install a special carbon monoxide detector.

Do bear in mind that these devices should, in the majority of cases, be installed 20 feet or more away from the furnace. They are usually placed in the habitable areas (dining rooms, bedrooms, hallways, etc.) and act as ‘the last layer of warning’.

Read: Why Furnace Blower Motor Is Making Noise When Starting?

Where Is My Furnace Exhaust Vent?

You should inspect and clean the intake and exhaust pipes from time to time. But to do that, you should first be able to find them.

The pipes are usually black or white and they exit the wall right next to your furnace. Remember where the pipes exit the building and go outside.

The exhaust pipes should be near to where the furnace is but on the outside. The chances are high that you’ll find the curved pipes next to the foundation, but, at times, they can be located near the roof.

How Close Can a Furnace Exhaust Be to a Window?

Of course, you wouldn’t want the exhaust gases to leave your house only to come back a minute later through a window.

That’s why it’s incredibly important to place the exhaust at least 4 feet away from windows, doors, and air inlet.

What Happens If Furnace Exhaust Is Blocked?

If your HVAC system is operating properly and only the exhaust is blocked, then the pressure switch will get triggered, and the unit won’t turn on until you get the exhaust cleared.

However, the safety device might not get triggered. In such a case, the combustion gases won’t be able to escape outside and they might get released into your house.

Read: What Does A Pressure Switch Do On A Furnace?

How Hot Does Furnace Exhaust Get?

Conventional furnaces that are 80% efficient will make the flue pipe extremely hot (the temperature might get up to 400 F). 

Highly efficient combustion furnaces take advantage of all the heat that is generated in the system. That’s why the exhaust that leaves the system might be merely warm.

By the way, combustion furnaces usually have PVC pipes as they don’t have to withstand extreme temperatures, but they have to be able to cope with the acidic condensate. 

What Does Furnace Exhaust Smell Like?

Usually, if you smell gas, you should get alarmed. However, if the smell is coming from the furnace exhaust pipe, then it’s completely normal.

If your window or door is open and the wind is blowing the exhaust fumes back into your house, then it’s okay to notice a slight gas smell. This is exactly how the system gets rid of the gas that hasn’t been fully used during the combustion cycle – through the exhaust pipe.