How to tell if furnace is leaking carbon monoxide

How to Tell if Your Furnace is Leaking Carbon Monoxide

Carbon monoxide is an odorless and colorless gas and very deadly. Many times, people don’t realize that some of the appliances they use in the home can produce this gas at toxic levels. When the power goes out in the home, many people will use a generator. However, we hear often of people dying from carbon monoxide poisoning from their generators. This is because the generator must be kept outdoors, due to the carbon monoxide levels that they produce.

Most homes here in Colorado and northern states where it is cold have a furnace that is used for many months of the year. How do you tell if your furnace is leaking carbon monoxide? There are many signs such as a yellow or flickering flame, moisture around windows or walls, and rust on the vent pipe. Now, some of these signs can be other things so it is best to have a professional check it out.

We will discuss why you need to know if your furnace is leaking carbon monoxide, signs of a leak, if you suspect there is a leak, symptoms, prevention, maintenance, and when to contact a professional.


Why You Need to Be Able to Tell the Furnace is Leaking Carbon Monoxide

As mentioned above, carbon monoxide is deadly. In fact, 430 people die each year from accidental carbon monoxide poisoning, and 50,000 end up in the emergency room due to accidental poisoning. Furnaces are great for heating up the home. They send the carbon monoxide outside the home, but if there is a crack or a leak then it comes into the home. Being in a small space with carbon monoxide can be deadly so learning about what to look for will help.

Signs That Tell the Furnace is Leaking Carbon Monoxide and the Location

There are a few signs that tell the furnace is leaking carbon monoxide. These are:

1. Yellow or flickering flame – this is usually a blue color so this is a sign that the unit may be burning carbon monoxide

2. Moisture around windows or walls – this is an indicator of a leaking furnace

3. Streaks of soot– black, brown or yellow soot stains may be indicating that carbon monoxide is leaking

4. Rust on the vent pipe – water and moisture may have leaked out, thus indicating a leak in the furnace

These are signs because it means that the burner is not receiving enough air so it is causing improper combustion. This then produces high levels of carbon monoxide.

These signs above will show up on the locations below:

Heat Exchanger

The combustion takes place in this part of the furnace. Then, the air is then sent out of the furnace by the flue pipe. It can develop cracks and allow the air to be circulated in the home instead of sent outside.

Exhaust Flue

The exhaust flue is the round pipe that goes from the top of the furnace and exits the home through the roof where it exhausts the air. Unfortunately, as the furnace ages, moisture can get into the flue pipe. Just like the heat exchanger, the moisture can cause rust to develop and lead to holes in the pipe. Thus, letting the carbon monoxide out into the home instead of it being carried out of the home.

Many times, carbon monoxide leaks come from unprofessional maintenance/installs and a lack of maintenance on units.

Call a professional right away if you notice any of these signs.

If You Suspect There is a Carbon Monoxide Leak

  1. Get your family and pets out of the home as quickly as possible
  2. Call the HVAC company and gas company
  3. See a doctor to have the family’s health assessed. Carbon monoxide poisoning is treatable, but only if caught early

Symptoms of Carbon Monoxide Poisoning

Short-term symptoms – frequent headaches, nausea, vomiting, dizziness, flu-like symptoms or even sick pets.

Long-term exposure – memory loss, brain damage, or heart complications. If you don’t have other reasons why this may be happening, contacting a doctor is a good idea and also a professional to check the levels in the home.

Carbon monoxide detector

Prevention of Carbon Monoxide Leaking From the Furnace and Other Places

  1. Regular Maintenance – If you have central air, you will have your HVAC unit checked each year. This is a great time to have your furnace checked out as well.
  2. Carbon Monoxide Detector– have a carbon monoxide detector on each floor of your home. This will detect if it is leaking from the furnace or other places in the home.
  3. Don’ts – don’t leave a car running in a closed garage, a generator running inside the home, anything gas-powered running further than 20 feet from an open window. In fact, it is best not to use any of those things in an enclosed space. Lastly, never use a grill, lantern, or camping stove inside a tent, home, or camper.
Air handler
Air handler

Maintenance of the Furnace and Air Handler

  1. Check the air handler to make sure it is working. You don’t want to find out that the float switch isn’t working, especially on hotter days. This will cause the entire system to shut down. Perhaps you notice the air handler is leaking water? Make sure you have this checked out. It could be that it is sweating or there might be bigger issues. We know that HVAC’s don’t last forever, about 10-15 years but maintenance can help them last as long as possible.

2. Hire a trained professional to check the furnace once a year and use a carbon monoxide detection device. This will be helpful and give you peace of mind.

3. Change the filter every 30-90 days, depending on if you have pets or smokers in the home. If your furnace is separate from the air conditioner unit, change these at the same time. If it is all together in one unit then you only have one filter to worry about.

4. Keep all vents clear of debris, including the ones on your roof.

clean the chimney
Clean the chimney so there are no clogs

5. Clean the chimney in the warmer months. You want to make sure there is no clogging in the chimney, as burning wood creates carbon monoxide. If it clogged, it goes back into the home. Summer maintenance is essential.

5. Install carbon monoxide detectors on every floor and change the batteries each year.

6. If you find you need repairs done or a new furnace, don’t try to do this yourself. It is a good idea to hire a professional. Furnaces last about 16-20 years so keep that in mind and plan to replace it if it is near that time or there are a lot of issues. It is better to be safe than sorry.

When to Contact a Professional

Hire a professional if you have any issues or concerns with your furnace. The furnace is very expensive and also dangerous to mess with unless you are trained. Leave it to someone who knows how to fix it to do the repairs. Additionally, contact your doctor right away if you suspect there is carbon monoxide leaking or go to your nearest hospital.


Carbon monoxide is nothing to play with. As mentioned earlier, it is a silent killer that is colorless and odorless. Keeping an eye on the areas above, plus installing a carbon monoxide detector will help. We will check your furnace when conducting a home inspection in the Castle Rock area. Think you may need our services? Leave us a reply below!

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Trackbacks & Pingbacks

  1. […] leak. However, it can happen with a furnace and gas appliances and when there is negative pressure. The warning signs of a carbon monoxide leak are yellow or flickering flame, moisture around windows … Being on the lookout for these signs will help. Carbon monoxide kills so you need to be careful and […]

  2. […] You will want to monitor the carbon monoxide if you have a furnace. One way to do this is by looking at the furnace. If you see a yellow or flickering flame, moisture around the windows or walls, or rust on the vent pipe you may have a carbon monoxide issue. […]

  3. […] flame, moisture on the windows or walls, or even rust on the vent pipe, the furnace may be leaking carbon monoxide. It is important to make sure the furnace is off and have this checked out right […]

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