How To Prevent Dirty Furnace Filters Releasing Carbon Monoxide

Carbon monoxide (CO) poisoning affects more than 50,000 people in America each year. Because of its debilitating health effects, it is crucial to know which home appliances contribute to its production. Since most CO poisoning occurs in winters, whether or not a dirty furnace filter is a source of carbon monoxide is a common concern.

Clogged furnace filters can cause carbon monoxide poisoning. Build-up in the furnace filter prevents adequate airflow, causing the heat exchanger to overheat. This leads to incomplete combustion, producing carbon monoxide (CO) gas, which is released from the cracks in the heat exchanger.

This article is a detailed overview of the production of carbon monoxide (CO) gas from dirty furnace filters, signs of CO leak, and its health effects. We then talk about essential safety measures to prevent CO poisoning. Lastly, we also discuss some interesting FAQ.

What is Carbon Monoxide (CO)?

Carbon monoxide, or CO is an odorless, colorless, flammable gas mainly produced through incomplete thermal combustion.

When the combustion fumes are inhaled, the CO concentration in the body replaces oxygen content, which leads to varying negative health effects.

How A Clogged or Dirty Furnace Filter Releases Carbon Monoxide

Dirt or blockages in a furnace filter disrupts the airflow, which causes incomplete combustion in the heat exchanger, this then results in the production of CO. To better understand the phenomenon, let’s first take a look at the working mechanism of a furnace and its filter:

How a Furnace Works

A furnace is a home appliance that works as a heating system by heating up and distributing warm air throughout the house.

A typical furnace functions by:

  • First, the cold air is drawn into the system.
  • Next, the furnace filter traps contaminants from the air and sends them to the exchanger.
  • The heat exchanger heats the air.
  • Finally, the hot air is dispensed into the room via a vent.

How A Clogged Filter Causes Carbon Monoxide Production

The filter serves to prevent the dust and debris from infiltrating the system. If not changed on time, the saturation in the filter hampers further filtration. This allows waste to accumulate in the unit, causing some parts to malfunction.

The build-up of dirt, dust and debris also resists the incoming air, and the inadequate airflow wears down the heating system.

All of these effects interfere with the combustion process, and consequently, CO gas is released.

How To Know If Your Furnace is Releasing Carbon Monoxide

CO is known as a silent killer because it’s colorless and odorless properties make it difficult to identify. However, there are signs you can look out for to ensure your furnace is not leaking the gas.

  • Soot Stains: Soot stains are one of the first signs of CO spread. Soot is a black, brown, or yellow powdery stain produced due to the incomplete burning of carbon. Since carbon is not adequately combusted, CO leaves soot stains on furnaces, furniture, and walls.

It is essential to mention that various activities such as excessive use of candles can also lead to soot. However, it is crucial to determine the cause quickly to prevent any potential harm.

  • The Smell of Exhaust Gasses: Although CO itself is odorless, exhaust gasses often accompany its emission. Therefore, look out for a smoke or a burning smell when operating a furnace.
  • Yellow Burner Flame: The furnace has a pilot light located inside the combustion chamber that usually appears blue. However, in the case of incomplete combustion or when adequate levels of oxygen are not available for burning, the pilot light flickers yellow or orange.

Apart from the above-mentioned signs, you can also check for CO leaks if anyone around the house starts experiencing related health symptoms simultaneously.

CO poisoning manifests in the form of the following initial symptoms:

  • Dull headache
  • Nausea
  • Vomiting
  • Blurred vision
  • Dyspnea (shortness of breath)
  • Confusion
  • Unconsciousness
  • Chest pain
  • Dizziness
  • Anxiety
  • Irritable behavior and unusual aggression in pets

The severity of symptoms depends on the extent of exposure. However, considering that even a minute concentration of CO is harmful, nausea and headache are experienced immediately within minutes of release.

Where Does Carbon Monoxide Leak From a Furnace?

CO can leak from two locations in the furnace; cracks in the heat exchanger and exhaust flue.

Cracks in the Heat Exchanger

As we know, the heat exchanger is the part where combustion occurs, and thus CO is produced within the chamber. Cracks in the heat exchanger are the most common points where CO leaks from.

There are 4 main causes of cracks in the exchanger:

  • Overheating: Overheating is one of the primary causes of premature cracks in the furnace. The clogged filters add to the resistance in airflow, and as a result the exchanger overheats. Over time, the heat stress causes the furnace walls to weaken and crack.
  • Undersized Ductwork: An improperly-sized ductwork is another cause of bends in the exchanger. It restricts the airflow, preventing the entire system from working adequately.
  • Normal Wear and Tear: Over the period of use, the furnace acquires wear and tear. Lack of maintenance then leads to cracks.
  • Large Furnace Size: An oversized furnace implies a system that is too big for the room and typically heats the space up in 10-15 minutes (short cycling). Although oversizing furnaces is a common practice, it is impractical and potentially dangerous in the long run—the short-cycling causes rapid expansion and contraction in the metal pipes, which results in cracks.

Exhaust Flue

The exhaust flue is a metal tube in the furnace that directs combustion gasses out of the house. Over the years, moisture and oxygen can cause the pipe walls to rust and crack. Allowing the CO gas to leak through into the house.

Can a Furnace Produce Carbon Monoxide When Turned Off?

A furnace cannot produce carbon monoxide (CO) when turned off. The furnace needs to be powered on to carry out combustion. Since combustion does not occur when the furnace is turned off, CO gas will not be produced.

It is, however, essential to remember that if other home appliances are releasing CO your furnace can still pull in the gas and circulate it through the cracks in the heat exchanger and exhaust flue.

How To Avoid Carbon Monoxide Release From a Furnace

Due to the highly debilitating and lethal effects of CO, it is essential to ensure that your furnace is not releasing the gas. Here’s what you can do to prevent CO production from the furnace:

1. Proper Installation

Malfunctioning in various parts of the furnace often arises due to improper installation of furnace motors or incorrect ductwork. Correct installation of the furnace is critical, and therefore, it is usually best to get a qualified professional to do the task.

2. Correct Sizing of the Furnace

Although oversizing (short-cycling) the furnace is a trend to achieve the ideal warm temperature quickly; it can be counterproductive and cause leaks in the system.

Here’s how you can estimate the correct furnace size for your room:

  • Calculate the area of the house in sq. ft.
  • Check for the climate zone that your home lies in. Each zone has a certain recommended heating factor.
  • Multiply the area by the lower number (heating factor) recommended for your location.

Remember also to consider insulation, ventilation, and the age of the home when sizing the furnace.

You can check with an furnace expert to estimate the furnace size suitable for your home.

3. Annual Maintenance

Adequate maintenance of an appliance is the key to keeping it running for a long time. With heating systems like a furnace that pose a danger for gas leaks, it becomes even more imperative to get it thoroughly examined and repaired.

Here’s what constitutes furnace maintenance:

  • Detect any cracks.
  • Check for potential CO leaks.
  • Repair any broken or malfunctioning parts like blower fan, motor, etc.
  • Clean the filter (every 90 days) if a permanent filter is installed.
  • Replace the filter annually.

4. Choose an Appropriate Filter

As we have established, furnace filters play a crucial role in maintaining the airflow, improving the air quality, and general optimal functioning of the system. Therefore, it is essential to choose a filter compatible with the size and design of your furnace.

Here are some of the aspects you should check for when selecting a furnace filter:

  • Types: Furnace filters come in various types such as fiberglass, pleated, high-efficiency pleated, electrostatic, disposable and permanent, etc. Each of these has a specific working mechanism and purpose. For example, the high-efficiency pleated filter is suitable for people with autoimmune diseases.
  • Sizes: Furnace filters are available in different sizes. The size of the filter compatible with your system depends on the furnace size.
  • MERV Ratings: MERV rating defines the thickness of a filter (the higher the rating, the denser the filter). While a thicker filter has a higher filtration capacity, it can also obstruct the airflow.

Filtrete have probably the largest range of quality furnace filters available. Once you know the type, size and MERV rating you require you can visit the filtrete store on to view the appropriate filter. Although, you should always check with a professional to determine the right filter for your furnace.

5. Carbon Monoxide Detectors

CO detectors are devices that sense a certain level of the gas and signal it via an alarm. Given the risk of CO leaks, it is essential to have a CO detector installed at home.

These are affordable and readily available from, such as this one from Kiddle.

Carbon Monoxide Poisoning

What is Carbon Monoxide Poisioning?

Carbon monoxide poisoning implies a set of symptoms triggered by inhaling the CO gas. The red blood cells (RBCs) in our bodies carry oxygen (bound to hemoglobin Hb) and deliver it to tissues. This oxygen is converted into energy and is thus essential for optimal functioning.

Hb has a 250 times greater affinity for CO than for oxygen. Therefore, CO soon displaces oxygen and binds to Hb in the red blood cells. As a result, the vital organs do not receive oxygen, causing symptoms. In the absence of prompt medical intervention, CO poisoning eventually leads to organ failure.

Health Complications

The common health effects of CO poisoning are:

  • Permanent heart damage because the heart is severely affected by low oxygen
  • Miscarriage because fetal hemoglobin has an extremely high affinity for CO
  • Fatigue and nausea due to disruption in oxygen supply
  • Unconsciousness
  • Anoxic brain injury may occur due to a complete lack of oxygen (It results in the death of brain cells within four minutes)
  • Death within minutes if CO concentration is high

Risk Factors

Following people are at higher risk of CO poisoning:

  • Infants and children because they breathe faster than adults and thus inhale more CO
  • Elderly because of compromised health conditions such as underlying heart diseases
  • Fetuses because CO crosses the placenta and binds to fetal Hb with a high affinity for the gas
  • People with heart diseases, blood disorders, or respiratory issues

How Much Carbon Monoxide Can Cause Poisoning?

There is no specific concentration at which CO gas becomes dangerous. In fact, inhaling even small amounts of CO is a health risk.

However, studies shows that CO levels above 150 – 200 ppm can lead to disorientation, unconsciousness, and even death.

How Long Does It Take To Get Carbon Monoxide Poisoning?

The concentration of carbon monoxide (CO) determines the time it takes for CO poisoning to manifest. Usually, people start feeling severe dizziness accompanied by chest pain and dyspnea within 1-2 hours of exposure.

The time it takes to get CO poisoning depends on the concentration of the gas. Typically, people start experiencing initial symptoms like nausea and headache within five minutes of exposure. Patients often present with shortness of breath, chest pain, or an unconscious state within 1-2 hours of exposure.

However, in extreme cases, CO poisoning may even kill a person within 5 minutes.


Can You Get Sick From a Dirty Furnace Filter?

A clogged furnace filter is incapable of adequately filtering dust, debris, bacteria, and allergens from the air. Breathing in the contaminated air can potentially cause sickness.

Apart from health effects, a dirty filter deteriorates the air quality and stagnates the air, causing mold and mildew build-up, odor in the house, and a change in the color of walls and furniture. To prevent this, regularly maintaining and replacing the filter is crucial.

Furnace filters do not only protect the system from infiltration or overheating, they also improve air quality, making it safe to breathe. Hence, dirty furnace filters understandably have a range of health implications, some of which are as follows:

  • Cold and Flu
  • Allergic reactions
  • Aggravated asthma
  • Increased chance of bacterial infections

A saturated furnace filter fails to trap allergens which may cause:

  • Fatigue
  • Tightness of chest
  • Headache

Theresa Orr

Theresa Orr is an Earth Scientist who specializes in determining past climates from rocks using geochemistry. Her passion for clean water, soil and air drives her to provide easy to understand information for everyone to read.

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