How To Prevent Dirty Furnace Filters Releasing Carbon Monoxide

Carbon monoxide (CO) poisoning affects more than 50,000 people across America each year. It’s surprising which appliances can make CO, increasing the danger in our home.

Clogged furnace filters can release carbon monoxide. 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 cracks in the heat exchanger.

This article has everything you need to know about how carbon monoxide (CO) gas can come from furnaces, signs of a CO leak, and its health effects. We’ve also got some safety measures to help prevent CO poisoning.

Furnaces And Carbon Monoxide Production

Carbon monoxide, or CO is an odorless, colorless, flammable gas mainly produced through incomplete thermal combustion. You can think of this as fuel that isn’t completely burned up, like wood in a campfire.

When we inhale these combustion fumes, the CO replaces the oxygen in our body, which causes health problems.

How Dirty Furnace Filters Release 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.

A typical furnace works by:

  • Cold air enters the system.
  • The furnace filter traps contaminants from the air and sends it to the exchanger.
  • The heat exchanger heats the air.
  • Finally, the hot air is sent into the room via a vent.

The job of the filter is to stop the dust and debris from entering the furnace system. When furnace filters aren’t replaced regularly they become clogged and can no longer filter out these contaminants. This allows waste to accumulate in the unit, causing some parts to malfunction.

The build-up of dirt, dust and debris also pushes back against the incoming air, causing low airflow, which wears down the heating system.

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

The dirty filter also deteriorates the air quality, which can cause mold and mildew build-up, and bad odors in your home.

How To Know If Your Furnace is Releasing Carbon Monoxide

CO is known as a silent killer because it’s colorless and odorless, which make it difficult to identify.

Carbon monoxide detectors are the best way to know if your furnace is leaking CO. However, there are signs you can look out for if you’re worried your furnace might be leaking CO 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. These soot stains are usually found on the furnace itself, as well as nearby furniture, and walls.

There are other activities such as excessive use of candles that can also lead to soot. But this is more likely near where you place the candles, and not on a furnace.

  • The Smell of Exhaust Gasses: Although CO itself is odorless, there are often other exhaust gasses that are released at the same time. Look out for a smoky or burning smell when using the 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 there isn’t enough oxygen available for burning, the pilot light flickers yellow or orange.

Other than these signs, there are also some health symptoms you can look out for.

CO poisoning can cause 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 tiny amount of CO is harmful, nausea and headache are usually experienced immediately within minutes of release.

If you suspect a CO leak at any time vacate your home immediately.

Where A Furnace Leaks Carbon Monoxide From

CO can leak from two places in the furnace: cracks in the heat exchanger, and the exhaust flue.

1. Cracks in the Heat Exchanger

As we know, the heat exchanger is the part where combustion happens, and CO is produced within this 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 the most common cause of 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: Incorrectly-sized ductwork can cause bends in the exchanger. It restricts the airflow, preventing the entire system from working properly.
  • Normal Wear and Tear: Furnaces naturally experience wear and tear over time. This can lead to cracks, but this is less likely to be the cause.
  • Large Furnace Size: An oversized furnace means that it’s 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.

2. 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.

When To Turn The Furnace Off

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

But remember, even if your furnace isn’t making CO gas if other home appliances are releasing it, your furnace can 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

Carbon monoxide is a gas that we definitely want to avoid. Here’s what you can do to prevent CO producing in your furnace:

1. Proper Installation

Incorrect placement of furnace motors or incorrect ductwork can lead to cracking, always use a qualified professional for installation and any major maintenance.

2. Correct Sizing of the Furnace

Although oversizing (short-cycling) the furnace is a trend to achieve the ideal warm temperature quickly; it can cause cracks and 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 to also consider insulation, ventilation, and the age of the home when sizing the furnace.

Obviously, you can also check with a furnace expert to estimate the best size for your home.

3. Regular Maintenance

Regular maintenance is the key to keeping your furnace running for a long time. Take the time to look over your furnace and:

  • Detect any cracks.
  • Check for potential CO leaks (you can use a detector for this)
  • 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 every 12 months.

4. Choose an Appropriate Filter

It’s important to choose the right filter for your furnace, as they can impact air flow. Thick filters can reduce air flow, and if this restriction is too strong it can lead to cracks in the furnace.

Here’s what you should check for when choosing a furnace filter:

  • Types: Furnace filters come in different types, including fiberglass, pleated, high-efficiency pleated, electrostatic, disposable and permanent, etc. Each of these has a specific 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 that will be compatible with your system depends on your 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 buy the filter online. But, if you are changing the type of filter you are using then you should check with a professional to make sure it’s the right fit for your furnace.

5. Carbon Monoxide Detectors

Carbon monoxide detectors can sense CO gas even at small concentrations, and will alert you via an alarm. It’s best to always have a CO detector at home – in most states these are combined with your fire or smoke detectors.

If your detector is ever triggered remember to leave your home immediately. It is impossible to know if it is a false alarm or not as CO is odorless and cannot be seen, and it’s important you move to a safe place outside the home.

You can read more about carbon monoxide detectors here.

Carbon Monoxide Poisoning

Carbon monoxide poisoning is a set of symptoms triggered by inhaling CO gas. The red blood cells (RBCs) in our bodies carry oxygen (which is bound to hemoglobin Hb) and deliver it to our tissues.

Our hemoglobin has a 250 times greater affinity for CO than for oxygen. This means the CO displaces oxygen and binds to the Hb instead. As a result, our vital organs don’t receive enough oxygen. Without immediate medical intervention, CO poisoning leads to organ failure.

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.

Health Complications

The common health effects of CO poisoning are:

  • Permanent heart damage because the heart is severely affected by low oxygen
  • Miscarriage
  • 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

These people are at higher risk of CO poisoning:

  • Infants and children because they breathe faster than adults and so 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

The Speed Of Carbon Monoxide Poisoning

The concentration of carbon monoxide (CO) determines how long it takes for CO poisoning to take effect. The higher the concentration, the faster the negative effects.

Typically, people start experiencing initial symptoms like nausea and headache within five minutes of exposure. With most feeling severe dizziness accompanied by chest pain, shortness of breath or falling unconscious within 1-2 hours of exposure.

However, in extreme cases, CO poisoning can kill a person in under 5 minutes.

Theresa Orr

Theresa Orr holds a PhD in Earth Science and 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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