Negative Air Pressure Equals Carbon Monoxide

Negative air pressure can cause carbon monoxide poisoning because it interrupts the flow of oxygen to a flame, causing incomplete combustion. A fire without enough oxygen creates CO, not the harmless CO2.

Negative air pressure is a crucial component in understanding how carbon monoxide can accumulate.  In carbon monoxide news this week, seventeen-year-old Jenna Fish succumbed to what is believed to have been an incidence of negative air flow. For her story, click here. This negative air low was believed to be a result of a fire dying out in a wood burning fireplace causing the radiator heat to kick on. Jenna had gone to the basement to sleep after messaging her boyfriend that the family’s snoring dog was disturbing her sleep. When her family, sleeping upstairs, woke up to suspected carbon monoxide poisoning, they discovered Jenna lifeless in the basement. The carbon monoxide had accumulated more densely in that area. The family said they had four functioning smoke and carbon monoxide detectors in the home, but none sounded the alarm.

What Causes Negative Air Pressure?

There are several reasons negative air pressure can occur. Negative air pressure occurs when the pressure inside your home is less than the air pressure outside your home. One of the main structural reasons is that our homes are more airtight. Since the seventies we have built homes to be more fuel efficient, tightening them up to reduce heating costs. This can decrease the air flow in our homes resulting in insufficient air/oxygen to allow for the proper combustion or venting of appliances and heating devices.

For more on negative pressure, click here.

Normally air moving out of our homes is replaced by air moving into our homes. But modern homes have reduced the amount of air coming in without necessarily increasing the amount going out.  This occurs because the movement of warmer air upwards, the use of exhaust fans and just humans using air to breathe.

Modern Homes Require Dedicated Fresh Air

In theory, a modern home has a high efficiency furnace which gets its combustion air from outside, while also exhausting air out the side wall of the house, as shown in the below picture. But when a house with a natural draft furnace (not high efficiency) gets tightened up, negative pressure problems can become more severe.

Negative air pressure may be avoided in tight modern homes with high efficiency furnaces which get combustion air directly from outside air. Outside air comes into this house directly from the down facing pipe here.

The average home creates a natural vacuum as warm air rises to the ceiling. But add in the many other ways we remove air from our homes, we can see the problems created. Bathroom fans, central vacuums, clothes dryers, and numerous other appliances remove air at a faster rate than it can be replaced.  Restricting the replacement air can cause a rise in carbon monoxide from furnaces, hot water heaters, wood stoves and fireplaces.

One might conclude that if your home is drafty then it is not at risk. But drafty homes are often signaling us that there is a greater flow of air out of the house and not that the airflow in has increased. The same effect is created. Consequently, equalizing the pressure in the house through proper air intake could actually reduce drafts.

More Draft Isn’t Always the Solution

However holes in the upper portion of the home, especially in the upper portion of the home, can create the same problems. Warm air rises and escapes through leaks in the upper portion of the house, creating more of a draw of cold air in the basement. If this airflow is less than the loss of air in the upper portions of the home it ultimately depressurizes the house and increases the risk of downdrafts which can raise the level of carbon monoxide.

Fireplaces function because warm air rises. Again that air must be replaced by incoming air. Certain weather conditions such as high winds can cause the chimney to malfunction, creating a downdraft.

Debris can also cause problems with proper ventilation. In the case of Jenna Fish, when the fire went out, the heating system itself created a depressurized state that allowed carbon monoxide to build up in the basement. Exhaust fans in bathrooms and kitchens can have the same effect.

Most of these instances of negative airflow are also called backdrafting or downdrafting. They can be detected by strips called Backdraft Indicators. Qualified heating contractors can use these strips to monitor the potential for negative air flow in the home. Unlike carbon monoxide detectors, these are not alarms. They are indicators to a professional of the patterns of air flow in the home and reveal areas of concern.

Never try to solve these negative air pressure issues yourself. Adding more exhaust pipes for heating may not be helpful as these tend to be highly affected by weather. Fan powered combustion kits are more effective but should only be installed by a professional as fixing one source may actually create problems at other sources. An overall look at the home’s air flow needs to be understood and considered.

Other Solutions for Negative Air Pressure

There are other measures which can be taken to decrease the possibility of depressurization in the home. Some of the ways one can reduce the risk are sealing up leaks and holes in the upper portions of the home and increasing air leakage in the lower portions through the use of combustion air and make-up air openings. Turning off exhaust fans can reduce depressurization. Sealing return ducts in the basement and closing return registers in the basement can reduce backdrafts. Opening supply vents to the basement. And simply, keeping doors open to rooms can reduce the risk of negative air flow.

One can look at a home as made up of general areas of pressure levels with the mid-range level likely being the most neutral pressure zone. The lower level or basement is generally the area where the air pressure falls below the air pressure outside. While the higher levels of the house would have higher pressure than outside as warm air is pressing against the ceiling. This is why your basement is the most likely place for problems. Furnaces, hot water heaters, and fireplaces located in the lower portion of the house are working harder against the negative pressure. So a fireplace in the basement is more likely to cause problems than one located above the neutral zone at a higher level.

It is important to note, however, that the neutral zone is a house is also impacted by other factors. If the basement is better sealed than the upper levels the neutral zone rises. If the basement is the leakiest place in the house the neutral zone drops. This is called the stacking effect and it determines the air flow and potential for problems due to depressurization.

We see this stacking effect also with chimneys, as in the Jenna Fish case. A fireplace installation below the neutral plane, in this case lowered by the heating system itself creates negative air flow if the flue has cooled. When a fireplace is not being used the flue is cold. However when the fire has died, the flue also cools down and the air in the flue begins to match the outdoor temperature. At this point even the most minor venting issues in the home, such as the heat kicking on, can cause negative air flow and the fireplace will backdraft into the home spilling carbon monoxide.

Why did it affect Jenna in the basement and only mildly affect her family? Several factors could explain this, including the location of the heat burning appliance. Most often CO levels will be highest closest to the appliance. Another factor may be that the rest of her family was sleeping in the upstairs bedrooms above the neutral zone where the air pressure was greater than atmospheric pressure while Jenna was sleeping in the basement below the neutral zone where the air pressure was below the atmospheric pressure. The backdraft bringing carbon monoxide into the area in levels high enough to have contributed to her death.

It is important to note that small damages to the heating venting and its connections can lead to the same type of dynamics in that carbon monoxide can be produced when the air going out exceeds the air coming in and combustion is compromised.  It is exacerbated if it occurs in combination with a home which is prone to negative backdrafts in addition.

What spoke to me about this particular news article is that the family had carbon monoxide detectors installed and they did not alarm to the danger. Not only to prevent the death of Jenna, but also to warn the family that they had been exposed to carbon monoxide. Though they did not suffer a lethal dose, one must question how the actual physics of air flow might affect the performance of detectors placed in the wrong location. We see combination smoke and carbon monoxide detectors placed in hallways outside upstairs bedrooms, typically on the ceiling. However that is not the only area they should be placed. Upstairs bedrooms are important because we presume that is where people are sleeping. Ironically, it may be the one place in the home where the atmospheric pressure is greater than the pressure outside.

We begin to understand why this might not be the only location they are needed. It seems a foregone conclusion that based on the stacking affect that precautions should be in place for the lowest areas of the home most at risk for negative air pressure.

Generally, CO alarms should be on every level and certainly on every level where there is a fuel burning appliance and of course on any level there is a fireplace. While rarely the cause of CO poisonings in the United States, they can be the cause, or as in this case, contribute to the cause.

I hope that Jenna’s death brings some awareness to the issue and perhaps that we see better information in the future about home safety. Our homes continue to become more efficient due to rising costs so understanding the dynamics of air flow is of utmost importance in the future.

Take aways:

Landlords Must Have Professional HVAC Maintenance

As important as all of these issues are in a residential single family home, landlords have these problems magnified. Almost all large apartment complexes try to do their own maintenance, with clearly unqualified people doing the work that should be done by licensed professionals. That is a recipe for a disaster, especially when apartments get to be older than the 20 year life expectancy of furnaces and other fuel burning appliances. Demand your landlords use professionals. Your life may be at stake.

This blog was written by Rebecca Martin.

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  1. […] Remember that air even if air can escape through vents in upper levels that lack of oxygen flowing to the flame can lead to the build up of carbon monoxide in lower levels of a structure. Carbon monoxide happens when the fire doesn’t have enough oxygen, even if there is a path for the smoke or exhaust. Click here for more.  […]

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