Danger of Negative Pressure in Carbon Monoxide Poisoning

When something subtle goes wrong to cause carbon monoxide poisoning, the culprit is usually negative pressure. Certainly, a broken or corroded exhaust pipe can cause a leak in the exhaust from any fuel burning appliance. But even in those cases, the reason for the corrosion may have been condensation from negative pressure. When fuel is burned inside, it is key to do two things: make sure that the fuel fuel burns clean and second, exhaust any harmful gases. When there is insufficient oxygen for the fuel to burn completely, carbon monoxide is formed. Even if there is carbon monoxide in the exhaust, the fumes should be safely exhausted up the chimney. However, when there is a problem with sufficient pressure up the chimney (negative pressure) the exhaust often doesn’t make it all the way outside the building. And when there is insufficient oxygen flowing to the flame, there is also a serious risk not only of incomplete combustion, but inadequate heat to prevent back up of the exhaust.

Proving the cause of negative pressure inside exhaust piping was the key element to recovering $3 million for clients of the Brain Injury Law Group. Below is an excerpt from the deposition taken by me, of a defense expert in a hotel poisoning case in Green Bay that settled in the summer of 2015. Shortly after the deposition below, the HVAC contractor involved agreed to pay more than half of the settlement. Negative pressure warning signs were the key element in such settlement.

It is critical to understand the relationship between negative pressure and carbon monoxide poisoning. The danger of negative pressure is explained by this summary of the physics of exhaust.

• The hotter the exhaust, the higher hot air can rise up a chimney.
• High efficiency hot water heaters suck almost all of the heat out of the exhaust, so that the exhaust gases are not hot enough to get up a vertical chimney. This is the reason that high efficiency appliances are exhausted out the side wall of a building, such as with a modern residential furnace.
• When exhaust isn’t hot enough, the exhaust condenses (meaning the gases turn to liquid, such as water and acid) before it leaves a building. Condensing gases cause corrosion to the vent system, which is what happened in the Days Inn.
• Further, in order to avoid the danger of negative pressure issues, hot water heater and other HVAC manufacturers should require a direct fresh air intake, again running from the sidewall of the building. In the Days Inn case, the contractor did not install a direct fresh air intake at installation of the new hot water heaters. Further, the HVAC contractor failed to install the fresh air intake later, despite warnings signs that the negative pressure in the hotel was causing damage to the equipment and visible evidence of corrosion and condensation in the building.