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Understanding CO, Part One

It’s a tradition in the appliance and HVACR service industry that there will be a high level of concern about carbon monoxide in the fall, with technicians fielding questions from the customers about alarms and requests for testing equipment for proper operation. And, as the winter wears on, people tend to forget about the concerns they had at the beginning of the season because their assumption is that the threat of a  (Carbon Monoxide, so identified with the chemical symbol “CO” since it is a product of carbon and oxygen) spill has been dealt with for another year, and they don’t need to be worried about it until the next heating season rolls around. The reason this is true is because most people equate a CO spill into a living space with a serious incident; the kind of thing you hear about on a news report about a furnace problem that results in someone getting killed.

However, from a technician’s perspective, there’s much more to the issue of CO safety, equipment operation and the alarms that are supposed to warn of an incident. One issue is what is commonly referred to as low-level carbon monoxide poisoning, which can be occurring in a residence or commercial building on an intermittent basis at any time of the year, and it goes largely unnoticed even though has an effect on the health of the building occupants. Consider the simplified building in Figure One.

Figure One

The first thing to understand about this building is that its tight construction, along with several other factors, is that it’s possible to create a slightly negative pressure in the structure. Hence, the collapsed look of the house. And, in this typical home, the gas furnace, which is a natural draft type, the and water heater  (which will almost certainly always be natural draft equipment), are in the basement along with the clothes dryer. On the main floor, there’s a range vent in the kitchen and a vent in the bathroom, and there’s a fireplace. And there is an attic vent system. Note what is happening to the vent system on the water heater and the furnace.

Both of these items, since they are connected to a common vent system, rely on a slightly negative pressure…likely in the range of -.02 to -.06 on water column-inch scale… to vent the by-products of combustion from the building. And when the pressure in the building is unaffected, or if there is either plenty of infiltration due to less-than-tight construction or a combustion relief system, this atmospheric vent system on these two appliances will operate as designed. However, in the event that the building pressure drops to a negative level, the vent system will back-draft, a condition we’re showing in our illustration.

One of the reasons for this condition is the vent on the clothes dryer, which expels approximately 150 CFM from the living space. Next is the kitchen vent, which, depending on the model, can send up to 250 CFM of air out of the building. In the bathroom on the second floor, the fan there, which is designed to vent moisture and odors, will also expel another 50 (or possibly more, depending on the design of the fan) CFM out of the building.

When you add the possibility that there could be infiltration from the living space up into the attic, and a possible open chimney damper on the fireplace, some, or all these factors, could create a negative pressure that allows for the back-drafting of any natural draft appliance. And, since carbon monoxide has a specific gravity of 0.98, meaning it is slightly lighter than air, an open basement door or infiltration from the basement to the living space will allow the CO to rise. And, since CO is so close to the weight of air, it will stratify and linger in the building.

Until next week….

Learn from yesterday….Live for today…..Look forward to tomorrow