Furnaces and hot water tanks are generally the largest sources of CO in the home because of the amount of fuel they consume. The amount of CO in combustion gas depends on the cleanliness and tuning of the burners, on the quality and quantity of combustion make-up air, and other factors. In the case of this home, the make up air venting has been purposefully blocked. When there is an inadequate amount of combustion make-up air, there is the potential for insufficient amounts of fresh oxygen that can cause larger amounts of CO to form. Unfortunately, I see this far too often.
Make up air ducts are basically 6 inch tubes that connect to a vent boot at the exterior of your home which purpose is to deliver fresh air to combustion appliances. Older make up air ducts are usually ridged sheet metal tubes, while in new construction they are wrapped in insulation and flexible. They are passive intakes that provide needed air to the home. To the unaware, make up air vents just seem to be a pipe that is not connected to their furnace or hot water tank, doesn’t contain any kind of damper, and delivers a constant flow of cold air into their home. Now, I know that in good spirit, when someone feels a constant draft coming into the basement or utility area that the first thing to do is to plug it until they figure out what needs to be done about it, but it’s a bad move.
When air is removed from a home due to combustion, it has to be replaced. Without the installation of a make-up air vent, the combustion air will get replaced from breaches in the building envelope. If the home has low air leakage then the only make up air will be from recirculated air in the home. When there are combustion gas vent pipe leakage problems, such as with poorly installed hot water tank and furnace venting the products, the combustion gases become part of the combustion make-up air. The amount of potential CO buildup in a home will depend on the amount combustion gas leakage and amount of outside air changes in a home. The more the combustion gas leakage, and the less the outside air changes, the more serious a potential problem can be. For this reason, one can see why it is so very important to have CO alarm protection or a detector that measures potential CO levels.
When a combustion air make up duct is properly installed, it will help to prevent the house from getting depressurized and from potential hazardous CO conditions.