Carbon monoxide expert witness Russell Keeler, PE, writes:
The push for greater and greater energy conservation has led to a variety of unintended consequences. One of these is that very high efficiency gas burning appliances such as furnaces and boilers can produce high, and perhaps toxic levels of carbon monoxide (CO). High efficiency appliances produce relatively low flue gas temperatures. These low temperatures can cause condensation of the flue gas. If the wrong type of flue is installed, the flue will corrode, and flue gas may leak into the room where the appliance is installed. Death from carbon monoxide poisoning has resulted in a number of cases over the last few years.
Burners need to be checked every year to determine the amount of carbon monoxide present. Anything over a trace amount of CO requires the retuning or replacement of the burner.