DEAR BARRY: You are part of a large group of folks who misunderstand the effects of a cracked heat exchanger in a forced-air furnace. The supposed "threat" of carbon monoxide is perpetuated by members of the heating industry who have managed to rope in many utility companies and home inspectors in promoting this fear.

When you consider that the heat exchanger is on the outlet side of the blower, it is more likely that circulating air would enter the exhaust stream than exhaust would enter the air stream and the home. This is the conclusion of a new study, but that information has apparently not come to your attention. –Dave

DEAR DAVE: According to a wise philosopher (Dennis Prager), scientific studies do one of two things: They agree with common sense, or they are wrong. In this case, common sense reflects what is routinely observed in furnaces with cracked heat exchangers. There are abundant cases of people having been hospitalized or having died from carbon monoxide (CO) exposure, where the source of CO was determined to be holes or large cracks in a heat exchanger.

From a scientific standpoint, the process is elementary. A heat exchanger is a metal container with a gas burner inside. When the burner is lit, the metal container becomes hot. Air is blown across the outer surface of the hot container, the air becomes warm, and the heated air is blown into the living areas of the home.

Meanwhile, the exhaust fumes inside the metal container rise through a chimney, known as a flue pipe, and exit at the roof. As long as the exhaust does not mix with the warm air, occupants of the home remain safe.

However, if the hot metal container has cracks or holes, combustion exhaust can enter the warm air stream in the following manner: …CONTINUED

As circulating air from the blower moves across the curved surfaces of the heat exchanger, air pressure is lowered. Reduced air pressure is what causes lift when air moves across the curved upper surface of an airplane wing. It is what enables a sailboat to travel upwind when tacking.

And it can cause a bicyclist to be drawn toward a truck that is passing on the highway. In a forced air furnace, low pressure in the warm air stream acts as a vacuum, drawing exhaust through the damaged metal surfaces of the heat exchanger.

Having inspected furnaces with damaged heat exchangers, I have seen tongues of gas flames lick through the cracks and holes and have observed black soot left on the air registers. These observations clearly outweigh the doubtful findings of an alleged scientific study.

DEAR BARRY: Everyone knows that a bedroom should have a door. But the home we just purchased has a bedroom doorway with no door. Is this room legally classified as a bedroom, or would it be regarded as a den or office? –Dan

DEAR DAN: Surprisingly, the building code makes no mention of doors on interior rooms of any kind, not even bathrooms. Obviously, it is unusual and impractical to have a bedroom without a door, but in this case the room could be regarded as a bedroom in need of upgrade, rather than a den or office.

To write to Barry Stone, please visit him on the Web at www.housedetective.com.

***

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