The house becomes depressurized. Carbon monoxide in the flue gases spill into the house. Lack of adequate combustion air increases the rate of carbon monoxide production, making the problem worse. The products of combustion from fossil fuel fired heating appliances cause several hundred deaths and thousands of health complaints each year. Continued exposure to carbon monoxide can cause permanent brain, nerve, or heart damage.
Some people require years to recover while other remain permanently incapacitated.
Direct vent units obtain combustion air from outside the house, and are less likely to allow the products of combustion to remain in the house. Because of the sealed design and forced exhaust flows, downdrafting does not occur. Extra safety switches ensure that venting is occurring before the burner will ignite. Furnaces, boilers, water heaters, and fireplaces are available as sealed combustion models. Sealed combustion units deliver more heat more efficiently at reduced costs.