The whitish vapor you see coming out the flue that is closest to you is indicative of a safety problem. It is also something an inspector sees a lot of. I was up on a roof and had identified the two flues to the right as having come from abandoned fireplaces. In that case, they should have been capped off. However, as I was up on the roof, suddenly the steam and distinctive exhaust gases from a gas burning appliance started coming out the one flue. This is an unsafe way to vent gas appliances. Full details of why this common, but old-fashion arrangement, is a problem are listed below. Sizing: A flue needs to be sized for the appliance it serves. The furnace and water heater in your home have a BTU (British Thermal Unit) rating usually listed in terms of thousands of BTU. It is the heat generated by the appliance that promotes venting. If you have a large, cold chimney and a small BTU appliance, it may not generate enough heat for the flue gases to rise inside the chimney. Adding a properly sized flue liner reduces the area to be heated by the appliance and therefore increases the venting capacity of the chimney.
Vapor: One of the byproducts of burning fossil fuels such as natural gas or oil is water vapor. If the chimney is unlined and cold, the water vapor will condense inside the chimney and the chimney will become wet. A wet chimney can stain the walls and ceiling of the home and rust out the metal flue pipes.
Gases: Other byproducts of combustion are nitrogen dioxide and carbon monoxide. When the water vapor produced by combustion combines with nitrogen dioxide, the result is nitric acid. Acid vapors can eat away at the mortar in the brick chimney. When the chimney passes through a home, if there is a problem like this, it can result in poisonous gases making their way into the home.
Efflorescence: This is a whitish mineral powder that forms on the inside or outside brick of the chimney that is exposed to the air. When water vapor soaks into the brick, the moisture migrates through the brick, usually to the exterior, where it evaporates, leaving behind minerals collected by the water. The minerals form a whitish powder that can usually be brushed off. In winter, the migrating water freezes inside the brick, resulting in damaged brick or a cracked chimney. For more information, check out this blog by James Quarello, home inspector.
Steven L. Smith
Bellingham WA Home Inspections
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