Rust on metal chimneys (C) Carson Dunlop Associates Explanation of Chimney Wet Time & Metal Chimney Corrosion

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Chimney wet time:

Definition of chimney wet time.

Modern metal chimneys, regardless of the metal used or other chimney construction details,have a significant potential for condensation and corrosion - conditions that can make a chimney unsafe and that lead to need for costly repairs.

Here we give an explanation of hidden safety hazards caused by rust and corrosion of metal chimneys and flues.

These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.

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Wet time and Corrosion in Chimneys and Vents

Rust on metal chimneys (C) Carson Dunlop AssociatesRecommended venting systems with draft hood appliances were developed in the 50's for atmospheric aspirated appliances. The efficiencies were low and approximately 35% of the energy input was wasted.

Newer mid-efficiency Category I appliances have a much lower flue gas temperature and reduced off-cycle losses.

These new furnace designs alter the vent gas temperature, dew point temperature, flow rate and dilution air in the vent.

The result is that in modern metal chimneys, regardless of the metal used or other chimney construction details, there is much greater potential for condensation and corrosion.

Factors that control the degree and duration of wet conditions in metal chimneys and vent systems are critical to the life of the appliance.

The Gas Research Institute Venting Project reported (and subsequently published in the 1996 National Fuel Gas Code) the following venting occurrences:

  1. The reduced dilution air increases the chances of more condensate to form in the vent.
  2. Reduced flue gas temperatures result in oversized vents not heating quickly which contribute to increased condensation.
  3. Reduced dilution air increases the maximum capacity of a vent.

Wettime or "wet time" is the time measured after start up that the flue wall temperature is below the dew point The "Wettime" of a mid-efficiency appliance is double that of a draft hood appliance.

Therefore, during the flue heat up time condensation forms inside the connector or flue. This moisture can dislodge soot products inside the flue which will fall to the bottom of the chimney or connector and become a corrosive media.

Rust and pitting: While rust is not as likely on a stainless steel chimney, corrosion can occur because of the corrosives in flue gases and in moisture that may develop inside of a chimney.

Carson Dunlop Associates [at REVIEWERS] sketch (above) warns us to inspect metal chimneys (of all types) for signs of rust and pitting.

Because metal flues corrode from the inside out, and because we on occasion find leaks between the layers of multiple-wall metal chimneys, discovery of these damage signs is important in evaluating chimney safety.

we have observed leaks and corrosion of metal chimneys from interior corrosion (described above) and also on occasion we have found exterior leaks into the interstitial space between the chimney walls due to loss of the rain cap or improper exterior installation or flashing.

Even a small amount of visible pitting on the exterior of a metal chimney or flue is likely to mean that the components are unsafe and need replacement.

Thanks to home inspector Arlene Puentes for technical editing - 4/2014

Reader Comment: Why B-Vents & other Gas Appliance Chimneys Rust & Corrode Near the Chimney Top

Type B Gas Vent (C) Daniel Friedman

30 August 2015 Gregh said:

Many experts seem confused as to why some B vent pipes tend to rust near the top under the rain cap. The rust has absolutely nothing to do with the age or condition of the gas appliance.

The rust is a result of the flue gas coming in contact with the outer shell of the B vent which is a type of galvanized metal. This outer shell is generally cool to the touch , so when the warm flue gasses contact the outer shell and condense on it over a long period of time rust will result as the flue gasses are highly corrosive.

Several manufactures became aware of this 40 years ago. Selkirk designed a rain cap around 1971 which incorporated a lower ring shield to direct flue gas away from the b vent pipe.

Both Selkirk and Ecco manufacturing sold a painted termination pipe from 1973-1985. By this time most cap designs used some type of shield to keep flue gas from drifting down onto the b vents outer surface. Also mid efficient furnaces are terrible for creating more corrosive vapor in flue gas as these appliances don't have a draft hood to introduce dilution air which would help dry the flue gas somewhat.

This comment was originally posted at TYPE B-VENT CHIMNEYS


Greg, thank you. Indeed condensate from gas fired appliances is quite corrosive. Understanding just where the corrosion shows up is both diagnostic and interesting. We'll edit your comments into the article on chimney wet time.


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