• If this is your first time visiting our NACBI Community Forum, Please go to the READ ME FIRST section for information about registering and helpful tips to use this site.
    Members Only sections are restricted and will indicate "No Content" until you are approved as a memeber of NACBI and logged in to the messgae board.

Condensation in chimney flue

MarcelCyr

Community Post Master
NACBI Member
Flue condensation problems are caused by hot gases cooling too quickly in the flue.
As the hot gases cool moisture in the gases condense and start dripping back down the flue. Flue condensation problems can be solved by making sure the gases are hot enough to be vented into the atmosphere before the moisture condenses in the gases. It could be a problem that requires a slight adjustment at the burner.
It is a good idea, when you observe a furnace flue condensation problem where the condensation is dripping back down the pipe, to call a professional for to analyze the combustion process and make the precise adjustments necessary to prevent this problem. Condensation dripping back down the flue causes flue to prematurely corrode and rust.
Flue condensation problems, if left alone, can cause the flue to rot and vent hot gases in an area where it could be very dangerous to the occupants of the building.


Installation of a new gas furnace with disregard to the size of the existing flue liner can contribute to the problem of condensation. If the flue is over sized for the appliance, the flue gases will cool before it exits the chimney and condensate.

Here is evidence to this affect. In this case, the chimney flue was 8"x12".

Flue gas contains a great deal of moisture. As long as the moisture leaves the building as vapor, all is well. But an over sized flue stays cool, encouraging the flue gas to condense. "For the flue gas from a natural-gas flame, the dew point is 136 degrees," says Carl Saunders, director of training at Utica Boilers in Utica, N.Y. "With a big masonry chimney, you won't have much chance of keeping the gas about the dew point."


DSCN2175 (2).JPG DSCN2184.JPG DSCN2185.JPG
 

MarcelCyr

Community Post Master
NACBI Member
Happens a lot up here when they switch from oil furnaces to gas fired. With all these efficient burning gas furnaces, they don't need a big flue, but you would think the installer of that new furnace would have known that.
 
Top