Stalled Draft at Furnace Flue

Furnace flue

The furnace is in a home originally constructed in the 1950’s.

Since that time, it has been extensively remodeled and updated.

The newer mid-efficiency furnace has a flue diameter of approximately 5″.

It quickly transitions to a connection to the original flue with a diameter of approximately 10″.

With the furnace activated, the gases from the furnace begin to condensate and drip onto the floor of the mechanical room within a few minutes. There is substantial corrosion at both the flue and furnace.

When changing the efficiency of a furnace or making any other changes to the furnace venting system, consideration must be given to the ability of the flue gases to escape with back drafting into the house or condensing inside the flue as the gases are not as hot as older low efficiency furnaces.

Higher efficiency furnaces extract more of the heat out of the waste gases which can lead to the condensation problem.