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Viessmann condensation issues

Hello all and thanks for your help in advance.  I have a Viessmann Vitogas 100 (with Vitotronic 200 controller) at my house in CT.  It was installed by the builder 5 years ago.

On certain (colder?) mornings, there is a strong downdraft in the gas flue.  The flue vents through a clay-lined vertical chimney.  The chimney is approx 35 feet tall and is on an outside wall of the house.  It extends above the roofline.  The effect of the downdraft is that the flue gases are pushed out from the draft hood, resulting in high humidity, heat and the presence of flue gas odor in the mechanical room.  There is also condensation on the outside of the flue piping leading to the the chimney.  Presumably this is a result of the high humiditiy level.  When this condition is occuring, the flue pipe is cold to the touch.

This issue seems to occur most in the early morning after I have had the heating circuit turned off for a period of time (during the night, to save energy).  It seems that the choice is either burn energy all night long or suffer with this flue gas/humidity.

A chimney sweep suggested that the chimney is too clod and the problem could be resolved using a new insulated steel liner dopped down the chimney and connect to the boiler. I also thought that another solution might be a power vent of some sort?

I appreciate any advice you might have on how to deal with the situation.  Also, any recommendations on Viessmann qualified contractors in CT.


  • Gordan
    Gordan Member Posts: 891
    edited November 2012
    This is a hazardous condition

    You are playing with carbon monoxide poisoning. These boilers are not supposed to be vented into an unlined masonry chimney. From page 14 of the installation manual:

    A corrosion-resistant approved liner

    must be installed in masonry or unlined

    chimneys. The liner should be insulated

    to prevent condensation of flue gas in

    cold weather.
  • puzzled99
    puzzled99 Member Posts: 3
    Viessmann condensation issues

    Thanks for your response. The chimney is lined with Terracota.  Is that not sufficient?
  • Gordan
    Gordan Member Posts: 891
    Insulated stainless liner is what you need

    Terracotta is still masonry. Even if your boiler were able to maintain proper draft, the water vapor in the flue gases would condense on the cold masonry, and the condensate, which is acidic, would eat away at the mortar joints eventually compromising the soundness of the chimney. (Probably causing moisture issues with whatever adjoins the chimney, as well.) The chimney was not designed to properly vent the higher (or even mid-) efficiency boilers we use today.
  • puzzled99
    puzzled99 Member Posts: 3
    Viessmann condensation issues

    OK, thanks. I will look into getting an insulated stainless steel liner.
  • CMadatMe
    CMadatMe Member Posts: 3,086

    Gordy hits the nail on the peverbile head.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Paul Pollets
    Paul Pollets Member Posts: 3,620
    edited November 2012

    You can also purchase a swivel wind vane cap to terminate the liner that greatly reduces any downdrafts, rather than the standard "shanty cap". Make sure the boiler is cleaned properly after the new liner is installed.
  • bob_46
    bob_46 Member Posts: 813
    Negative pressure

    I would check and make sure that you don't have an exhaust fan or vented

    appliance that is creating a negative pressure in your house .
This discussion has been closed.