Viessmann condensation issues

puzzled99

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

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

Viessmann condensation issues

Thanks for your response. The chimney is lined with Terracota.  Is that not sufficient?

Gordan

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

Viessmann condensation issues

OK, thanks. I will look into getting an insulated stainless steel liner.

CMadatMe

Unpuzzled

Gordy hits the nail on the peverbile head.

Paul Pollets

Termination

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

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 .