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Pro & Con of direct vent and outside air supply

We have both long and short developed piping length configurations of coaxially vented Vitodens out there and not one of them has gone into a fault code during these types of weather patterns.

I’ve seen what happens when this crystaline air condition occurs in New England. I always prefer to vent through the roof wherever possible. However, more often than not we end up choosing to side - wall vent the Vitodens. I always pull in outside combustion air from the annular coaxial vent kits as to not cause any air infiltration into the house.

The presence of huge clouds of steam at the vent termination may be a sign that we are not fully condensing the flu gasses in the heat exchanger (where we want) but in the vent piping and outlet itself. This can only be corrected by lowering return water temperatures but will still occur during a call for domestic hot water as well as when it gets really cold and dry outside. Although not recommended by the factory for this purpose, I’ve chosen to try out some of their “Cold Weather Termination Kits” for one of our multiple Vitodens installations to push the inevitable steam further away from the house. These kits consist of a 45º Elbow on the interior (vent) pipe facing downward and the deletion of the stainless steel cage. Too early to tell how this setup will perform for us but it was developed for cold “Alaskan winter” conditions.


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Comments

  • Perry_2
    Perry_2 Member Posts: 381
    Pro & Con of direct vent and outside air supply

    Do I direct vent with outside air supply to a new boiler - or should I use indoor air for supply?

    The current rage in the US is to use outdoor air supply for "sealed" cumbusition units. Yet, I understand that this is not normally the case in Europe where the designs - if not the guts - of condensing boilers originate.

    In my discussions with a regional Vitodens rep I have found out that, in the case of the Vitodens 200, that they do not use outside supply air in Germany because in the cold winter that a condition of "crystaline air" forms (very small particles of ice suspended in the air); and that this ice then melts into water on the boiler inlet causing problems with air inlet sensors and controls.

    The result is that Viessmann only allows the Vitodens 200 to use outside air in the US or Canada in an arrangment that normally would warm up the inlet air, if only a bit, on a running Vitodens 200 (the inlet air being warmed by the exhaust).

    Idealy, you would also want some length to the vent system for this to work best (but am limited by maximum length as well due to combustion air fan limits). However, you are still faced with a potential issue during startup of the boiler if it cycles on and off.

    Alternately, you can use indoor air for supply, and a single pipe (exhaust only) for the boiler.

    Now I understand what crystaline air is - and we do get that here in the winter.

    So; any comments on the pro's and con's of using the dual pipe vent system and pulling outside air, versus using indoor air and only exhausting the boiler outlet air.

    I do understand that this issue may not apply to many parts of the US.

    Perry
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Interesting

    I have always thought outside air superior because in the US most boiler rooms share the Laundry room space. The bleaches and other nasties in the air get sucked into the fossel fuel heat exchangers, and cause more corrosion there, when inside air is used. Also the higher negative pressure of the house caused by using inside air causes more infiltration of cold air which results in lower humidity which causes dry skin and shrinking glue joints in the furniture and cracks in the crown molding and so on... If the ice crystals are a concern I would think a concentric vent would be worth it's price for warming the combustion air. WW

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  • Solarstar
    Solarstar Member Posts: 82


    I'm somewhat interested in this as well since I"ll be running the vito vertically about 20 feet vent with an equivelent vent length close to the max specified around the 30 foot mark. Guys...do I need to be concerned Re this or is the longer vertical vent a good /preferred choice.?? thanks Paul
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,385
    The

    Vitodens venting instructions must be followed to the letter.

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  • hr
    hr Member Posts: 6,106
    If you use inside air

    you may need to install combustion air vents. Typically high and low. Check the codes in your area, generally 1 square inch for every 1,000 btu input. This gets to be a fairly large opening into the heated space.

    I have seen snow drifts pile in front of low combustion air vents in homes. Seems like a very large energy waster? More so if the hot water tank is in that room.

    Think I would preheat the intake venting before I put two 25 square inch holes into the mechanical space.

    Been there, done that and have frozen pipe stories a plenty due to this.

    I understand your concern but I have not had, or heard, of any problems with sealed combustion along those lines.

    hot rod

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  • When it comes to combustion air for oil burners, directly using outside air (as in an air boot ) the fluctuating tempature of the air is a concern. The colder the air the more dense it is and this will change the fire. It will have an effect on a gas burner as well.
  • Perry_2
    Perry_2 Member Posts: 381
    Vitodens venting instructions allow

    either the concentric vent or a single pipe vent where the Vitodens pulls inside air for combustion air.

    Perry
  • Perry_2
    Perry_2 Member Posts: 381
    Is the roof high enough?

    My current air supply for my existing boiler (135,000 Btu/hr) is a annualar opening arround the chimeney that runs up to the attic. Lots of nice cold air comes down that in the winter...

    I assume that if it can support my existing boiler and water heater; that it will support a smaller boiler.

    Of course, I was hoping to largly seal that opening up - one of my air leaks in the house... Something I could do If I direct vent with combustion air.

    Perry
  • Frost


    I prefer to vent through the roof wherever possible but find that more often than not, we end up choosing to side-wall vent the Vitodens. I’ve always chosen to pull outside air as to not cause air infiltration into the house.

    I’ve seen what happens when this crystaline air condition occurs in New England. (see attachment)

    The presence of steam at the vent termination is a sure sign that we are not fully condensing the flu gasses in the heat exchanger (where we want) but in the vent piping and outlet itself. This can only be corrected by lowering return water temperatures but will still occur during a call for domestic hot water. Although not recommended by the factory for this purpose, I’ve resorted to using a “cold weather termination kit” for one of our multiple Vitodens installations to push the inevitable steam further away from the house. This kit consists of a 45 deg. Elbow on the interior (vent) pipe and the deletion of the stainless steel cage. (see attached) Too early to tell how this setup will perform for us but it was developed for Alaskan conditions.

    None of our coaxially vented Vitodens have ever gone into a fault code during this type of weather pattern. We have both long and short developed piping configurations.

  • Perry_2
    Perry_2 Member Posts: 381
    Thanks

    Gary:

    I appreciate the information.

    Perry
  • ALH_4
    ALH_4 Member Posts: 1,790


    In my opinion the best venting option is sealed concentric through the roof. You're as far as you can get from dust and other pollutants. The warm exhaust is naturally buoyant. You do not have to have large openings to outdoors in your mechanical room as hot rod pointed out. I like sealed combustion in most situations, but have vented Vitodens' both ways.

    -Andrew
This discussion has been closed.