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Distance between intake and exhaust vents?

sunlight33
sunlight33 Member Posts: 338
I have a Viessmann 200-W 125kBTU boiler, the manual says to keep a minimum of 10 inches between the two. From your experience if space is not an issue what's the typical distance you use in your installations?

Comments

  • Derheatmeister
    Derheatmeister Member Posts: 1,081

    I have a Viessmann 200-W 125kBTU boiler, the manual says to keep a minimum of 10 inches between the two. From your experience if space is not an issue what's the typical distance you use in your installations?


    It depends...We like to get some distance in order to avoid Exhaust gas recirculation.
    IMO the more distance between the Intake and Exhaust the better.
    However, Make sure that the piping is:
    1. Balanced Equal runs on the Exhaust/Combustion air.
    2. You do not have the Intake and exhaust in different pressure Zones.
    3. Drawing Combustion air from the roof and exhausting as a side wall vent,
    4. Total developed lengths are in check.
    5. Drawing in air near a Hottub or other chemicals.
    Follow viessmanns venting requirements but do not drink the concentric venting Coolaid..
    Hope this helps.
  • sunlight33
    sunlight33 Member Posts: 338
    Thanks. Another question, for intake pipe I can use either PVC DWV or Sch 40 right? Do you prefer one over the other?
  • Lance
    Lance Member Posts: 157
    Here is what I was taught. 1) Follow the manufactures' instructions. IF you don't you void warranty. 2) Follow the code, if you do not you get cited and if something goes wrong you are liable for damages. 3) Know the engineering and principles related to this. Too close and too far can have negative results. Look for the goldilocks sweet spot. It is what many manufacturers had to find out the hard way. Also you cannot ignore circumstances like nearby structures, landscaping, predominate wind patterns. In the city we had a large courtyard totally enclosed except for above which was open sky. Due to the 6-8 story buildings air recirculation prevented some equipment from working properly. Pipe type is a code issue. Some materials will fail as they cannot meet requirements of the equipment or go against the "CODE", or how the equipment passed certification to be built.
  • captainco
    captainco Member Posts: 471
    I never could understand the close proximity of the intake and flue. On a one pipe furnace installation they are farther than 10" apart? Definitely different pressure zones. Going back to the 80's I don't remember any installation that had a problem because the two pipes were far apart. Do these instructions come from theory, opinion or actual field testing. Some of the ones I am speaking about were condensing oil furnaces. Even this manufacturer called me to ask why I wasn't having problems with their furnace. I said following your instructions didn't work in actual field conditions.
    Funny, that having been involved in many courts cases over the years, almost every one of them involved equipment that was installed according to manufacturers recommendations and Code approval. The result is, if you don't verify what you are doing works, it is still your fault.

    Yes, goes against common beliefs but I have always believed in being mechanically correct versus politically correct!

    I think of the commercial on TV, "I am pretty sure it will work!" Pretty sure?
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,175
    @captainco

    I agree about the different pressure zones on a 1 pipe exhaust system with inside air intake. The only rational I have (which is probably wrong) is that with a 1 pipe system the fan has less pipe resistance to overcome so that being in 2 pressure zones isn't an issue.

    With a two pipe system the fan has more pipe resistance to overcome so they want the intake and exhaust in the same location......just guessing
  • Derheatmeister
    Derheatmeister Member Posts: 1,081

    @captainco

    I agree about the different pressure zones on a 1 pipe exhaust system with inside air intake. The only rational I have (which is probably wrong) is that with a 1 pipe system the fan has less pipe resistance to overcome so that being in 2 pressure zones isn't an issue.

    With a two pipe system the fan has more pipe resistance to overcome so they want the intake and exhaust in the same location......just guessing

    One of the reasons why some Manufacturers do not want different pressure zones is due to the possibility that someone may take combustion air from prevailing winds which would be a high pressure zone and Exhaust on the opposite side off the building (Low Pressure Zone)
    IMO If the building is shaped correctly it can (Just like a Sail) even create a negative pressure zone/Vacume.
    Even while the equipment is not operational all this can Trigger a safety exhaust pressure switch that may justifiably lock up the boiler.
    If the reversed install is performed by Exhausting into the prevaling winds/combustion air from low side the Draft inducer may not be able to overcome the resistance which again may lock up the equipment.

    Taking combustion air from inside the building can sometimes be Problematic due to the Noise levels and contaminats such as lint or Chemicals.

    When ever possible we try for a well balanced system (Equal Piping lengths) Exhaust and Combustion air in a "Neutral" but not "Stale" zone with as much distance between both as possible..
  • captainco
    captainco Member Posts: 471
    The only wind that I have seen that might trip a pressure switch is from a hurricane or tornado. The only vents that are somewhat affected by the wind are concentric. I found years ago that using a swing tee or plumbers tee on the flue nullifies the wind effect. Combustion air pipe doesn't matter.
  • Derheatmeister
    Derheatmeister Member Posts: 1,081
    edited July 9
    captainco said:

    The only wind that I have seen that might trip a pressure switch is from a hurricane or tornado. The only vents that are somewhat affected by the wind are concentric. I found years ago that using a swing tee or plumbers tee on the flue nullifies the wind effect. Combustion air pipe doesn't matter.

    Winds blowing off of the mountains at high altitude will trigger a pressure switch...A Concentric venting systems will regurgitate on inself..In our enviroment the tee ALSO prevents Dry snow from blowing into the cabinets and ruining the Electronics/Fans..

    BTW: The earlier Viessmann Vitodens 200, WB2A model also has a feature that will automaticly ramp up the Fan speed in order for the boiler to be able to provide proper exhausting of Flue gasses when it is venting into a high wind situation..

    At over 10'000 feet during a low fire/Low Modulating situation with High winds we have seen this Blower ramp way up in order to provide proper venting !
    Zman
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