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  1. #1
    Join Date
    Dec 2011
    Location
    bedford ind
    Posts
    1,092

    make-up air ducted with combustion air

    Just got back from a call on a gov. bldg that has the make-up air ducted together with the combustion air vent. The system is old, but the metal duct is in good shape.
    the two ducts join right before the vent to the outside.
    There is a water heater and two 80 % furnace in an 8 x 12 sealed room sharing an 8 x 20 combustion air vent with the buildings fresh air make-up vent.
    Isn't this a code violation?

  2. #2
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,198
    Consider the OA louver as having a plenum attached. Would there be any chance of any pressure differences in the 2 duct runs if compared with 2 seperate louvers/ ducts right next to each other?
    The combustion air is make up air as well

    Seems the answer is no but I see why this might give you a question to pause and consider
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  3. #3
    Join Date
    Feb 2012
    Location
    New York City
    Posts
    106
    It sounds like there's a likely code violation here, but you'll need to review a lot more details (combustion system capacity, operating conditions, relative pressures, etc.) and what code is being followed in the jurisdiction.

    Check the fuel gas code for your specific situation. The system interaction bit is probably covered in a blanket statement that the combustion air shall not be installed in a way in which it won't work right. In terms of the minimum area for combustion air, that will be determined based on the specifics of your situation. It could require anywhere from 2 sq in per 1 MBH to 1 sq in per 4 MBH. The more the design goes away from "nice" the more free area you'll need per MBH. Also typically you need a high and low opening.

  4. #4
    Join Date
    Dec 2011
    Location
    bedford ind
    Posts
    1,092
    This is a system about 25- 30 yrs old. There is a 4ton blower on one system and a 3 ton on the other. The units are not twinned and both returns are connected .
    There is no possibility of a low vent, i asked about that.
    I have several concerns about the system. Another company started the job because of a mold problem. unfortunately their bid did not include re-installing the equipment they took out. They were a cleaning company.
    I was assured there would be NO inspection.And they will not re-do the duct. Money is tight.
    I'd walk away from this, but I do work for a few of the city departments. I guess I'm really just kind of dumbfounded.

  5. #5
    Join Date
    Feb 2012
    Location
    New York City
    Posts
    106
    Hmmm... I'd try to separate the ducts at the louver. Also if the room or building arent super well sealed, the infiltration can help you.

  6. #6
    Join Date
    Dec 2011
    Location
    bedford ind
    Posts
    1,092
    They just foam insulated the interior, then they are insulating the 2 x 6 walls with 6" batts. The one thing they did that I suggested.

  7. #7
    Join Date
    Feb 2012
    Location
    New York City
    Posts
    106
    D'oh! Just be on the record with your observations and reommendations and client directions not to proceed. Put in a CO alarm or two for safe measure and move on. If it looks really bad and you're really worried you could anonymously report the situation to the buildings department or fire marshall.

  8. #8
    Join Date
    Dec 2011
    Location
    bedford ind
    Posts
    1,092
    Co alarm is a good idea.Thanks. That makes think. There are no smoke alarms in the building either!

  9. #9
    Join Date
    Feb 2012
    Location
    Upstate SC
    Posts
    84
    The code that was in effect when the property was given an occupancy certificate governed the design. Though it might not meet current design standards, there is probably nothing wrong with it provided the duct is properly sized.

  10. #10
    Join Date
    Dec 2011
    Location
    bedford ind
    Posts
    1,092
    Occupancy certificates are not done here.

  11. #11
    Join Date
    Jan 2009
    Location
    Etters PA.
    Posts
    517
    I would be concerned that the resistance of the damper louver would cause the fresh to be drawn in from the furnace room .

    Sent from my PC36100 using Tapatalk

  12. #12
    Join Date
    Dec 2011
    Location
    bedford ind
    Posts
    1,092
    that was a thought. They are going to leave things as they are. Before, the building was limestone with cracks all over. Now they have foamed it tight. I think it will pull more from the furnace room.

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