1. Welcome to Hvacproforums, an interactive community exclusively for HVAC professionals. The collective years of HVAC experience and the knowledge our forum members share make this an invaluable resource for those in the HVAC industry. We have a lot of fun, but make no mistake, we are pros. Hvacproforums is self supporting ...there are NO advertisers here. Do-it-yourselfers and Homeowners are encouraged to seek information elsewhere. Those involved in the HVAC industry are urged to register. its 100% free and easy, Once registered you will have access to the technical forums, Be sure to click over to the new members forum and introduce yourself after registering!

Where's the water coming from.

Discussion in 'Commercial' started by Rather B Riding, May 10, 2017.

  1. Rather B Riding

    Rather B Riding Behind bars

    Messages:
    608
    Sorry I got busy and no time to pay attention to my own thread. Finally got some PM time.
    It is warm here so I won't do a combustion analysis till fall PM. I may write it up for a powered fan on the 14". It really needs it. As Zman said, the hot water tank can't be drafting properly.
    So there are some really good points on this thread. When I first started this account, I tried to catch 1 back drafting because I know it's happening. Which brings me to what I believe is going on. I may be wrong which is why I posted the thread in the first place. All I have is evidence of water I have not seen the actual cause. So I think when 1 or a certain combination of furnaces are running some of the others are backdrafting. The steam in the flue gas from the one that's running is condensing in the ones that aren't.
    This common flue thing has bothered me for years. I remember doing unit replacements for a real estate customer. They were duplex units. We were taking out 80% natural draft furnaces and installing induced draft 80% furnace on 1 side of the duplex. Now there were 2 furnaces and 2 hot water heaters all going into the same flue but only one appliance was induced draft. It had to have caused issues.
     
  2. icy78

    icy78 New Member

    Messages:
    524
    @Chuck is dead on here. I see the same at my work place. Pretty sure im the only one using a CA. (And im not good at it) It has really opened my eyes to what I missed before. In my opinion it's a mandatory tool for heating work. There are many calls where I would not have seen an existing problem or understood the potential for a problem, without that CA.



    Sent from my SM-G930V using Tapatalk
     
  3. tuna

    tuna New Member

    Messages:
    2,915
    Bottom line...I don't believe the mechanical code allows so many appliances to dump into a big pipe like that. You didn't install it. Not sure what I would suggest. It probably should be redone with individual or multiple flues through roof. :dunno:
     
    Rather B Riding likes this.
  4. Veloci_lobster

    Veloci_lobster New Member

    Messages:
    25
    Location:
    Minnesota
    This is my thought as well, as the pipes enter the larger sections of pipe the flue gasses will cool down and the water will condense and rust out the pipe, and or induced draft blower. I had a similar problem with a furnace I own.

    Sent from my XT1565 using Tapatalk
     
  5. Daryl_Dixon

    Daryl_Dixon New Member

    Messages:
    3,780
    You raise a good point. Guess how many combinations of running characteristics you get when there are six furnaces vented together?

    X o o o o o
    o X o o o o
    o o X o o o
    o o o X o o
    o o o o X o
    o o o o o X
    XX o o o o
    X o X o o o
    X o o X o o
    X o o o X o
    X o o o o X
    o X X o o o
    o X o X o o
    o X o o X o
    o X o o o X
    o o X X o o
    o o X o X o
    o o X o o X
    o o o X X o
    o o o X o X
    o o o o X o
    o o o o X X
    o o o o o 0
    X X 0 0 0 X
    X 0 X 0 0 X
    X 0 0 X 0 X
    X 0 0 0 X X
    X X 0 X X X

    ..any many more...

    Well, you get the point.

    You're going to have a really hard time recreating every possible running combination.
     
  6. Daryl_Dixon

    Daryl_Dixon New Member

    Messages:
    3,780
    oooooo
    xooooo
    xxoooo
    xxxooo
    xxxxoo
    xxxxxo
    xxxxxx
    oxoooo
    oxxooo
    oxxxoo
    oxxxxo
    oxxxxx

    I give up.
     
  7. Daryl_Dixon

    Daryl_Dixon New Member

    Messages:
    3,780
    Wait, was there a water heater too?

    so 7 items?

    That exponentially increases the odds
     
  8. Daryl_Dixon

    Daryl_Dixon New Member

    Messages:
    3,780
    Come on, Buddy, you have 50,000 different combustion tests to run.

    Why don't you have out your CA yet?

    It amazes me that guys don't get down to the brass tacks right out of the gate and run 50,000 different CA tests!

    :p
     
  9. icy78

    icy78 New Member

    Messages:
    524
    Think about this. Does a given quantity of fluid move more easily thru a small pipe, or a larger pipe?
    That flue needs to be sized for ALL the appliances running simultaneously.
    Situation: no appliances running. Laminar flow is happening in the flue. Now one appliance fires. The cool air traveling downward on the inside surface of the flue (and into the mechanical room) makes a path for the combustion products of the just fired appliance to most easily follow. Eg back thru the appliance flu which at that moment exhibits the least resistance to flow. So now you have flue gases not exiting, but recirculating back into the mech room.
    If the firing appliance is equipped with a draft hood, you can also have the situation where, if you have too much draft, the draft hood allows too much air thru it and creates an eddy current or air curtain , causing spillage of combustion products out of the hood itself or the appliance. (The easiest path for the combustion products to travel now are back into the mech room to replace the dilution air that went up the flue.)
    To determine the flue draft you'd need a highly accurate draft guage. Not the one on your CA.
    To see if your flue is big enough, fire all the appliances (once you've made them safe) and watch the draft guage. Once it settles out, you then shut off an appliance, if the draft then rises, the flu is too small. (Because you are now trying to vent less thru the same size flue) equates to less resistance.

    There's more to it than that, but that is a realistic senario.

    Sent from my SM-G930V using Tapatalk
     
  10. R/HVAC

    R/HVAC selfemployed

    Messages:
    470
    Location:
    W-S , NC
    I would have to use b-vent per our code to pass inspection to start with. I find the different approaches members use in problem solving good to read. Keeps me from getting tunnel vision when confronted with different situations. Still learning on this end.
     
    icy78 and knave like this.
  11. iceout70

    iceout70 New Member

    Messages:
    25
    If some flue gases are back drafting into the space and being reburned do you find when you are in the mechanical room when you see water any reading of CO in the space? If you do get a hi CO reading or any CO reading then it means it is back drafting and you are breathing that in. Be carefull.

    Sent from my XT1254 using Tapatalk
     
  12. freeworld

    freeworld Freon Jockey

    Messages:
    174
    Location:
    Great Northwest
    128 Combinations I think...
     
    Daryl_Dixon likes this.