low pressure at vents

Discussion in 'HVAC Heating & Cooling' started by toohott7718, Sep 23, 2010.

  1. toohott7718

    toohott7718 New Member

    Messages:
    20
    Location:
    Missouri
    I removed the cover from the blower and it doesnt seem that anymore air pressure is at any vents
  2. jaycanter

    jaycanter New Member

    Messages:
    5
    Location:
    mi
    That says the problem is in the supply an 8x20 duct at 1600 cfm for 4 tons of cooling is .25 static which is o.k but leads me to believe it's the taps off of the supply. how many runs come off & what size are they?
  3. toohott7718

    toohott7718 New Member

    Messages:
    20
    Location:
    Missouri
    there is 25 total runs and there all 6"
  4. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    Based on what you said 1600 CFM and 25 six inch vents the six inch ducts are fine.
    (~64CFM thru a 6" duct is sufficient, assuming you unit is OK, it is your supply and return size.)
    Note: the mention of .25 SP is .25" per hundred feet of 20"x8" straight duct. This does not count the grill pressure drop, filter loss, elbows,transitions, 6" duct, plus the similar losses for the return system. I doubt if your unit puts out 1600CFM at a .75" external SP but something less.
  5. toohott7718

    toohott7718 New Member

    Messages:
    20
    Location:
    Missouri
    how exactly would I know what cfm my furnace puts out, the guy who installed it said it should be 2000cfm, but Im not sure and like I said before these guys didnt know what they were doing as I know now. but as far as for the sp how would I know what my system is at. is there a meter that would test that. Im not sure if the meter I plan to buy will tell me that but Im going to get a meter that tells me how much cfm each vent is putting out. thanks again
  6. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    Try searching for "measuring external static pressure" on the web. One page that gives you an idea is http://contractingbusiness.com/enewsletters/cb_imp_70061/

    To give you an example for air volume of air look at the following: http://eetd.lbl.gov/ie/pdf/LBNL-51550.pdf#search=""garbage bag" "air flow" "homeowner""

    The contractor should have paperwork similar to that for the Trane equipment below, giving you an idea of what your unit is capable of. As an example look on pg 8 of this web page for a Trane unit http://www.trane.com/CPS/Uploads/Us... Systems/Small Splits/22-1774-17_10012009.pdf (static pressure vs cfm, note the heating section if external to the AHU also has a pressure drop assciated with it that has to be overcome by the AHU).
  7. toohott7718

    toohott7718 New Member

    Messages:
    20
    Location:
    Missouri
    so what size duct would you reccomend for the supply and the return? or do you need a drawing of what my setup looks like
  8. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    Need a plan view rough sketch of the trunk lines and the approximate location of each of the 6" duct in reference to each other and approximate length in feet between the 6" ducts totaling the 48' of trunk line. (Some of the 6" ducts may be acrossed from each other or off set.)
    You still have my email if it would be easier. I can accept pdf older versions of autocad or smartsketch.
  9. toohott7718

    toohott7718 New Member

    Messages:
    20
    Location:
    Missouri
    I emailed the sketch to you and I will also post it here, it might be too big a file to upload but I'll try. thanks again!!

    Attached Files:

  10. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    25 supplys at 6" apiece adds up to (28" x 25= 700 inches of air. But the main is only 8x20= 160 inches. That means the MAIN DUCT is much TOO SMALL to handle this amount of supply takeoffs.
    You need a pro to come in and size the supply AND return properly.
    Last edited: Oct 2, 2010
  11. toohott7718

    toohott7718 New Member

    Messages:
    20
    Location:
    Missouri
    I have already had 4 supposed pro's out here, I cant afford anymore pro service calls!! That's why Im here. Do you think if I had the money I would come to a forum for help?? Everyone I had out here said everything was fine, what a waste of my money.
  12. toohott7718

    toohott7718 New Member

    Messages:
    20
    Location:
    Missouri
    it seems to me that gator37 is the only one that has truly given me any real answers, and didnt give me the run around like all the other service pro's I had out here. So thanks again gator37, I really appreciate it
  13. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    Yes ,Gator37 has given good advice. it's the same advice given by me and some others .
    Your MAIN TRUNK duct is either not large enough to supply those 25 6" takeoffs.
    Or it also could be that those 6" takeoffs are not required. Pehaps 4" or 5" would be sufficient to supply these rooms.. i have been in Hvac for almost 50 yearsand I know what i'm talking about.
    Get a real pro to come in and look it over. this time call in an hvac rep, not any old plumber/carpenter or butcher, but a real live hvac (heating/ cooling)pro, not a "supposed" pro.
  14. toohott7718

    toohott7718 New Member

    Messages:
    20
    Location:
    Missouri
    Again, if I could find a real pro I wouldnt be here. The first one was the guy who put the system in, and the other 3 I found out of the phone book, all local guys too. You act like Im looking through the newspaper or I'm just calling anyone who thinks they can do hvac work, Im calling straight out of the phonebook. Isnt that people normally do, use the phone book?? I got more info from emailing gator37 than here is what I was trying to say. I know the 6" ducts arent the problem because I have shorter runs that are also 6" and they still dont get enough pressure, so it has to be the supply duct and or the return duct
    Last edited: Oct 3, 2010
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