return air
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Thread: return air

  1. #1
    Join Date
    Jan 2005
    Posts
    16
    I have a 5 Ton heat pump duel fuel propane unit with an electronic air filter in a new 2300 sq ft house. After four months of running with standard fiberglass filters, I stopped using them when the construction dust diminished.

    The two foot by three foot metal return grill sings when the unit runs without the filter. I am wondering if there is adequate return vent size for the job. The actual clear opening size of the return duct that goes through the floor is 14 X 19 inches.

    The single return is in a hallway with no other return ducts.

    I wish I had known about this web site before, because I would have been a better informed consumer. I would have requested more return air ducts. When the bedrooms are closed off, there is noticeable air pressure resistance when the doors are opened.

    Is my return undersized? Would it be something that should be fixed to increase efficiency and longevity of the unit?

    I thank you.

    Nladan

  2. #2
    Join Date
    Feb 2005
    Location
    In a van by the river
    Posts
    618
    For a 5 ton unit you would need 2000 cfm air, and correct me if I'm wrong guys, but there is no way you are getting that out of a single 14 x 19 inch opening. It would atleast have to be 20 x 25 or something to that extent.
    ## + years in the field never made you a know-it-all This industry is far more diverse than you are

  3. #3
    Join Date
    Jan 2005
    Location
    Shawnee, OK
    Posts
    69
    According to Paul Lang of Basics of Air Conditioning - Third Edition page 178. A return grill should be 36 x 24
    for 1990 to 2400 cfm.
    Fuzzy

  4. #4
    Join Date
    Apr 2001
    Location
    Camel City, NC
    Posts
    6,232

    Return

    2 returns are required for homes over 1600sqft. where I live.
    Be safe not fast. body parts don't grow back

  5. #5
    Join Date
    Jul 2004
    Location
    midwest
    Posts
    2,868
    Return duct to small= Noise at return grill
    The only thing you can do is make the return duct larger.
    Bending the return grill fins out straight will help on the noise but with that large of grill it would not be a problem with the correct duct size. If I would have done it the duct would be 20x20 and lined on the inside with duct liner for noise. How close the unit is to the return and the number of turns the return duct makes is something we always check also. If the unit is close with no turns in the duct it will be loud.

  6. #6
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    Originally posted by nladan

    The two foot by three foot metal return grill sings when the unit runs without the filter. I am wondering if there is adequate return vent size for the job. The actual clear opening size of the return duct that goes through the floor is 14 X 19 inches.

    The single return is in a hallway with no other return ducts.

    When the bedrooms are closed off, there is noticeable air pressure resistance when the doors are opened.

    Is my return undersized? Would it be something that should be fixed to increase efficiency and longevity of the unit?
    The contractor who sized your return used a rule of thumb friction rate to size it. The problem with this lies in the fact that there are no allowances for the extra turns & fittings in the design of the return duct system.
    The only way to be for certain the return is sized properly is to have a Manual D calculation of the duct system performed.
    If it is a a short run of duct it may work. The only way you will know for sure is to run the calculations & then have the duct system tested.
    You should have your contractor test the total external static pressure of the equipment to insure the fan has the ability to deliver the proper airflow.
    The grille problem you are having sounds like it may be associated with high face velocities creating a resonate tone when the blower is running. You may have to add a second return to resolve this problem.
    You can provide an easy fix for the air pressure difference in your rooms by adding transfer ducts between the rooms & the area where the central return is.This will help to neutralize the room to room pressures when doors are closed off. If you have natural draft gas appliances in your home this negative pressure created from closing doors can create a situation where the appliances could spill flue gas due to the flue pipe being used as fresh air inlet.

    For a list of contractors who have been trained in how to deal specifically with the type of problems you are having check out http://www.nationalcomfortinstitute.com

    Hope this helps.
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  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    14X19,2000cfm,@ .1 Friction Rate and 1150 feet per minute,what a suprise.You can bet the supply ducts were sized the same way,so they are too small also.

    Have someone check the External Static Pressure,compare it to the mfrs. fan data chart,see how many btus of cooling and heat pump heting you are losing.Document and ask them to fix it or pay X% of your electric bill.

    Check the heat rise of the furnace,low air flow,high heat rise out of specs,equals a cracked heat exchanger in the future.

  8. #8
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Adding returns

    A word of encouragement -- if your duct system has a plenum, it may be relatively easy to just add a couple of new returns and route them to the bedrooms. I had an undersized return (which is apparently pretty common) and struggled for awhile thinking how to enlarge that one return. The proposals for that approach would have cost a ton of money. In the end it was just simpler, cheaper, and better functioning to add new returns and leave the existing one as it is.

    This greatly helped my ESP problem and gave me more filter area so the system is less impaired by effective filters. The only real drawback from a homeowner's point of view is the new returns are ceiling located and I need a ladder to reach them.

    Here is a $15 gadget which tells you when a filter needs changing:
    http://www.google.com/url?sa=U&start...dak.html&e=747
    Scroll down until you see "General G-99 air filter gauge". With that you could use a filter which really stops airborne stuff (dare I say 3M?? <g>) and still not have problems when it loads up and becomes more restrictive. I am pretty certain it will pay for itself because you will change filters when they are *needed*, not just because the calendar says X number of days. You do have to watch the dial and change when it tells you to, of course.

    What I've found is with my older restrictive return, a certain brand restrictive filter added about 0.3 inches water column to the ESP -- bad news. When I got more return and filter area, the penalty dropped to a small fraction of that. With the restrictive return, I *needed* to run loose spun filters for good results, with the enlarged return I am free to choose what filters I want (although I still don't use those notorious 3M's). I am hoping you will be encouraged by the possibilities to fix this presumed problem, so that several things are greatly improved for you.

    Best of luck -- P.Student

    [Edited by perpetual_student on 03-05-2005 at 12:26 PM]

  9. #9
    Join Date
    Jan 2005
    Posts
    16
    Thanks for your responses. When I get a few minutes I will digest more what you have written. For now, I have two questions.

    What is a plenum?

    Who do you get to do a meaure of the External Static Pressure,compare it to the mfrs. fan data chart,see how many btus of cooling and heat pump heting you are losing.

    Would that be someone listed at http://www.nationalcomfortinstitute.com? If so, I went to that site and couldn't find a list of members.

    Thanks.

    NLadan

  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Plenum,is a "large "section of duct,attached to the supply and return of the equipment.What was suggested is to run an additional return from there.


    A qualified HVAC contractor,can test it,be specific when you call. i.e. Can you test the External Static Pressure of my suppy and return duct and do you have the mfrs data ,to tell me how many cfms the system is moving?Plus do you have the detailed cooling charts to show the capacity reduction,if the cfm's are low?Not all will be able to do this.

    Try emailing NCI,they may be able to refer you to a member.

    What city and state are you located in??

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