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  1. #14
    Join Date
    Jul 2005
    Posts
    64
    Because of the lack of a TXV to adjust of superheat, closing vents in a CapTube system would be much worse, especially when heat loads are low... Correct?

  2. #15
    Join Date
    Dec 2002
    Location
    in a house, Appomattox, Va.
    Posts
    3,191
    Difference between zoning and registers is that zoning duct (wehope) was designed to operate with dampers closed, such as each branch sized for total load, or a bypass damper and protective controls to prevent the floodback/freezeup issues.

    Closing floor registers moves air up stairs because they were the path of less resistance, and by closing there is less resistance upstairs vs the closed register. but, as resistance in system increases, airflow across the coil decreases. Most systems are not designed to be adjusted in this manner (not that they thought about such things, leaving out balancing dampers and proper duct sizing)

    Bottom line is, fix the duct so it won't damage the equipment.
    Col 3:23


    questions asked, answers received, ignorance abated

  3. #16
    Great Info.

    So closing registers won't improve airflow in other registers? Or are you talking only about 2 story homes.

    I live in a one story home and keep the washer/dryer and walk in closet registers closed. Since it's Wye Branched with the bedroom next to it, wouldn't it help a little?

    On another note, when we zone system we do include a bypass damper with a counterweight and all the main runs are electronically dampened as well as manuals on the main runs. I thought this was in case one zone remained closed and the other opened it would relief into the R/A.
    This sounds interesting

  4. #17
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,000
    Bypass dampers are used to reduce noise in the supply duct. They do not protect by increasing air flow across coil. You are actually causing the equipment to reach operational limits faster when the bypass opens. In cooling, you are dumping cold air in the return. You are reducing the load on the evap by preconditioning the return air. Best zoned systems have supply ducts that can handle the airflow without using a bypass.

    Shutting dampers will help non-zoned systems force the air to other registers. However, it only works to a certain point. It's best to get the duct sized properly with summer-to-winter balance dampers.

    [Edited by jrbenny on 08-04-2005 at 06:58 PM]

  5. #18
    Join Date
    Dec 2002
    Location
    in a house, Appomattox, Va.
    Posts
    3,191
    Closing registers will increase airflow at other registers, but at the expense of reduced airflow at the coil or furnace heat exchanger.
    Col 3:23


    questions asked, answers received, ignorance abated

  6. #19
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Depends on the duct design.we have tested ours and some can be partially losed without raising the static and reducing air flow.


    When the Friction Rate is say .08 ,and you set your ductolator,for the cfms and fR,and it comes out to a 7.25" duct,so you go to an 8" duct.Because of this ,they can usually carry a little more air,so the system stays below the design static,when a few grilles are partially closed to balance things out.

  7. #20
    'Not that closing half the registers is a good idea, but I'd think a system with TXV might be less susceptible to floodback...
    '

    Reply: Joe, your observation is correct ; With a TXV system, the valve will throttle down under low load to maintain its superheat setting as sensed at its bulb location (at the exit of the evaporator). Low load can occur via a low cooling load in the house , keeping the house cooler than normal , or, via restricted airflow whether it be a dirty air filter or blocked registers decreasing the airflow across the evaporator surface. Metering devices that employ a fixed orifice or cap tube are more prone to floodback by closing alot of registers.
    Dave

  8. #21
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Figuring this out

    Just so I can get to understand this better...

    What would have to be done so that a homeowner could be free to open and close the registers freely? I'm thinking of my parents' home where 1) they do close registers for little used rooms, and 2) there is little room under the doors, the carpet fits snugly so there is virtually no return air path when those doors are closed. Despite these faults, they have long lasting equipment and supernaturally low electric usage.

    Just trying to figure out what is right and wrong in principle, plus how to apply that to practice.

    Thanks in advance -- P.Student

  9. #22
    Join Date
    Apr 2004
    Posts
    7,680
    Originally posted by JoeSix
    I'm curious, but doesn't a TXV help with this? (e.g. if the coil gets very cold, doesn't it meter less refrigerant into the coil?

    (No, I'm not advocating blocking registers, just wondering about those dehumidifying stages of the air handler operation which also reduce air flow over the coil)
    Fist off, no it doesnt help significantly for several reasons. TXV's are sized for nominal capacity and when you reduce the capacity the Valve does not work properly causing it to hunt flooding and starving the coil. Also, by lowering the amount of refrigerant, you also lower the pressure which lowers the saturation temperature. Now you have a freeze condition which will eventually cause floodback.


    Now, the reason its not a problem with ECM motors is because they are proper air flow more often than a PSC motor. If you have it set for 1200 CFM, it will deliver 1200 CFM as long as your static isnt above 1" or so.
    A PSC motor at higher static has a tremendes loss of airflow to begin with. With the ECM you can knock it down 15% because you know the airflow will be there. With a PSC, you have literally no control over the airflow and you are at the mercy of the curve. You may think the design is 900 CFM only to find out you only got 600 and want to reduce it futher.

  10. #23
    Join Date
    Jun 2002
    Posts
    460
    Originally posted by docholiday
    Now, the reason its not a problem with ECM motors is because they are proper air flow more often than a PSC motor. If you have it set for 1200 CFM, it will deliver 1200 CFM as long as your static isnt above 1" or so.
    A PSC motor at higher static has a tremendes loss of airflow to begin with. With the ECM you can knock it down 15% because you know the airflow will be there. With a PSC, you have literally no control over the airflow and you are at the mercy of the curve. You may think the design is 900 CFM only to find out you only got 600 and want to reduce it futher.
    OK, but we aren't talking about 15% here in some cases even in properly operating systems.

    Both Trane and Lennox (that I know of -- I'm sure the others do this too) offer a dehumidification stage that runs the system blower at 60% or so of nominal for the first 7 or so minutes or so at a time (and I have a Lennox Dehumidistat that will do this on demand, which lasts the entire cycle when it's damp).

    So, instead of 800 CFM with my 2-ton case, I'm only getting around 500 CFM.

    Why doesn't this cause a problem?

  11. #24

    Re: Figuring this out

    Originally posted by perpetual_student
    Just so I can get to understand this better...

    What would have to be done so that a homeowner could be free to open and close the registers freely? I'm thinking of my parents' home where 1) they do close registers for little used rooms, and 2) there is little room under the doors, the carpet fits snugly so there is virtually no return air path when those doors are closed. Despite these faults, they have long lasting equipment and supernaturally low electric usage.

    Just trying to figure out what is right and wrong in principle, plus how to apply that to practice.

    Thanks in advance -- P.Student
    REPLY: It largely depends on the size of the trunk ducts and branch ducts , plus, the size of the furnace drive package ; if the trunk and branch ducts are adequately/generously sized (which few are..) then it becomes less of a problem to close some registers off. Same applies to a generously sized furnace blower/motor that can handle the added static of some registers being closed to divert the air to the other open registers . You have to take each residential system into account, but, the vast majority of duct systems (especially in large developments, condos, townhomes, etc...) are cheapened by cutting corners on the size, the number of branch ducts, the thickness of the sheetmetal, the tightness of the connections, and the overall design (unfortunately).

  12. #25
    Join Date
    Apr 2003
    Location
    Northern Virginia, Fairfax County
    Posts
    641
    Very interesting thread that nicely explains why my Trane XL19i A/C with the Trane XV80 furnace, with both have the ECM or "variable speed" fan and blower motors work so nicely.

    Two years ago, my contractor who knows my house, TOLD me to shut most of the register capacity downstairs in my two story house and that the static pressure would NOT be a problem.
    Al

  13. #26
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    3,944
    Originally posted by JoeSix
    Originally posted by docholiday
    Now, the reason its not a problem with ECM motors is because they are proper air flow more often than a PSC motor. If you have it set for 1200 CFM, it will deliver 1200 CFM as long as your static isnt above 1" or so.
    A PSC motor at higher static has a tremendes loss of airflow to begin with. With the ECM you can knock it down 15% because you know the airflow will be there. With a PSC, you have literally no control over the airflow and you are at the mercy of the curve. You may think the design is 900 CFM only to find out you only got 600 and want to reduce it futher.
    OK, but we aren't talking about 15% here in some cases even in properly operating systems.

    Both Trane and Lennox (that I know of -- I'm sure the others do this too) offer a dehumidification stage that runs the system blower at 60% or so of nominal for the first 7 or so minutes or so at a time (and I have a Lennox Dehumidistat that will do this on demand, which lasts the entire cycle when it's damp).

    So, instead of 800 CFM with my 2-ton case, I'm only getting around 500 CFM.

    Why doesn't this cause a problem?
    Doesn't it take about 10 minutes for a system to reach full efficiency? I belive the 7 minute delay just allows the coil to get cold a bit faster. I personally put a timer/relay on my A/C system to switch the blower from low to high speed @ 7 minutes and it helped a lot with dehumidification Delta T only decreases about 1 degree when high speed kicks in. Normally it won't hit the high blower speed unless it's over 95 outside or the thermostat setting has changed. Oddly enough our power bills don't seem to be any higher either.

    I have had ot go behind the previous homeowner and increase the size of a few ducts to get tempatures relatively even in the house. The heat is about 2X oversized and the A/C is about 50% oversized. We are the 2nd owner of the house, the people we bought it from were HVAC contractors!! You would think an HVAC contractor would have his own system set up right ...

    [Edited by 54regcab on 08-06-2005 at 09:37 PM]

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