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  1. #14
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    A 1" gap on a standard 30" wide door will allow approx 47cfm to pass through.

  2. #15
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    Quote Originally Posted by jtrammel View Post
    I understand that but the force of pos pressure on a room caused by having supply air with no return is not near as great as the force of having a return that is ducted to the equipment which has the force of the fan to pull the air, therefore if you have a 6" supply forcing air in then you need a larger return than one that would pull ~100 cfm if connected to the return side of equipment via ductwork.
    Not true. For residential, and most commercial applications, it is the pressure differential that forces air from one point to another. HVAC system air is only pushed by the blower on the supply side and not pulled from the return side. That is why we refer to the negative side of the system as "return" air and not "suction" air.

    As long as there is an opening between the area being pressurized by the HVAC system and the area being depressurized by that same system, air from the positive side is going to travel through that opening to the negative side. Just close a door to a room with no return air ducting, turn on the HVAC blower and feel around any openings in the door to feel just how much air will push through those openings and at what rate.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  3. #16
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    Quote Originally Posted by jtrammel View Post
    A 1" gap on a standard 30" wide door will allow approx 47cfm to pass through.
    At what SP? What is the SP of 47 cfm of air going through a 1" door?

    You have been led to some false beliefs.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  4. #17
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    Oct 2010
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    Anderson, South Carolina, United States
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    If there is not an easy enough pathway for the return air to get back to the filter grille it will cause that room to bottle up the excess supply air causing pos pressure in that room and will push the air out through any hole in the thermal envelope because there is not adaquet space for the air to flow under and around the door. It also causes the other side of door to be neg and pull in air from any hole in the thermal envelope

  5. #18
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    Quote Originally Posted by jtrammel View Post
    If there is not an easy enough pathway for the return air to get back to the filter grille it will cause that room to bottle up the excess supply air causing pos pressure in that room and will push the air out through any hole in the thermal envelope because there is not adaquet space for the air to flow under and around the door. It also causes the other side of door to be neg and pull in air from any hole in the thermal envelope
    Not necessarily so. If there is no way for supply air to exit a room, the supply air will simply stop flowing into that room. Only if there are severe leaks in the room going to the outside ambient air wil the conditions you describe occur. Obviously, if there are major leaks to the outside of the house and not enough controlled leakage from the area to the return path, some things need to be addressed.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  6. #19
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    This explains how these scientists and engineers tested it and what they found. I had this problem at my own house that i rented at one time. The doors were undercut but there was a noticable pressure difference when door was closed. I talked my landlord into paying me to onstall a ducted return to the master bedroom and solved the issue. Since then my company has invested in blower door, duct blaster, flow hood, ir camera etc and have found many many master bedrooms like that. If its not feasible to run a ducted return we will jumper duct with flex to deaden sound transmission and if you dont oversize the duct then you still have a pressure difference from the rooms. I by all means dont know everything about these types of problems but i do attempt to solve them all the time.http://www.bestofbuildingscience.com...3-4_p42-45.pdf

  7. #20
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    Quote Originally Posted by jtrammel View Post
    This explains how these scientists and engineers tested it and what they found. I had this problem at my own house that i rented at one time. The doors were undercut but there was a noticable pressure difference when door was closed. I talked my landlord into paying me to onstall a ducted return to the master bedroom and solved the issue. Since then my company has invested in blower door, duct blaster, flow hood, ir camera etc and have found many many master bedrooms like that. If its not feasible to run a ducted return we will jumper duct with flex to deaden sound transmission and if you dont oversize the duct then you still have a pressure difference from the rooms. I by all means dont know everything about these types of problems but i do attempt to solve them all the time.http://www.bestofbuildingscience.com...3-4_p42-45.pdf
    Well, you are certainly over thinking this one and making a proverbial mountain out of a mole hill. Our industry has been doing very well with undercutting doors and installing jumper chambers without having to spend a lot of time and money for such testing unless situations call for it. Personally, I have not run into a residential system that cannot be resolved without having to bust the bank.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  8. #21
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    Anderson, South Carolina, United States
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    Nobody's busting the bank. We make profit and satisfy the customer. That's what it's all about

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