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  1. #1
    Join Date
    Apr 2011
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    22

    Confused Self contained ceiling air conditioner

    I came across a ceiling unit manufacturer on the web stating that one self contained ceiling air conditioner(1 or 2ton) should not be connected to multiple rooms (supply cold air to multiple rooms). It also stated that if some one supplies the cold air to more than one room, then unbalanced airflow will occur creating positive or negative pressure within the room.

    I got 3 questions. Your reply to all is appreciated.

    1) Can someone please explain what it means " Unbalanced airflow will occur creating positive or negative pressure within the room"

    2) how unbalanced air flow will occur and how it creates positive or negative pressure within the room?

    3) what is wrong if unbalanced airflow creates positive or negative pressure within the room? Does it effect the air conditioner performance in any way?

  2. #2
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    For example... you have 3 bedrooms and a 2 ton system with a return in bedroom 1 but supply registers in bedroom 2 and 3. Here is what you get.

    Bedroom 1: Supply 267 CFM - Return 800 CFM = Negative 533 CFM in bedroom 1.
    Bedroom 2: Supply 267 CFM (no return) = Positive 267 CFM in bedroom 2.
    Bedroom 3: Supply 267 CFM (no return) = Positive 267 CFM in bedroom 3.

    So to fix this you can just put a return in each room.

    Slightly positive or negative is okay but too much could starve your AC of air and then the coils will freeze.

  3. #3
    Join Date
    May 2012
    Location
    San Diego, CA
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    352
    Also know that longer duct runs = more static pressure which means less of the CFM will reach the rooms.

  4. #4
    Join Date
    Apr 2011
    Posts
    22
    Thanks for a quick reply. What you said make sense, but why the manufacturer is stating that it should NOT be connected to multiple rooms. Seems like they don't want it that way. They could have said it is okay to connect to multiple rooms if and only if you have equal number of supply and returns. But they are against it. I wonder why and if there are any other reasons.

    By the way negative air pressure may cause the unit coils to freeze. What is the effect of positive air pressure to the room and or the ac?

    Any feedback is appreciated!

    Thanks Guys!!

  5. #5
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    The other reason that I mentioned is the AC fan is not powerful enough to push air through a duct system if you use ducts to convey the air to multiple rooms. For example, if your fan only have 0.25 external static pressure and your duct static pressure loss is 0.25 then you won't get any air to your rooms. The manufacturer probably has their reasons to not connect multiple rooms.

    I think what happens, someone correct me if I am wrong but when a room is too negative pressured then the room is pulling air from other places to equalize itself. Since the return is also pulling air, the fan has to work harder to suck air and blow it through the coils, if not enough air passes through the coils, then they get too cold and freeze over.

    When it is too positive pressure you just get air leaking into other spaces. In a closed system where no make up air/outside air/or exhaust air is introduced then when one room is positive another has to be negative.

    Anyways, you probably also don't want to connect multiple rooms because what happens if one room is hot but the Ac and thermostat is in another room that is empty and cool, then you won't get the AC to turn on. I guess that can happen with any system with only one zone and one thermostat.

  6. #6
    Join Date
    Mar 2007
    Location
    southeast u.s.
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    71
    If it's a ductless system then adding duct will go against the way it's engineered and there will always be problems. If it's intended to have ductwork then I would imagine multiple rooms wouldn't be a problem, so long as it's sized and ducted properly.

  7. #7
    Join Date
    Apr 2011
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    22
    The unit I am talking about is ductless.

    I am also thinking that even if there are returns wherever there are supplier, still I think there is not a perfect way to supply equal amount of air and also get the same amount of return air from multiple rooms. I mean the Evap fan cfm for example is 1000 and with 2 ducts going to two rooms requiring 500 cfm each may not receive 500 cfm. One room may receive more than the other. And the same thing with the return side. Chances are that the room that is receiving more cfm will return less cfm (positive pressure) and the room receiving less cfm will return more cfm back to the unit (negative air pressure). Please correct me if I am wrong. Is there any way to make sure both the rooms receive and return 500 cfm?

    Hcong- regarding your comment on the static pressure.. "... You won't get any air into the room", what will happen to the air that enters from the AC unit then? I mean where will the air go if it does not come out of the duct into the room?

    You guys are good!

    More responses will help me understand better!

    Thanks

  8. #8
    Join Date
    May 2012
    Location
    San Diego, CA
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    Quote Originally Posted by Samart View Post
    The unit I am talking about is ductless.

    I am also thinking that even if there are returns wherever there are supplier, still I think there is not a perfect way to supply equal amount of air and also get the same amount of return air from multiple rooms. I mean the Evap fan cfm for example is 1000 and with 2 ducts going to two rooms requiring 500 cfm each may not receive 500 cfm. One room may receive more than the other. And the same thing with the return side. Chances are that the room that is receiving more cfm will return less cfm (positive pressure) and the room receiving less cfm will return more cfm back to the unit (negative air pressure). Please correct me if I am wrong. Is there any way to make sure both the rooms receive and return 500 cfm?

    Hcong- regarding your comment on the static pressure.. "... You won't get any air into the room", what will happen to the air that enters from the AC unit then? I mean where will the air go if it does not come out of the duct into the room?

    You guys are good!

    More responses will help me understand better!

    Thanks
    Sorry, i meant the farthest supply register in the duct won't get any air if the duct is too long or has too many bends (aka static pressure loss).

    Since it is a ductless, I know some ductless (like the multi-zone splits) units that can have multiple fan coils with refrigerant going to them. That will allow you to have one condensing unit and an a/c in each room.

    Answer to how to evenly distribute air... I know commercial have adjustable dampers and I am sure residential have some sort of adjustable grilles or dampers too. My boss has always told me if your ducts are sized correctly and your supply and return grille necks are also sized correctly, then you can have physics help you get even air distribution. Also, remember air likes to travel in the path of least resistance so any straight shots will likely get more air.

    But I think the main point is I wouldn't recommend going against manufacturer instructions, even us engineers follow what the manufacturer wants.

  9. #9
    Join Date
    Jul 2011
    Location
    East coast USA
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    1,000
    Quote Originally Posted by hcong View Post
    For example... you have 3 bedrooms and a 2 ton system with a return in bedroom 1 but supply registers in bedroom 2 and 3. Here is what you get.

    Bedroom 1: Supply 267 CFM - Return 800 CFM = Negative 533 CFM in bedroom 1.
    Bedroom 2: Supply 267 CFM (no return) = Positive 267 CFM in bedroom 2.
    Bedroom 3: Supply 267 CFM (no return) = Positive 267 CFM in bedroom 3.

    So to fix this you can just put a return in each room.

    Slightly positive or negative is okay but too much could starve your AC of air and then the coils will freeze.
    you cant be negative in a room that doesn't have enough air supplying it. It only becomes negative if your returning more than what your supplying.

    1) Can someone please explain what it means " Unbalanced airflow will occur creating positive or negative pressure within the room" (if you add or return more air than the room needs)

    2) how unbalanced air flow will occur and how it creates positive or negative pressure within the room? (if you add or return more air than the room needs)

    3) what is wrong if unbalanced airflow creates positive or negative pressure within the room? Does it effect the air conditioner performance in any way?

    an unbalance system will effect the over all operation. Air flow, temps, and units pressures will effect the efficiency of the unit. More so if you introduce outside air in the mix.

  10. #10
    Join Date
    Apr 2011
    Posts
    22
    The unit I am talking about is NOT ductless. By mistake, I mentioned ductless in my last reply.

    The unit I am considering to buy comes with one wall mount thermostat. Can
    I go against the manufacturer's recommendation and connect it to 2 rooms and make sure there is balanced air flow: same inlet and outle cfm? Can I convince the manufacturer with that arrangement?

  11. #11
    Join Date
    Jul 2011
    Location
    East coast USA
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    1,000
    Quote Originally Posted by Samart View Post
    The unit I am talking about is NOT ductless. By mistake, I mentioned ductless in my last reply.

    The unit I am considering to buy comes with one wall mount thermostat. Can
    I go against the manufacturer's recommendation and connect it to 2 rooms and make sure there is balanced air flow: same inlet and outle cfm? Can I convince the manufacturer with that arrangement?
    you need to look at the ducting requirements. This will tell you the max range and size you can run a supply and return duct, if the two rooms are within that range than i don't see why not. does this unit have an air cooled condensor? if so where will you exhaust the air? the unit specs should also tell you how far you can run a duct for exhaust. get the installation manual for the unit it will answer all your questions

  12. #12
    Join Date
    May 2008
    Posts
    30
    Can you post the piece of equipment you are referring too? The reason they may specify only one room is because of duct length restraints. Many ducted indoor units on mini-splits are only able to flow the proper amount of air with very short duct runs and very low static pressures. This could be why the manuf. specifies only one room.

  13. #13
    Join Date
    Apr 2011
    Posts
    22
    Based on the spec the maximum duct length ( evap supply and return) recommended by the manufacturer is 18ft straight length. The max ext static pressure for Evap fan is 0.15 IWG and 350 cfm.

    Max duct length for cond exhaust and return is 10' straight length.

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