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  1. #1
    Join Date
    Sep 2003
    Posts
    86

    fresh air to fan coil unit

    We need to get some fresh outside air to a fan coil unit in the ceiling of a clinic.There is a 14 x 14 duct with no damper of any kind connected to an outside air duct(about 36 x 36) on the roof of this 4 floor building.The fan coil unit is on the first floor.The return air duct is open in the ceiling space,also with no damper or control.What I am wondering is why wont the outside air mix with the return air.Will adding a damper to the outside air duct and the return duct allow both to get pulled into the unit?What is the science behind the fact there is no cold outside air being pulled in.The nurses are always complaining that this office has always been warm.They would like to run the A/C but it is too cold for that.Any thoughts are appreciated.

  2. #2
    Join Date
    Jan 2003
    Location
    USA
    Posts
    1,683
    >What is the science behind the fact there is no cold outside air being pulled in.

    Path of least resistance.

    The return is open to the ceiling, no resistance. OA is a long duct run to the roof, plenty of resistance. If the building is positive pressure it would only fight the unit pulling OA.

    Add the econo dampers. That will close off the RA and force the fan to pull OA.

  3. #3
    Join Date
    Sep 2003
    Posts
    86
    Thanks very much for the reply.I thought that it may be something similar to that,but that's why I thought I would ask.I wonder if I shut down the fan,would the outside air come(Just to prove that we can get it)Thanks again

  4. #4
    Join Date
    Sep 2007
    Location
    Michigan
    Posts
    290
    Quote Originally Posted by mark7 View Post
    Thanks very much for the reply.I thought that it may be something similar to that,but that's why I thought I would ask.I wonder if I shut down the fan,would the outside air come(Just to prove that we can get it)Thanks again
    Your probably going to need a controller on your fan coil if doesn't already have one. Add two dampers so your able to control based on temp. The OA isn't likly to just fall into the space when you turn the fan off. The building should be in a posistive from other equipment pulling OA rooftops, AUHs.
    "Yeah I can figure out whats wrong with it, but you were here first and there isn't room for two, plus it's cold up here, I'll be in the van"

  5. #5
    Why would it be to cold to run a/c ?

  6. #6
    Join Date
    Sep 2003
    Posts
    86
    Thanks for the input.I realize we will have to have a controller to control the new dampers for a decent mixed air temp.That being said, why wont the cold outside air come into the space thru the open return duct with the blower off?It is the only unit in the office.There is some bathroom exhaust fans I believe, but wouldn't that make for a negative condition?

    The reason it is too cold for a/c is there are no crankcase heaters on the compessors.We are talking below freezing temps, and it does not seem to make sense to run a 60 ton York chiller for a 1500 square foot office.There is
    no other area that needs it.There are 6 scroll compressors on the chiller system with glycol used rather than water.Thanks again for any replies.I really appreciate it.

  7. #7
    Join Date
    Sep 2007
    Location
    Michigan
    Posts
    290
    I did say, isn't likly. Maybe it will pull if your exhausting. I don't see the point if you know the duct goes to the OA. Anyway good luck with the work.

    Quote Originally Posted by mark7 View Post
    Thanks for the input.I realize we will have to have a controller to control the new dampers for a decent mixed air temp.That being said, why wont the cold outside air come into the space thru the open return duct with the blower off?It is the only unit in the office.There is some bathroom exhaust fans I believe, but wouldn't that make for a negative condition?

    The reason it is too cold for a/c is there are no crankcase heaters on the compessors.We are talking below freezing temps, and it does not seem to make sense to run a 60 ton York chiller for a 1500 square foot office.There is
    no other area that needs it.There are 6 scroll compressors on the chiller system with glycol used rather than water.Thanks again for any replies.I really appreciate it.
    "Yeah I can figure out whats wrong with it, but you were here first and there isn't room for two, plus it's cold up here, I'll be in the van"

  8. #8
    Join Date
    Mar 2007
    Location
    West Palm Beach
    Posts
    172
    Just a thought ! we have seen the same kind of scenario, what we did was run a o/a duct right up to the return of the fan coils with a damper, (within 6" or so) that did the job for us.

  9. #9
    Join Date
    Sep 2003
    Posts
    86
    Thanks for the input.I really appreciate it.

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