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  1. #1
    Join Date
    Jul 2008
    Location
    Southern California
    Posts
    139

    VAV Reheat System

    We just started servicing a building that has two large VAV package units and a hot water boiler with pump all located on the roof, this system is very common for the area. Downstairs in the suite it has your typical VAV boxes with pneumatic thermostat, damper actuator, velocity controller and HW reheat valve. Here is the part I've never seen, all the hot water reheat coils are located in the high side duct work about two feet before the inlet of each VAV box. One of the problems we are having is the box is never seeing the 1" WC it requires for the velocity controller to operate because the air hits the hot water coil first causing a pressure drop, the velocity controller always keeps the damper open because its seeing low static at the pitot tube ports. If it was one or two zones I would just think someone screwed up but every perimeter zone is this and it looks original from possibly late 70's early 80's. Its a 150 ton system so there are alot of zones. What were finding is the isolation valves to most of the reheat coils have been closed over the years. Has anyone ever come across a system like this and if so did it work ?
    Effort is the 1st step towards failure.

  2. #2
    Join Date
    Jun 2004
    Location
    Eugene, Oregon
    Posts
    1,018
    Is there a reheat coil for each VAV or for a Zone? What's the duct static? Are coils plugged. What pressures are you seeing are you seeing at the air flow measuring stations?
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten". --Benjamin Franklin
    "Don't argue with an idiot, they will drag you down to their level and beat you with experience". --Mark Twain
    http://www.campbellmechanical.com

  3. #3
    Join Date
    Dec 2002
    Location
    SouthEast NC ICW & Piedmont Foothills
    Posts
    7,635
    off hand; can't say I've seen what you are describing, but, are those coils clean-one can only assume it worked correctly once upon a time.
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  4. #4
    Join Date
    Jan 2013
    Location
    San Gabriel Valley, California
    Posts
    32
    What is the problem? Keep it simple, too hot. , too cold , no airflow???

  5. #5
    Join Date
    Jan 2005
    Location
    steeler nation
    Posts
    2,055
    if it were me, i would pick a couple coils, take a pressure drop, clean the coil and remeasure pressure drop. its possible these coils have never been cleaned and are plugged.
    IV IV IX

    use your head for something other than a hat rack.......Gerry

  6. #6
    Join Date
    Mar 2011
    Location
    North Carolina Piedmont Area
    Posts
    449
    I have never seen hot water coils at the inlet of any VAV boxes. But that does not mean that it will not work so long as they are placed at least 3 pipe diameters ahead of the VAV box so that the airflow will stabilize. This is important so that the flow ring, static pick up tubes or velocity air sensor does not experience turbulence or flow separation.

    You will also need to consider that you have set the controller correctly so that proper air flow can be obtained. This is normally accomplished by the equal friction method or static regain. In addition you will have to establish proper main duct static pressure two thirds down stream of the unit.

    Equal friction method usually require pressure-independent control capability and/or static regain which requires that you maintain constant static pressure.

    You should contact the Vendor or specific brand of VAV boxes which are in place and ask them for their recommended guidelines.

    Enclosed is an attachment describing typical VAV boxes and there related applications or installation guidelines.



    But I am still learning and looking for a new mentor.
    _______________________
    In a strict sense troubleshooting is not part of the repair..........understand the symptoms and you will find a solution.
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  7. #7
    Join Date
    Oct 2005
    Location
    B.C. Canada
    Posts
    855
    Quote Originally Posted by alcomech View Post
    But I am still learning and looking for a new mentor.
    _______________________
    In a strict sense troubleshooting is not part of the repair..........understand the symptoms and you will find a solution.
    Really giving up on Feng? Give the poor guy another chance... lol

  8. #8
    Join Date
    Mar 2011
    Location
    North Carolina Piedmont Area
    Posts
    449
    If this trade were simple every one would be a Pro.

    I have to start at the bottom.. Simple is not an option

  9. #9
    Join Date
    Jul 2008
    Location
    Southern California
    Posts
    139
    Thanks for the response, It looks like it will work if you set it up as pressure dependent and get away
    from the velocity controller. I was mainly curious if anyone has ever seen an installation like this, I
    haven't and I have been in a lot of buildings in the last 37 years. Thank you again.
    Effort is the 1st step towards failure.

  10. #10
    Join Date
    Jun 2004
    Location
    Marietta,Ga.
    Posts
    13
    I had a building with same dumb setup. My primary issues were the duct liner was degrading and blocking the coils. An apprentice with some tin snips, duct tape and a toilet brush was the approach I used.

  11. #11
    Join Date
    May 2004
    Location
    SE Pennsylvania
    Posts
    915
    I would agree with the blocked coil theory. Never seen them mounted before a VAV terminal but I had a similar issue with blocked coils and low airflow. Ended up cutting an access panel in, put a large drain pan under it, hooked a garden hose to drain the water out of the pan and went at them with a garden hose. Kind of pain getting hoses in and out of an office area but much quicker and easier than pulling the coils.

  12. #12
    Join Date
    Mar 2010
    Location
    Tidewater Virginia
    Posts
    101
    Quote Originally Posted by psycho-metric View Post
    I had a building with same dumb setup. My primary issues were the duct liner was degrading and blocking the coils. An apprentice with some tin snips, duct tape and a toilet brush was the approach I used.
    Does anybody else use a brass bristled grill brush to clean coils? I pull the scraper blade out and it keeps my hands away from sharp fins (I miss the old ones with the bristles slanted forward though).

  13. #13
    Join Date
    Mar 2010
    Location
    Tidewater Virginia
    Posts
    101
    I've only run into this setup once in a 1972 building. It turned out the pneumatic VAV boxes were a modular design assembled in the field with s-cleats and drives holding the sections togather. Each box kit included two blank end caps (cut in and tap transitions as needed) a cased damper assembly and a cased HW coil with external actuators and valves. It turned out the installaing contractor got thrown off the job at the completion of the third floor and another finished the fourth floor and started the building, so every 4th floor VAV was assembled with the HW coil upstream of the damper. I got involved when someone tried to install an electric VAV in place of the HW and burned out three heat packages due to limited air flow and someone discovering where the high temp reset button was! That was a fun two hour call which turned into 10 days with a helper...

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