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  1. #1
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,289

    CV Ahu with Reheat

    Got a job that we are replacing a AHU. It's for a big salon. CV AHU that has reheats with old Barber Coleman stats. The AHU has an old XL50 that only controlled Occ and UnOcc. It has never worked right. The staff has just let the AHU run in occ all the time and they constantly adjust the DA spt. And in the summer shut down the Reheats.

    So we are replacing the air handler. I'm going to throw a Sypder on it for some better control. I'm thinking a simple space sensor to control cooling and reset discharge air based on return temps. Setup electric heat to handle MWU and unocc heating.

    Or should I just reset the discharge air based on OA.

    The only time I have dealt with CV with reheat control is labs and OR's, so it's alot easier to control.

    BTW- No VAV's

  2. #2
    Join Date
    Oct 2007
    Location
    Huntsville, AL
    Posts
    385
    Why control off discharge air on a constant volume unit?


    Im like a mushroom, they keep me in the dark and feed me crap.

  3. #3
    Join Date
    Nov 2006
    Location
    SE Wisconsin
    Posts
    299
    I would reset supply air based on return and keep the OA out of that part of the equation.

    You stated the unit has electric reheat. Will it be staged or SCR?

  4. #4
    Join Date
    Aug 2009
    Location
    Prattville, Alabama
    Posts
    1,749
    I don't get it, why be concerned with resetting discharge air temp on a constant volume AHU? Shouldn't it be trying to increase cooling (stages of cooling or chill water flow, which is it anyway?) the further the space temp is from setpoint? Sounds like resetting supply air would be working against that. I would appreciate any enlightenment.

  5. #5
    Join Date
    Jul 2008
    Posts
    1,421
    Quote Originally Posted by ascj View Post
    .... The staff has just let the AHU run in occ all the time and they constantly adjust the DA spt.
    Sounds like a good plan to me too ... Cascade control works very well in large spaces.

    Assuming this AHU is CHW cooling I'd consider letting OAT control the Min DA sp too...

    so DAsp is reset by RA but as OA goes high the minimum DAsp allowable can go lower. On a cool day maybe DAsp can go as low as say 16C ...on a hotter day DAsp could go as low as something like 12C. This way if there's a full cooling call on a cooler day ( maybe at startup or something) the system wont blast the occupants of the space with an super icy breeze.

    for those askin the question WHY?
    https://controls.engin.umich.edu/wik...ascade_Control
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  6. #6
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,289
    This is a 40ton DX CV AHU w/ staged electric heat. It feeds the 1st of a salon/spa. There are 10 zones controlled by electric reheat packages. No Vav's.

    I've dealt with this setup alot in OR's and Pharma Labs. CV AHU's with electric reheats are used due to the required ACH's and/or room static pressures requirements. I reset DA based off RA or zone temps(if they are tied into the BMS). They normally have tight humidity control and keep the room setpoints the same, year round.

    So now I have this setup at a salon. No humidity control and zone temps not tied into BMS. The problems I fore see are:
    1) Employees setting room setpoints lower in the Summer and higher in the Winter. I run into this alot in our buildings, that don't have means of humidity control. This throws DA reset based off RA and/or space temps, out the window. I end up with DA with OA reset.
    2) Looking at the space.....I foresee very different loads. There is locker rooms, saunas, nail studio, massage rooms, and lounge areas. And of course, I have no load calculations and history off what has been changed in the building over the years.

    The existing AHU didn't supply proper comfort levels and the building engineer just dealt with it. Now there is new engineer and the existing AHU's remote condenser deteriorated.....so we installed a new AHU. The existing controls are off a XL50 that is beat and no database for.

    So I'm thinking DA reset off OA during occupied periods and standard heat/cool based off single zone sensor during unoccupied periods?

  7. #7
    Join Date
    Jan 2008
    Location
    Near Philly
    Posts
    501
    Is there anyway to get better control of the reheats? If not, in a couple of years the system will probably be back in the same state as it is now. What you are proposing to do will work but, it's an energy hog. These type of systems are for critical controlled environments. It's a salon, somebody should to talk to whoever is paying the E-Bill and get them on the right track.

  8. #8
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,289
    I got talking to maintenance.....they set the reheats for 72 degrees and leave them. They all have lock boxes. So I ended up programming occ heat/cool to run off return air(69/72) and unocc heat/cool to run off a space sensor.

  9. #9
    Join Date
    Jan 2008
    Location
    Near Philly
    Posts
    501
    Quote Originally Posted by ascj View Post
    I got talking to maintenance.....they set the reheats for 72 degrees and leave them. They all have lock boxes. So I ended up programming occ heat/cool to run off return air(69/72) and unocc heat/cool to run off a space sensor.
    You're playing the cards as they were dealt, good job and good luck.

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