Results 1 to 10 of 10

Thread: why 55deg?

  1. #1
    Join Date
    Jan 2009
    Location
    Indiana
    Posts
    3,228

    why 55deg?

    Maybe this is more for the controls folks but here it goes......

    Tomorrow I'm to meet the controls guy for a building we take care of. Some work was done on the chilled and heating loop valves. This office building sits empty and we are to keep it at a semi steady temp in order to keep it alive - ceiling tile wont droop and wall paper stays on the walls type thing.

    The controls guy insist the air leaving the handler is to be set at 55 deg and the VAV's with re heat loops heat the air back up. To me this is a tremendous wast of energy. Why would he want to do this on a empty building...why would he want to do this on a occupied building? There are 8 main air handlers in the building. This will be the only one set up like this because of the new controls. The others the leaving air is determined by the space temp.

    I currently have each air handler coming on for two hours a day in order to circulate air and dehumidify. Each unit is on at a different time.

  2. #2
    Join Date
    Oct 2001
    Posts
    6,966
    there will be no heat load as occupied would be at but still the VAVs will make there setpoints 72F whatever and the reheats will bring it back if they drop below that SP.as the boxes go to setpoint minimums the air pressure will hit the static SP met to 1.5" and the IGV or VFD will back the air off the discharge air will chill out and the 3-way will loop the chilled water back to the chiller and unload itself but not shut off..your question to him is what is NSB "night setback" typical 50F(AHU should actually cycle with a call for heat) is set at and what about MWU "morning warm up" typical 68F..most typical occupied building run 1.5" staic at 55F DA to satisfy a VAV stat at 68F within the space..industry standard
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  3. #3
    Join Date
    Mar 2011
    Location
    Manchester NH
    Posts
    208
    I agree with what you are saying, I take care of 9 buildings that are occupied. We are in New England and it gets cold here in the winter. We set out discharge air on the RTUS to as high as 64 deg in the winter time. All three of us engineers on the site agree about this. Why heat up 55 deg air to say 80 deg when you can heat up 64 deg air. Just seems more efficient. Not that this answers your question just my .

  4. #4
    Join Date
    Jan 2005
    Location
    steeler nation
    Posts
    2,131
    55 deg with reheat is a traditional way to do vav. its easy and provides good comfort and dehumid in a building. however,this isn't 1960 any more. energy is expensive. this is why most good control installations will reset the discharge air temperature based on zone demand. a simple way is to reset discharge air based on outdoor air temp. this however requires alot of tuning and a intimate knowledge of the building. another simple way is to use return air temperature. if return air temp is 68 degrees why would you be dumping 55 deg air? the building is obviously cold.

    other ways to reset DAT are based on vfd speed. if the fan is backing down the boxes are closing so cooling demand has decreased.

    actual zone demand can be looked at also. things such as terminal load -how far the zone is away from setpoint can be used to save energy.

    The control contractor may have reasons he did not tell you for not resetting the discharge air temperature. Maybe he has stock in the utility companies!
    IV IV IX

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

  5. #5
    Join Date
    Oct 2001
    Posts
    6,966
    the biggest killer with a VAV systems is that the building perimeter is super tight heating or cooling seasons especially in the winter.the lighting,electronics,people,coffee pots..are all heating sources to be contained and used.thats why MWP temp.is 68F comes on hour before the troops come in cycles air to catch the lighting heat and then once 68F is made on that return the ahu goes into occuppied.back in the old days there was never DAY heating with VAV,but now it is part of the occuppied cycle on drafty buildings.....next time you see a 250W flood light in a showroom contract space hit it with a laser thermometer....thats some heat note this the perimeter VAVs with heating wet or resistance type always shut supply air off and pull in return or ceiling air..thru those unit mounted filters thats why most ceiling are the full returns egg crated...and when you do NSB or MWU the supply fan will go to full open IGV or 60 hz on the VFD to get as many air cycles to make setpoints and call the VAVs to full open asap..reheating of cold discharge is not an engineers design goal.mint designs i've worked on is a building with interior rooftops and perimeter rooftops or AHUS.dead of the winter 20F outside the perimeter can supply straight out heat and the interior can do econo cooling where perimeter VAV with heat aren't installed...but a loose building kills that vav aplication bigtime..and watch those stand alone vav boxes or discharge registers with stats within each grill...how do you achive nsb or mwu??
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  6. #6
    Join Date
    Mar 2008
    Posts
    16
    depending on what type of BMS system you have you may be able to implement supply air temperature reset. This measures the outside air temperature as well as polls each vav box to see its damper position. If most dampers are closed, i.e there is too much cooling, then the DDC will increase the supply air temperature typically to a maximum of 68 degrees.

  7. #7
    Join Date
    Mar 2011
    Location
    Northeast Missouri
    Posts
    61
    The co I work for services several vav systems and we have the same energy efficiency arguments. Viceman is right, its not 1960. We vary our dat based on outdoor ambient. The facilities we service have seen a cost savings over the past couple summers/winters.

  8. #8
    Join Date
    Jan 2009
    Location
    Indiana
    Posts
    3,228
    He did the final commissioning Friday. The system is called "Talon". He was able to get the system down to 60deg discharge. Though he is uncertain if the valve is operating correctly and a much warmer day is needed to do a full check out. He did leave it so we could set the leaving air of the unit to the temp we wanted. Come summer we will see how the humidity is doing in the rooms and adjust accordingly. Again comfort is not what we are going for..keeping the walls from warping and paint from pealing is the only goal as it is a empty building.

    Thanks for all the insight.

  9. #9
    Join Date
    Apr 2007
    Location
    Ohio
    Posts
    8
    Reseting the discharge air setpoint based on load or OA conditions work as long as the humidity levels are maintained. Once the humidity levels exceed 55-60% the discharge setpoint should go to 55 deg F to dehumidify.

  10. #10
    Join Date
    May 2009
    Location
    Bardstown, KY
    Posts
    94
    Set up a discharge reset off of the average space temp.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event