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  1. #1

    Cooling an open plenum commercial building

    I am a layperson with no HVAC expertise and need some information from the experts.
    My office is located in eastern PA in a seven year old commercial building rated as Class A commercial construction. It is a 45,000 square foot building on three floors. There are about 14 feet between the concrete floors. On each floor, the suspended ceilings are 9 feet, leaving about 5 feet of open plenum return above the ceiling.
    Every summer, during hot weather (85 to 95 degrees), we experience times when the air conditioning is so poor that we are sweating during the day. We then call maintenance and the problem is corrected.
    I have had individuals, who claim to have HVAC expertise, tell me that's just the way it is with open plenum returns - sometimes the cooling just isn't good enough and I should learn to live with it and sweat. This certainly does not seem right to me. It seems to me the problem stems from something else, possibly with the way the system is managed.
    I have worked my entire career in office buildings and have never been told that maximum available air conditioning may still leave me sweating. A relatively new system in a relatively new building should work well and keep occupants cool. When you set your thermostats to 65 degrees and the room temperature is between 80 and 85 degrees, something is not right.
    I would appreciate input from those of you who have the real expertise.

  2. #2
    Join Date
    Dec 2004
    Location
    newton,mass.
    Posts
    6,109
    It's called comfort cooling for a reason, your system has problems, it should give you the desired temp at all times ... sixty five is not possible though. Plenum returns are very common and work well when set up properly. They may have undersized the A/C, or there may be and unanticipated high load. Do you have a large computer load or exhaust air need?


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  3. #3
    Join Date
    Dec 2009
    Posts
    147
    Worst thing to do is crank the temperature WAY down.
    Everyone thinks the lower you set the temperature the faster it will fall.
    70F is the perfect temperature for most people
    Either way it sounds like your unit is under sized
    best of luck

  4. #4
    Thank you for your comments. I do not expect the room temperature to reach 65 degrees, however, out of desperation, we set the thermostats to 65 degrees to see how low the temperature would drop. In that instance, the air coming from the vents was 78 to 80 degrees. My conclusion: I agree that the system needed maintenance.
    I appreciate the heads up on the potential computer load. In my office, that is minimal and we do not have any other heat generating equipment. Thanks again.

  5. #5
    Join Date
    Feb 2010
    Location
    Cal
    Posts
    1,596
    If you are not responsible for the building, the details don't really matter. All that matters is that the problem seems avoidable and needles due to the quick resolution once the facilities dept is made aware. Fixate on the symptoms/outcome, not the root cause. With the best intentions, occupants will often have "diagnosed" the problem without a complete understanding of the system functions, only to be far from correct.Your lease may include specified indoor conditions and occupied hour time frames.

    If you are responsible...it's difficult to determine online. Outside air intake and solar loads from glass can put a big dent in capacity. The outside air volume could be manipulated/lowered to reduce load sacrificing indoor air quality leading to a quick fix. A building of that size could have 80 tons of cooling and easily lose/waste 20 tons cooling OSA. Hopefully, your complaints don't stem from aggressive energy conservation/control strategies...another quick fix.

    Zoning, capacity, control strategies, glass, etc all complicate and impact each building differently.

    HVAC is one of the biggest reasons for tenant dissatisfaction and turn over, convince them to be proactive without telling them how to do their job.

  6. #6
    Join Date
    Dec 2009
    Posts
    147
    Quote Originally Posted by btuhack View Post
    If you are not responsible for the building, the details don't really matter. All that matters is that the problem seems avoidable and needles due to the quick resolution once the facilities dept is made aware. Fixate on the symptoms/outcome, not the root cause. With the best intentions, occupants will often have "diagnosed" the problem without a complete understanding of the system functions, only to be far from correct.Your lease may include specified indoor conditions and occupied hour time frames.

    If you are responsible...it's difficult to determine online. Outside air intake and solar loads from glass can put a big dent in capacity. The outside air volume could be manipulated/lowered to reduce load sacrificing indoor air quality leading to a quick fix. A building of that size could have 80 tons of cooling and easily lose/waste 20 tons cooling OSA. Hopefully, your complaints don't stem from aggressive energy conservation/control strategies...another quick fix.

    Zoning, capacity, control strategies, glass, etc all complicate and impact each building differently.

    HVAC is one of the biggest reasons for tenant dissatisfaction and turn over, convince them to be proactive without telling them how to do their job.

    Ya what he said

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