Hysteresis
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Thread: Hysteresis

  1. #1

    Hysteresis

    Hi there,

    I work for my state government and as such usually work in a large building. The respective facilities managers always turn off the blowers, heat, or air conditioning at 5:00PM, ostensibly as a cost-saving measure.

    Two questions:

    1) As many people work in the buildings past 5:00PM, how dangerous does it become with the ventilation turned off?

    2) Does it really save money to turn off the heat/air conditioning every day? It seems as if it would take less energy to keep a huge volume of air at a constant temperature than it would to radically change the temperature of that volume once every 24 hours.


    Thanks,

    Eric

  2. #2
    Join Date
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    Usually most larger class A office buildings do not shut down after the "Occupied" Hours. They go into "Setback" which moves all of the setpoints to a greater hysteresis band.
    Most buildings here only setback to 78 degrees [cool 65 heat] in order to support any operating workstations in the tenant areas and prevent humidity from curling the paper in printers/copiers.
    There are considerable energy savings by going into setback.

  3. #3
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    You may be able to easily justify overthrowing this practice if you have any sort of smoke purge system, which you probably do.

    There's not a doubt in my mind that would be HIGHLY against fire code. Fires do not only happen during working hours =P

  4. #4
    Join Date
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    When a building goes "unoccupied" you are saving money in energy costs.

    BUT, unoccupied does not mean "OFF" completely. It is only a period of time

    that a building can "relax" a little. Typically I would never adjust temp

    setpoints by more than 3 degrees during unoccupied modes. You don't

    want to "shock" the building and then try and recover it by using lots of

    energy to get it back to your desired "occupied" requirements. Most BAS

    have a "optimal start" program that takes into consideration at what time and

    at what temp you want to go "occupied" and whether or not the system was

    able to do it in time. So the next "unoccupied to occupied" change it will

    compensate and make it happen flawlessly.

  5. #5
    Join Date
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    Western NY
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    There are many different control strategies building managers use to save money, always depends on the site and application (and some trial and error). This is the "Bible" of controls, known as the "Gray Manual". Make a pot of coffee, sit back and relax....it's a long read, but well worth it.

    http://customer.honeywell.com/Techli...s/77-E1100.pdf
    "Excellence is the gradual result of always striving to do better"
    -Pat Riley

  6. #6
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    Wow... 518 pages. Thanks for the link, guess I have something to do on rainy days now! =D

  7. #7
    Join Date
    Aug 2003
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    225
    Yes it saves ($$$) money, if done in the right manor, and right way.

    Also I agree a “Big” change is not good – better to let things gradually rise and fall (Temps) and maybe have a “Maxim” Area/Floor/Building temperature –(and bring units on as needed) so units don't have to work twice as hard ' to bring (pull down/etc) it to a nice comfortable range (warm up/cool down time should be included in any strategy) for the "CUTOMER"

    Also better to “stager” all equipment off/on (5 min to a 1 hour?) – so as not to burden the electrical Equipment / system, and Chillers? / etc

  8. #8
    Join Date
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    Location
    Southern California
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    I am on the side of the facility manager with regards to saving energy. In some climates you may have condesation issues that prevent you from maximizing your saving by turning the equipment fully off. But whenever and wherever you can shut stuff off you are saving energy.
    A basic explanation is that heat transfer is a function of differential temperature. Once the inside and outside of your facility are nearly the same temp no heat goes in (or out). If you run your equipment in setback then you are maintaining a temperature differential and are gaining heat. That means overnight you are constantly rejecting heat that would not have been there if the conditions were allowed to equallize. That along with the heat generated by the pumps and fans makes it certain that you save energy by shutting your equipment off durring unoccupied periods.
    Again condensation and the peak capacities of your particular system determine just how far you can go with this strategy. Here in Southern California we shut our stuff off Friday and by the time the equipments starts the interiors are a brisk 100*. Not a problem when humidity runs in the teens. JRinjax would have moss growing on the walls if he tried that.

    Just my .02.
    Mike

  9. #9
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    Quote Originally Posted by Quixote View Post
    I am on the side of the facility manager with regards to saving energy. In some climates you may have condesation issues that prevent you from maximizing your saving by turning the equipment fully off. But whenever and wherever you can shut stuff off you are saving energy.
    A basic explanation is that heat transfer is a function of differential temperature. Once the inside and outside of your facility are nearly the same temp no heat goes in (or out). If you run your equipment in setback then you are maintaining a temperature differential and are gaining heat. That means overnight you are constantly rejecting heat that would not have been there if the conditions were allowed to equallize. That along with the heat generated by the pumps and fans makes it certain that you save energy by shutting your equipment off durring unoccupied periods.
    Again condensation and the peak capacities of your particular system determine just how far you can go with this strategy. Here in Southern California we shut our stuff off Friday and by the time the equipments starts the interiors are a brisk 100*. Not a problem when humidity runs in the teens. JRinjax would have moss growing on the walls if he tried that.

    Just my .02.
    Mike
    Mike,
    We have moss grow on the walls anyway [inside and outside]. We are now back to a more normal weather pattern [after several years of drought]. It gets up into the mid 90s each day with bright sunshine 90%+ RH, then we have intense thunderstorms with 30-40 MPH winds/hail, then it is Sunny with 100% RH for the rest of the day. Any hygroscopic surfaces on the building exteriors cause mold growth inside the structure.

  10. #10
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    Sounds like a little slice of heaven JR.

  11. #11
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    whec720
    Great link, very very usefull, thanks.

  12. #12
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    Quote Originally Posted by JRINJAX View Post
    Usually most larger class A office buildings do not shut down after the "Occupied" Hours. They go into "Setback" which moves all of the setpoints to a greater hysteresis band.
    Most buildings here only setback to 78 degrees [cool 65 heat] in order to support any operating workstations in the tenant areas and prevent humidity from curling the paper in printers/copiers.
    There are considerable energy savings by going into setback.
    Any of the High Rises I have worked in have a BMS system that shuts down tne AC after hours. The leases will have section dealing with hours of AC and charge for after hour AC and some also have hours for lighting.
    Old snipes don't die they just loose their steam

  13. #13
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    Jan 2003
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    Quote Originally Posted by Talz View Post
    You may be able to easily justify overthrowing this practice if you have any sort of smoke purge system, which you probably do.

    There's not a doubt in my mind that would be HIGHLY against fire code. Fires do not only happen during working hours =P
    Two seperate control systems. You can shut down the HVAC without affecting the smoke purge system.
    Old snipes don't die they just loose their steam

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