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  1. #1

    Is anyone able to explain Supply air reset

    Hi, im new here. My question is, i have heard the phrase supply air reset but i dont know what it means. Is anyone able to explain it please.

    Thanks heaps,
    Aaron.

  2. #2
    Join Date
    Dec 2006
    Location
    Hartford, Connecticut
    Posts
    442
    Assuming this is for a rooftop unit or air handling unit. It means the supply air temperature or some people refer to as discharge air temperature (leaving air temperature) from the unit will reset as needed to maintain temperature. If this is on a DDC system you would have a process variable like space temperature or return air temperature.

    So the heating & cooling will work as needed (depending on lockouts, limits, deadbands, etc.. in the program) to reset the SAT / DAT based on a variable like space temperature or return air temperature.

    A simple reset would look like this;

    Space Temp
    75
    70

    Supply Air Temp
    55
    65

    So if the space is 75 deg. we want a 55 deg. supply in a cooling mode, and on the other end if the space was 70 deg. we would want a 65 deg. supply. Many variables. Does system have reheat, is it constant volume or VAV etc.. The reset would usually be proportional in between those setpoint values.

    Gotta get back to work. Hope that helps.

  3. #3
    Join Date
    May 2007
    Location
    Wiscoinsin
    Posts
    73
    It is also possible to reset the DA based on OA. I.E. if it is 80 outside we may need cooler supply air than if it is 70 outside. So the discharge air would be adjusted based on what is "needed" to satisfy the space.

  4. #4
    Everything can reset the DAT. I've seen OAT, RMT, fan speed, VAV damper positions, terminal loads .... you name it.

    And ratio based reset like twisted pair described is not the only way. Periodical increasing/decreasing setpoint is another possibility.

  5. #5
    Join Date
    Dec 2006
    Location
    Hartford, Connecticut
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    442
    zarembad and leaflying,

    I agree with both of you. Based on the general question I thought I would answer it with some common or generic examples. I am sure there are probably even more ways.

    He did not elaborate on what type of system this would apply to. So I figured my example would be a good start, but you both point out that there are many ways to accomplish supply air reset.

  6. #6
    Join Date
    Mar 2005
    Location
    Virginia
    Posts
    1,144
    VAV Damper Pos resetting DATstpt? WOW! Never seen that one before. Havent got the slightest idea how that would be proper...

    On a VAV unit:
    I reset my Static Pressure Setpoint based on VAV Damper Position, and reset the DAT setpoint based on terminal load. I also have time limits on how fast the setpoints can change. Example: Static setpoint can not change more than x-"w.c. per x-time, etc...
    Without going nuts like adding another variable in there for "riding the fan curve", that about as good as it gets IMHO.
    “It is impossible for one to learn what one thinks they already know"

  7. #7
    Join Date
    Nov 2006
    Location
    SE Wisconsin
    Posts
    341
    Quote Originally Posted by codewriter View Post
    On a VAV unit:
    I reset my Static Pressure Setpoint based on VAV Damper Position, and reset the DAT setpoint based on terminal load. I also have time limits on how fast the setpoints can change. Example: Static setpoint can not change more than x-"w.c. per x-time, etc...
    Without going nuts like adding another variable in there for "riding the fan curve", that about as good as it gets IMHO.
    I've not been doing this as long as some but I think I'm seeing a potential for conflict there. Resetting static stpt based on damper position? Why wouldn't you let the damper change based on load & vary the VFD or guide vanes at the AHU to keep the static constant? The way I'm reading your statement you're changing 2 variables in the same equation and I'm curious how you keep making them add up.

    Seriously, not being a wise guy.

    Thanks.

  8. #8
    Join Date
    Aug 2005
    Posts
    145
    Quote Originally Posted by chesehd View Post
    I've not been doing this as long as some but I think I'm seeing a potential for conflict there. Resetting static stpt based on damper position? Why wouldn't you let the damper change based on load & vary the VFD or guide vanes at the AHU to keep the static constant? The way I'm reading your statement you're changing 2 variables in the same equation and I'm curious how you keep making them add up.

    Seriously, not being a wise guy.

    Thanks.
    I could see if the VAV is at 100%, and not achieving max airflow, this would be consider a starved box. The static pressure would increase to eliminate this starved box condition. OK

  9. #9
    Join Date
    Apr 2002
    Posts
    11,808
    Discharge air control, in some cases, will have an air handling unit supplying air at a neutral temperature that does not really heat or cool the space.

    As the space starts to warm up or cool down, you can reset or change the supply air temperature to compensate.

    Example a make up air unit in winter would normally supply room temperature air to the space, however if the space was starting to cool off the air temperature could be reset up to say 90F, to warm the space up.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  10. #10
    That's actually a pressure-dependant VAV system. No air flow measurement on box.
    Quote Originally Posted by codewriter View Post
    VAV Damper Pos resetting DATstpt? WOW! Never seen that one before. Havent got the slightest idea how that would be proper...

  11. #11
    okemechtech had a pretty good explaination. The other side of the coin is, trying to keep all damper positions as open as possible, i.e. if all dampers ae below 80%, decrease static pressure to drive them open and save energy.

    From what I read, Japaness goes even further on this road. They have so called "variable static pressure" systems that don't have static pressure sensor at all. Fan speed is controlled by damper positions.

    I acutally tried that in a small system, with high pressure protection. Not bad at all.
    Quote Originally Posted by chesehd View Post
    I've not been doing this as long as some but I think I'm seeing a potential for conflict there. ......

  12. #12
    This is kind of stealing the topic. But I am gonna do it anyways.

    The other way of VAV system fan speed control I know in Asia is to calculate the total box air volume / design air volume. I havn't tried that myself but I can tell the change of the speed will be faster and not as smooth as other methods.

    But the craziest idea stays in America. Thomas Hartman in his all VFD controlled system (from chiller to pump to fan), suggested that every little change of the cooling load will be evenly shared by all analog controlled device. I've not figured out how to exactly apply this to a real system.

    Mr. Thomas Hartman has some famous patented control strategies. Has anyone here heard of or tried his stuff?

  13. #13
    Join Date
    Nov 2006
    Location
    SE Wisconsin
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    341
    Quote Originally Posted by okemechtech View Post
    I could see if the VAV is at 100%, and not achieving max airflow, this would be consider a starved box. The static pressure would increase to eliminate this starved box condition. OK
    Not trying to be rude, just exploring this train of thought further.

    With the exception of engineering the system with some diversity built in, if you had the AHU & VAV'S TAB'ed properly wouldn't you be able to maintain a constant static setpoint regardless of damper position? Even if all the VAV's were at a minimum position the VFD on the AHU should ramp down to maintain the setpoint. Conversly, as the boxes open the VFD ramps up to meet the demand. Why complicate things by changing multiple factors?

    Carnak: Thank you. I already understood the DAT reset, I've just not heard about resetting the static setpoint prior to this and am trying to play this out.

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