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  1. #1
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    Constant Cycling/ Web ctrl/ ALC

    Ok bare with me....

    About a month ago we noticed that one of the units was cycling the Compressor about every 10-15 min, even with the space being within in the set point.

    For example; Space temp would be at 72 with a Heating SP of 70 and a cooling SP of 75, and a hysteresis of .5

    From this information alone we know that the space is within SP and the compressor should NOT come on, yet it does.

    Upon reviewing the whole building i realized that out of 9 RTU's 7 of them are experiencing or rather, behaving the same way.

    I have attempted to play with set points, hysteresis, sensor settings and reviewing through the logic but I cannot come up with a conclusion.

    Below I will be posting a screen shot of what I am seeing, hopefully some of you can help.

    Thanks in Advance
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  2. #2
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    On the top graph you can see that the zone temperature never surpasses the cooling set point, on this particular unit I have it set to a Heating Sp of 68 and a Cooling Sp of 77 with a hysteresis of 2, meaning cooling should really not come on till it hits at least 79 degrees, am I correct????? if so, we can clearly see that it never surpasses 77 degrees and yet the compressor is cycling about every 20-25 min on this particular chart.

    Please tell me what I am being oblivious too.

  3. #3
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    Is there dehumidification control on these unit? That could account for the cooling coming on when the zone is between temp setpoints.

    I can see a few spots where the yellow zone temp is peeking over the cooling setpoint. I would increase the deadband from 0.5 to 1.0 on the cooling enable. It looks like you're just barely coming over the cooling setpoint and the DX is riding the min on/min off timer block.

    Speaking of which, you could also increase the time of your min off stage time to reduce your cycles per hour.

    How many stages of cooling are coming on at a time?

    If your cooling setpoint is 77F then the cooling will come on at a temp above 77.0F (like 77.1F) and turn off at 2 deg below the setpoint (75F).

    Ken

  4. #4
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    Quote Originally Posted by chesehd View Post
    Is there dehumidification control on these unit? That could account for the cooling coming on when the zone is between temp setpoints.

    I can see a few spots where the yellow zone temp is peeking over the cooling setpoint. I would increase the deadband from 0.5 to 1.0 on the cooling enable. It looks like you're just barely coming over the cooling setpoint and the DX is riding the min on/min off timer block.

    Speaking of which, you could also increase the time of your min off stage time to reduce your cycles per hour.

    How many stages of cooling are coming on at a time?

    If your cooling setpoint is 77F then the cooling will come on at a temp above 77.0F (like 77.1F) and turn off at 2 deg below the setpoint (75F).

    Ken
    Hi Ken,

    Thank you very much for you response, I dont know if we have dehumidification control, but looking at the logic there is a micro block for humidity... check out the pic and tell me what you think.

    Would you mind going a little into detail how a humidity controller would in this case come into play? Are you saying that cooling is engaged because humidity levels are too high?

    Yeah, it barely crosses or lingers at the line, would increasing it to 2 instead of .5 be a good idea? so that if Sp is 77, cooling would not come on till lets say, 79.1...?

    As well my timer block is set up for 1 min, maybe i should increase it to 3-5 minutes?

    As far as cooling stages, this particular unit is on one stage but we have some of the with 2 stages.
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  5. #5
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    That block is calculating the outside air dewpoint. Shouldn't have anything to do with your DX staging on and off.

    I was asking about dehumidification in the unit because if it exceeds the indoor setpoint then yes, there could be a call for the DX.

    Can you show the rest of the program on here? Or, perhaps, when you're looking at the logic page select the print icon on the top, right corner and print the program as a .pdf file, then post it here?

    Ken

  6. #6
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    Quote Originally Posted by chesehd View Post
    That block is calculating the outside air dewpoint. Shouldn't have anything to do with your DX staging on and off.

    I was asking about dehumidification in the unit because if it exceeds the indoor setpoint then yes, there could be a call for the DX.

    Can you show the rest of the program on here? Or, perhaps, when you're looking at the logic page select the print icon on the top, right corner and print the program as a .pdf file, then post it here?

    Ken
    Thanks again for your reply Ken

    Here are the screen shots of the whole logic page.
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  7. #7
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    Ok, so if the deadband in your setpoint block is 0.5F then that means your color will change from green to yellow when you hit the cooling setpoint (in this case 77.0F). The color will change back to green when your zone temp is 0.5F below your cooling setpoint. This is relevant, I promise.

    Looking at the cooling control, part of the conditions for enabling the cooling to run is the color output from the Setpoint block. Changing the deadband from 0.5 to something higher, like 1.0 or whatever you want will allow the cooling to run longer and (ideally) reduce the cycling of the DX.

    Part of the issue not discussed yet would also include the load in the building. If it's an empty shell, waiting for equipment and people to fill it and give the space some heat to remove that could explain some of the problem. The units are not necessarily short-cycling as they appear to be running for 20 or so minutes at a time with a reasonable gap between starts and stops.

    If your motions sensor is built in to the wall sensor (as shown by the BSVI named Sensed Occupancy) then this is probably a fairly new project. Have you had any discussions with your local dealer about your concerns?

  8. #8
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    Quote Originally Posted by chesehd View Post
    Ok, so if the deadband in your setpoint block is 0.5F then that means your color will change from green to yellow when you hit the cooling setpoint (in this case 77.0F). The color will change back to green when your zone temp is 0.5F below your cooling setpoint. This is relevant, I promise.

    Looking at the cooling control, part of the conditions for enabling the cooling to run is the color output from the Setpoint block. Changing the deadband from 0.5 to something higher, like 1.0 or whatever you want will allow the cooling to run longer and (ideally) reduce the cycling of the DX.

    Part of the issue not discussed yet would also include the load in the building. If it's an empty shell, waiting for equipment and people to fill it and give the space some heat to remove that could explain some of the problem. The units are not necessarily short-cycling as they appear to be running for 20 or so minutes at a time with a reasonable gap between starts and stops.

    If your motions sensor is built in to the wall sensor (as shown by the BSVI named Sensed Occupancy) then this is probably a fairly new project. Have you had any discussions with your local dealer about your concerns?
    Wow, thanks Ken very informative I appreciate it very much.

    It seems like I have mis informed myself in regards to how the hysteresis or "dead band" works.

    I was under the impression that, if i have a hysteresis of 2 with a cooling set point of 77, the unit would not engage cooling until it reached 79 degrees or more, (77 + 2).

    What you are telling me is that the way it works is, if heating sp is 68, heat will come on when the temp reaches 68 and will not turn off till the space is 70 or higher, and with cooling if sp is 77, cooling will engage at 77 and will not turn off till space is 75 or below, when the hysteresis has been set for (2).

    Am I understanding you correctly?

    This building is not very big, but does have a lot of empty spaces and at times not much traffic or " heat loads".

    Why would the lack of heat loads cause cooling? Shouldnt it be opposite?

    In regards to the sensors, they were installed but i have disabled them hoping it would resolve the problem.

    The RTU's have been here for years, but the installation of controls (ALC) is in fact recent. It was installed about 6 months ago.... I have indeed contacted the installing company and I am still waiting to hear from them.


    Thanks again Ken, you being extremely helpful.

  9. #9
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    Looks like you're on track with the deadbands now.

    The lack of heat load in the space would contribute to the short cycling because you just have open air, no heat sources adding heat to the space. The low load conditions would not cause cooling, the just wouldn't be adding heat to the space. Lacking any additional indoor heat beyond what you're gaining from solar and ambient conditions, the indoor air will cool off much faster.

    Look at it like this, if you had an electric space heater running in the room it would take the cooling process longer to bring the zone back down to the setpoint, right? Same idea with having a room or building full of people. The people produce the heat, the RTU removes the heat. The people adding heat while the RTU is removing the heat just makes the RTU work longer to achieve the goal (reaching setpoint).

    I'm glad to help, we are all here to give and get. Gotta call it a night, still need to get up in the morning.

  10. #10
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    Quote Originally Posted by chesehd View Post
    Looks like you're on track with the deadbands now.

    The lack of heat load in the space would contribute to the short cycling because you just have open air, no heat sources adding heat to the space. The low load conditions would not cause cooling, the just wouldn't be adding heat to the space. Lacking any additional indoor heat beyond what you're gaining from solar and ambient conditions, the indoor air will cool off much faster.

    Look at it like this, if you had an electric space heater running in the room it would take the cooling process longer to bring the zone back down to the setpoint, right? Same idea with having a room or building full of people. The people produce the heat, the RTU removes the heat. The people adding heat while the RTU is removing the heat just makes the RTU work longer to achieve the goal (reaching setpoint).

    I'm glad to help, we are all here to give and get. Gotta call it a night, still need to get up in the morning.
    Thank you Kind Sir!

    Have a good night sleep!!

    Stay safe

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