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Re: [EnergyPlus_Support] Return Air Setpoint Control Strategy

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Return Air Setpoint Control Strategy
From: Fred Buhl <wfbuhl@xxxxxxx>
Date: Wed, 11 Apr 2007 14:35:48 -0700
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Jose,

Sorry - I should have answered this in the first place. The simple 
controllers in EnergyPlus
operate by iterating the air loop side of the air loop / zone equipment 
combination. Furthermore, zone
temperatures are not updated until the full HVAC (system, plant) 
solution is complete. Another way of
saying this is that the zone temperatures are held fixed while the HVAC 
system is simulated. This
has some good features; a bad feature is that the return temperature 
will not vary significantly
during an HVAC simulation. Thus the simple controller, by varying the 
water flow rate through the coil,
will not influence the return temperature. So you are right, you can not 
control off of the return temperature
and the return temperature setpoint.

The only way forward would be to use a Setpoint Manager; one could write 
a setpoint manager that
would look at the return temperature and adjust the setpoint at the coil 
outlet to try to maintain a given
return temperature. Getting this to work well would be somewhat 
difficult. Otherwise you are left
with what Mike suggests - using zone temperatures as surrogates for the 
return temperature and using
one of the exisiting set point managers.

Fred Buhl

Jose Coutinho wrote:
> Thanks a lot Mike,
>
> However, from the I/O manual, It looks that CONTROL objects that actuate
> on Air Loop coils can not sense setpoints at nodes that are located
> downstream after the Zone Equipment subloop. Is this correct? If it is
> so, solutions 1-3 are not possible to implement. Actually, your first
> suggestion is exactly what I tried to model on the first hand.
>
> Anyway, thanks again for your suggestions. After all, it is always
> important to realize that we are thinking the right way.
>
> Cheers.
> Jose'
>
>
>
> Michael J. Witte wrote:
>   
>> This is not a supported control strategy in EnergyPlus, but I will add
>> it to
>> the suggested enhancement list.
>>
>> Here are some approaches you could try, but I do not know if they will work.
>>
>> 1. Use SET POINT MANAGER:SCHEDULED to place the desired return air control
>> temperature as a setpoint on the Air Loop Inlet Node. In the
>> CONTROLLER:SIMPLE
>> object for the cooling coil, set the Control_Node to be the Air Loop Inlet
>> Node.
>>
>> 2. Same as 1, but use the ZoneEquipGroup Outlet Node instead of the Air
>> Loop
>> Inlet Node. These nodes both represent the same physical point in the
>> system,
>> but they are updated at different times in the iteration sequence, so
>> you might
>> find one to behave better than the other.
>>
>> 3. Use SET POINT MANAGER:WARMEST to place a setpoint on the cooling coil
>> air
>> outlet node, and set the controller control node to that node. If this is a
>> draw-thru system, then place this setopint at the fan outlet and use a SET
>> POINT MANAGER:MIXED AIR to place an adjusted setpoint at the cooling
>> coil air
>> outlet node.
>>
>> 4. Choose the dominant zone on the system and use it to control the
>> supply air
>> temperature with SET POINT MANAGER:SINGLE ZONE COOLING.
>>
>> Note - whatever approach you use, be sure to set the cooling priority
>> (in the
>> ZONE EQUIPMENT LIST) for the DIRECT AIR units to be 1, and the unit
>> vents to be
>> cooling priority 2.
>>
>> Mike
>>
>> On 7 Apr 2007 at 11:31, coutinho301272 wrote:
>>
>>     
>>> Dear All,
>>> I have several zones served by a single constant volume AHU (cooling
>>> only) plus individual cooling unitvents on each zone. The air system
>>> has cooling prioriry over the UnitVents and provides fresh air.
>>> UnitVents will complement cooling needs on each zone.
>>>
>>> I wonder how can I control water flow rate in the central cooling coil
>>> based on return air temperature, ie, by setting a target temperature at
>>> the Air Loop Inlet Node.
>>>
>>> Thanks a lot in advance,
>>> Jose'
>>>
>>>
>>>
>>>
>>> The primary EnergyPlus web site is found at:
>>> http://www.energyplus.gov <http://www.energyplus.gov>
>>>
>>> The group web site is:
>>> http://groups.yahoo.com/group/EnergyPlus_Support/
>>>       
>> <http://groups.yahoo.com/group/EnergyPlus_Support/>
>>     
>>> Attachments are not allowed -- please post any files to the
>>>       
>> appropriate folder
>>     
>>> in the Files area of the Support Web Site.
>>>
>>> EnergyPlus Documentation is searchable. Open EPlusMainMenu.pdf under the
>>> Documentation link and press the "search" button.
>>>
>>> Yahoo! Groups Links
>>>
>>>
>>>
>>>
>>>
>>>       
>> ========================================================
>> Michael J. Witte, GARD Analytics, Inc.
>> EnergyPlus Testing and Support
>> EnergyPlus-Support@xxxxxxxx <mailto:EnergyPlus-Support%40gard.com>
>>
>>
>>     
>
>
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
>
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
>
> Attachments are not allowed -- please post any files to the appropriate folder in the Files area of the Support Web Site.
>
> EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.
>  
> Yahoo! Groups Links
>
>
>
>   


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Follow-Ups:
- Re: [EnergyPlus_Support] Return Air Setpoint Control Strategy
  - From: Jose Coutinho

References:
- Re: [EnergyPlus_Support] Return Air Setpoint Control Strategy
  - From: Michael J. Witte
- Re: [EnergyPlus_Support] Return Air Setpoint Control Strategy
  - From: Jose Coutinho

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