[Bldg-sim] Baseline Sizing

[Vinay Devanathan]

Paul,

Thanks for the clarification. But in my case, I am just using a
AirTerminal:SingleDuct:Uncontrolled object. I am not able to define a
minimum supply airflow with this object. Do you have any suggestions on how
I can model in this situation?

Thank you very much.

Regards,
Vinay

On Tue, Jul 13, 2010 at 9:56 AM, Paul Raftery <p.raftery at gmail.com> wrote:

> Hi Vinay,
>
> I presume by 'not higher always' you mean an object which varies airflow at
> the zone level depending on load (say you are modelling a VAV box in that
> zone). EnergyPlus doesn't look at the exhaust airflow requirements when
> figuring out the supply airflow to the zone. It calculates supply airflow
> based on what is needed to maintain space conditions assuming that this
> airflow does not exceed the max or min design airflow (take a look at the
> Engineering Reference included in EPlus documentation).
>
> So the minimum supply airflow must be greater than the exhaust airflow at
> each timestep.
>
> You can do this easily by ensuring that the VAV box max design airflow
> multiplied by the VAV box minimum airflow fraction is greater than the
> exhaust fan airflow.
>
> Regards,
> Paul
>
>
> On 13 July 2010 14:30, Vinay Devanathan <vinay.devanathan at gmail.com>wrote:
>
>> Paul,
>>
>> Thanks for your reply. I understand that the supply airflow should be
>> equal or greater than the exhaust airflow. But, the design airflow from
>> auto-sized runs give me higher CFM than my exhaust fan CFM. But, the problem
>> maybe, it is not higher always. Is there any method using which I can
>> simulate my Zone Exhaust fan operation proportional to the supply fan
>> operation so that the exhaust fan doesn't pull out a constant CFM?
>>
>> Thank you.
>>
>> Regards,
>> Vinay
>>
>>
>> On Tue, Jul 13, 2010 at 4:21 AM, Paul Raftery <p.raftery at gmail.com>wrote:
>>
>>> Hi Vinay,
>>>
>>> You are getting that error because EnergyPlus does not currently take
>>> exhaust fans into account when autosizing zone airflows. In your case,
>>> EnergyPlus sized the supply airflow for that zone lower than the exhaust
>>> flow rate, and that's why you are getting that error.
>>>
>>> To fix this, you can 'force' the supply airflow to be equal to or larger
>>> than the exhaust airflow at all timesteps. You can do this by setting the
>>> autosize value in the airflow parameter for the air terminal unit object for
>>> that zone equal to the exhaust airflow. For example, where you are using
>>> constant volume reheat:
>>>
>>> !-   ===========  ALL OBJECTS IN CLASS:
>>> AIRTERMINAL:SINGLEDUCT:CONSTANTVOLUME:REHEAT ===========
>>>
>>> AirTerminal:SingleDuct:ConstantVolume:Reheat,
>>>     ObjectName,                        !- Name
>>>     Schedule,                        !- Availability Schedule Name
>>>     OutletNode,                        !- Air Outlet Node Name
>>>     InletNode,                        !- Air Inlet Node Name
>>>     autosize *(change this to exhaust airflow)*,
>>>  !- Maximum Air Flow Rate {m3/s}
>>>     HWInletNode,                        !- Hot Water or Steam Inlet Node
>>> Name
>>>     RHCoilType,                        !- Reheat Coil Object Type
>>>     RHCoilName,                        !- Reheat Coil Name
>>>     autosize,                        !- Maximum Hot Water or Steam Flow
>>> Rate {m3/s}
>>>     0,                        !- Minimum Hot Water or Steam Flow Rate
>>> {m3/s}
>>>     0.001;                   !- Convergence Tolerance
>>>
>>> Regards,
>>> Paul
>>>
>>> On 12 July 2010 16:46, Vinay Devanathan <vinay.devanathan at gmail.com>wrote:
>>>
>>>> Hello All,
>>>>
>>>> I am using EnergyPlus to model a building according to ASHRAE App. G for
>>>> baseline case. I included Zone exhaust fans in the baseline as advised, in a
>>>> fashion similar to what is prescribed in the proposed design. But, when I
>>>> auto-size the model with this Zone Exhaust Fan object, I get a warning
>>>> message saying -
>>>>
>>>> **   ~~~   **   there is unbalanced exhaust air flow.
>>>> **   ~~~   **   Unless there is balancing infiltration / ventilation air
>>>> flow, this will result in
>>>> **   ~~~   **   load due to induced outdoor air being neglected in the
>>>> simulation.
>>>>
>>>> And, I think the design loads are increased due to this, which would
>>>> probably cause over-sizing.
>>>>
>>>> Any help in solving this issue would be greatly appreciated.
>>>>
>>>> Thank you.
>>>>
>>>> Regards,
>>>> Vinay
>>>>
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>>>>
>>>
>>>
>>> --
>>> Regards,
>>> Paul
>>>
>>>
>>> =====================
>>> Contact Details
>>> =====================
>>> Paul Raftery, BEng(Hons) (Mech), Fulbright Fellow, IRCSET Scholar
>>> Informatics Research Unit for Sustainable Engineering (IRUSE) Galway,
>>> Department of Civil Engineering,
>>> National University of Ireland, Galway,
>>> University Road,
>>> Galway,
>>> Ireland.
>>>
>>> Landline: +353 91 49 3086
>>> Mobile: +353 85 125 4999
>>> Skype: praftery
>>>
>>
>>
>
>
> --
> Regards,
> Paul
>
>
> =====================
> Contact Details
> =====================
> Paul Raftery, BEng(Hons) (Mech), Fulbright Fellow, IRCSET Scholar
> Informatics Research Unit for Sustainable Engineering (IRUSE) Galway,
> Department of Civil Engineering,
> National University of Ireland, Galway,
> University Road,
> Galway,
> Ireland.
>
> Landline: +353 91 49 3086
> Mobile: +353 85 125 4999
> Skype: praftery
>
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