Baseline Sizing

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Last post

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

Vinay Devanathan's picture
Joined: 2011-10-02
Reputation: 0

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

Paul Raftery's picture
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Joined: 2011-10-02
Reputation: 0

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

Vinay Devanathan's picture
Joined: 2011-10-02
Reputation: 0

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

Paul Raftery's picture
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Joined: 2011-10-02
Reputation: 0

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

Vinay Devanathan's picture
Joined: 2011-10-02
Reputation: 0

Hi Vinay,

No problem. Would you mind describing the overall system that you would like
to model?

For example, is it a single zone system where the exhaust air flow varies
proportionally to the supply flow rate (as you mentioned before)? If so
model the exhaust fan as a return fan and set the outdoor air economiser to
NoEconomiser.

Regards,
Paul

Paul Raftery's picture
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Joined: 2011-10-02
Reputation: 0

Paul,

Yes, it is a single zone system. As such, I do not have an OA Economizer.
But, if I already have a return fan, should I just give that fan the CFM of
Zone Exhaust Fan also? Then again, it would result in the same problem
right?

On a different note, if a proposed design has a return fan and is Multizone
system and the baseline is requiring a single zone system. Do I include a
return fan for a each single zone system?

Thank you so much.

Regards,
Vinay

Vinay Devanathan's picture
Joined: 2011-10-02
Reputation: 0