# Re: [EnergyPlus_Support] odd autosizing results

```In a nutshell the sizing logic is as follows:

1.  Based on the ZONE SIZING supply temperatures, determine the loads and
therefore the flow rates required to meet the zone heating and cooling loads.

2.  Using the max of the flow rates for required by the zone, calculate the
peak heating and cooling coil loads required to take the design day mixed air
temperature and bring it to the specified heating/cooling supply temperature as
specified in the SYSTEM SIZING object.  Also, if your sizing factor in SIZING
PARAMETERS is not 1.0, this multiplier will be added.

2b.  When using DX equipment, there is an assumed ratio of airflow to capacity
used, so, the flow rate or capacity may be increased to maintain a valid ratio.

3.  Why did you specify flow/system?  This essentially overrides the loads
determined by zone sizing and forces a flow rate on the system.

Mike

On 5 May 2004, at 10:42, Ian Doebber wrote:

> I have some odd results using the HVAC autosizing feature for a furnace and
> central air conditioner system for my residence and am curious how Energy Plus
> is handling this situation.  I am autosizing everything I can for both heating
> and cooling but specifying a flow/system of 700 cfm.  The zone sizing output
> spreadsheet shows that I need 11,925 W heating and 4,049 W cooling.  Yet the EIO
> file shows that I need 7,542 W heating with 1100 cfm and 4,802 W cooling with
> 590 cfm.  First question is that my EIO file shows that I need lower heating
> (7,542 W < 11,925 W) since it calculated it with a higher flow rate (1100 cfm >
> 700 cfm) and similarly a higher cooling (4,802 W > 4,409 W) since it calculated
> it with a lower flow rate (590 cfm < 700 cfm.  Is this correct.
>
> My second question concerns how then Energy Plus sizes my equipment.  The EIO
> file states that with a 700 cfm flow rate, I will have 9,629 W heating and 6,040
> W cooling.  This doesn't make sense.  My heating capacity then does not meet my
> maximum zone load of 11,925 W and cooling capacity then is too big for my
> maximum zone load of 4,049 W.  Where is Energy Plus calculating my equipment
> capacities from.
>
> Thanks
> Ian Doebber
> Virginia Tech
>
> [Non-text portions of this message have been removed]
>
>
>
> 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.
>
>
>
>
>
>

========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx

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