Why is autosizing leading to undersized system?

asked 2022-12-13 15:29:00 -0500

updated 2023-04-06 21:44:50 -0500

I am stymied. My auto-sized system is coming out grossly undersized (which is strange, since it almost always comes out oversized). I know this topic has been covered here.

Both the Component Load Summaries (AirLoop and Zone) agree. But then, System Sizing is showing half of what is required. Plus, why does this show mixed air at 32°F for cooling on summer design day?

Please, can anyone help?: image description

The output variables for the sizing runs even show the system is using significantly more cooling than what the System Sizing Information is reporting: image description

The Zone Component Load Summary and Airloop Component Load Summary appear to agree on the correct value.

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answered 2022-12-13 15:52:07 -0500

rraustad

13599 ●72 ●12 http://www.fsec.ucf.edu/

updated 2022-12-14 12:33:45 -0500

What E+ version is this? The mixed air temp isn't 32 F, it's 0 C which means it was not calculated. Obviously with a 72 F inside/98 F outside condition the mixed air temp can't be 32 F, so it must not have been calculated by the program. Also obvious is with a 32 F mixed air temp and 52 F supply air temp the system cooling coil should show 0 W. So something else is happening. I can't provide more insight without more info. Can you attach the idf?

Update: In the Coil Sizing Details report the "coil" inlet air temperature is reported as 77.89 F. So there is an issue with reporting the air loop mixed air temperature for this (or other) configuration(s). Since this is air loop equipment the coil/system load is not necessarily equal to the zone load because of the outdoor air component, and heat gain to system return, the target system supply air temperature, etc.

Does zone TZ:02-FCU-33-TERRACE 219 control the zone temperature to the thermostat set point temperature?

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OpenStudio Application 1.4.0 EnergyPlus 22.1.0 here is the idf: https://drive.google.com/file/d/1am9k...

Ski90Moo ( 2022-12-14 00:06:22 -0500 )edit

I was able to run your file and provide additional information (Update in the answer above) based on that review.

rraustad ( 2022-12-14 12:34:55 -0500 )edit

Yes, TZ:02-FCU-33-TERRACE 219 controls zone temperature to the thermostat setpoint temperature. This is load calculations only, so the thermostat is constant 70 for heating and 72 for cooling. Since it is an air loop, the air loop sizing should be larger than the zone load, especially with outside air latent cooling (in San Antonio), right?

Ski90Moo ( 2022-12-15 13:28:46 -0500 )edit

Well, you're right, an air loop cooling coil will be larger if the mixed air enthalpy is higher than the zone. I did not check the zone air flow vs air loop air flow to see if they were the same but I suspect they are since this is a single zone air loop. Bottom line is I haven't found a reason yet for the coil size difference.

rraustad ( 2022-12-15 13:43:42 -0500 )edit

The reason I asked if the zone temperature was maintained was that if with this system the zone temperature is held at the set point then the cooling coil size does not need to be larger. I would expect if this cooling coil is undersized then the zone temperature will not be maintained at the peak condition. This information is helpful in figuring this out. If you run this simulation on the design days you would see if the zone temp is held at set point.

rraustad ( 2022-12-15 13:51:56 -0500 )edit

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Why is autosizing leading to undersized system?

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Why is autosizing leading to undersized system?