Interesting Zone Temperatures

asked 2016-04-06 11:44:25 -0500

Dustin
1055 ●3 ●14

Hi all,

I am seeing some strange behaviour in a model I have created. Basically I am seeing a significant temperature increase right at the time my AHUs stop supplying fresh air. That being said I have my occupanct, lighting, and electrical equipment all going into setback an hour before I see the interesting temperature increase.

A screenshot of the results can be seen below, with the highlighted values being what is concerning me. image description

The setpoints to the right of the highlighted values are still active because in the building I am modelling the occupied and AHU OnOff schedule differ. The AHUs are on from 05:00-17:00 while the occupied setpoints are active from 07:00-21:00. It seems very strange that I see an increase in temperature at 17:15 when the only HVAC item that should be active during the unoccupied times are the radiant heaters that only heat the space if it get below the setback temperature, these would be active in unoccupied from 21:00 to 07:00.

I know this may be hard for someone to point me in the right direction but any help is greatly appreciated.

Thanks.

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@Dustin Do you have plenums? Or an attic?

Jeremy ( 2016-04-06 12:24:04 -0500 )edit

Hi Jeremy I do not have any plenums or attic spaces. Might be good to note that the rise in temperature at this time only occurs during the summer months.

Dustin ( 2016-04-06 12:41:09 -0500 )edit

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answered 2016-04-06 12:58:52 -0500

rraustad

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

updated 2016-04-06 13:00:16 -0500

It's not uncommon for the solar load to lag several hours behind the peak solar. It takes time for the solar energy to pass through the construction materials. HVAC loads are typically highest 2-3 hours after the sun has reached it's peak. I suspect the solar component is causing this result. It is also common to allow the HVAC systems to operate during occupied hours. If HVAC systems do not need to run they won't. You could switch between constant fan operating mode and cycling fan if that option is available with your HVAC system.

I can think of several ways to diagnose if solar is the culprit. One is to report the inside surface temperature of the zone walls to see if that temperature is higher than the zone temperature. You could also turn off solar and run the simulation again. I am not sure of the easiest way to do this, but I do know you can edit the weather file and lower the direct solar component or add cloud coverage to minimize the impact.

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Thanks @rraustad I definitely looked into this issue regarding the solar effects being the cause of my temperature changes. I do believe this is causing the issue but that being said, my model is becoming significantly overheated during the unoccupied times which is causing a lot of concern for me. I have tried multiple things but to no avail and am trying to work through the issues. Thanks and if you have any comments feel free. I appreciate the response.

Dustin ( 2016-04-08 13:03:01 -0500 )edit

Turning off the AC is not the answer. Allow the AC to operate as needed and then figure out ways to reduce HVAC energy. For example, daylighting, operable blinds, heat recovery with OA, shading, etc.

rraustad ( 2016-04-08 13:24:43 -0500 )edit

I am modelling an existing building that has been in operation for a couple of years now so this is where I am struggling. I understand when sizing a HVAC system you would define your schedules to ensure the building is being kept at a reasonable temperature. That being said where I am modelling a new building and fairly new to Energy Plus/OpenStudio is there an parameter in the model I may be missing that could be causing this to occur? Thanks again I appreciate the responses.

Dustin ( 2016-04-11 06:34:52 -0500 )edit

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