Simulation based on ON and OFF cycles of the HVAC.

asked 2022-11-15 16:45:29 -0600

Kaycee_fresh's avatar

updated 2022-11-16 10:57:32 -0600

I am trying to simulate the ON and OFF cycles of the HVAC so that i can ascertain its heating and cooling time (time without the HVAC) over a 24 hour simulation. For this simulation i have considered a 10 degrees dead band temperature, 30 degrees setpoint temperature, 0 degrees outdoor dry bulb temperature, 45 degrees supply air temperature from the VAV terminal system. Also, only heating has been considered, hence i turned off the cooling coils in the model. My problem is that when i look at the output of the zonal air/mean temperatures(please see west zone), the results do no not seem to corroborate expected results based on practical experiments in a real building scenario.

For one, the HVAC heats up the zone(west zone ) very quickly and the temperature of the zone decreases sharply at OFF time (without the HVAC). For a building without cracks and openings( fully closed building), though it is expected that the building will heat up quickly but i am not very sure why the zone air/mean temperatures decrease sharply too. Please can anyone give me a logical reason why i have this type of result?

Second, observe at ON Time(HVAC), the zonal temperature quickly rises and stabilizes(Please see attached plot). Is there a form of PID Control going on? I am not sure why this happens. In reality, it is expected that the zonal temperature would gradually increase until it reaches the setpoint temperature. Is there a clear explanation for the sharp increase?(Please atached photo) Thanks as always. image descriptionI have attached the IDF file and weather file herein:

Please any help would be greatly appreciated.

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EDIT: Sorry, I misunderstood the question.

shorowit's avatar shorowit  ( 2022-11-15 19:35:43 -0600 )edit

Hi @shorowit , Thanks for your reply, but the links you directed me to don't seem to answer my questions. I have already implemented the ON and OFF Cycles using EMS, but I need some explanation regarding the behavior of the system. You can take a look at my IDF file to get some clarity. For this application, i have set the number of timesteps to 60(does this have any effect on the simulation results?).

Kaycee_fresh's avatar Kaycee_fresh  ( 2022-11-16 03:27:15 -0600 )edit

I only see the weather file on the google drive. Have you checked that the heating coil is fully on during the on period? The zone temp decay can be impacted by many things in the simulation such as ground heat transfer (see Site:GroundTemperature:BuildingSurface), zone air capacitance (ZoneCapacitanceMultiplier:ResearchSpecial), zone internal thermal storage (InternalMass), infiltration (ZoneInfiltration:DesignFlowRate), etc. I agree the rise/decay seems rather sharp. Are you bringing in OA with the air loop? What is SupplyT when coil is off? Are you reporting the amount of zone infiltration?

rraustad's avatar rraustad  ( 2022-11-16 07:50:55 -0600 )edit

@rraustad Oh my bad, I have updated the link accordingly. Please see link

Kaycee_fresh's avatar Kaycee_fresh  ( 2022-11-16 10:58:27 -0600 )edit

@Kaycee_fresh the google drive should include the idf file

rraustad's avatar rraustad  ( 2022-11-16 11:07:03 -0600 )edit