OpenStudio Cooling Load is Zero

asked 2021-05-26 14:14:30 -0500

updated 2021-05-27 12:50:32 -0500

I'm running into this problem with a more complex OpenStudio model, but I've replicated the problem in a simple one-zone model. As far as HVAC goes, all I've done is make an air handling unit with two fans and water coils connected to district heating and district cooling. Its temperature is managed by a scheduled setpoint manager set a constant 12 C. The zone is served by the AHU with a VAV reheat. The zone's cooling setpoint is 21 C and the zone's heating setpoint is 20 C. The model runs, but only shows results for heating; no cooling. Perhaps there is a setting somewhere preventing the building from turning on cooling?

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I would look at the water coil heating and cooling energy variables (COIL SPECIFIC) (look in the .rdd file to find a list of the ones you may need). Check if the water coil cooling energy is 0, since this may indicate that there is no cooling energy from the coil to the air in the air loop. In other words, the water coil is never extracting any heat energy from the air.

The water coil is connected to a plant loop on the demand side, serving an air loop on the supply side to condition air before delivering it to the zone.

sashadf1 ( 2021-05-27 08:28:54 -0500 )edit

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answered 2021-05-27 09:37:53 -0500

codybond

371 ●8 ●6 http://www.bractlet.com/

If you're autosizing the coil parameters, check your EIO file to check the capacity of your cooling coils.

If the capacity of your cooling coil is = 0, then you either have a problem with your zone loads (actual loads like people/lighting and thermostat design temps) or with your sizing parameters (Sizing:Zone/Sizing:System).

If the capacity of your cooling coil is > 0, then you have some sort of operational problem. Is your AHU making setpoint, or is it just blowing hot air because the cooling coil is not coming on?

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Hi, I know this is 2 years from the original post, but I am having a similar problem where my cooling coil capacities are 0. Could you elaborate on how you can trouble shoot the issues with the "zone loads" or with "sizing parameters" in order to fix this issue?

Thanks for your time.

vgmvp02 ( 2023-07-27 14:19:05 -0500 )edit

I'm not exactly sure where to find this in OS, but in general each coil is autosized based on the parameters you feed it via the Sizing:System table. The most notable in this case being: Central Cooling Design Supply Air Temperature and Central Cooling Design Supply Air Humidity Ratio. These two values set the enthalpy_out for each coil. If you've set up a scenario where the enthalpy_in (to the coil) is < the enthalpy_out then the coil will size to 0 W because there is no cooling to be done. This is all assuming you are autosizing coil capacity.

codybond ( 2023-07-28 11:09:20 -0500 )edit

So what sets the enthalpy_in? That's happening mainly in the zones that are connected to the air loop. If you have not added any load/heat into zones connected to this air loop via people/equipment/lighting/windows/etc, then enthalpy_in will be < enthalpy_out and coil size will go to 0 W.

codybond ( 2023-07-28 11:11:44 -0500 )edit

Hi Cody, I'm trying to set up a way to model server heat dissipation and air conditioners nearby for a datacenter. I have checked the .eio file and found out that my 4 cooling coils each have cooling capacities greater than 0. For some reason, under the HVAC Load Profiles Tab, I do not see any heating or cooling loads. I've modelled the server heat dissipation using Electric Equipment Definitions in the Loads Tab. I've also made sure to input those Definitions in the Spaces Tab under 'Loads'. What else can I do to troubleshoot?

vgmvp02 ( 2023-08-09 16:00:49 -0500 )edit

Hmm, this sounds more like an OpenStudio inputs issue, which I'm not very well versed in. You might want to open up a separate ticket and include some screenshots of what you're seeing. There are a lot of OpenStudio experts on Unmet, so I'm sure you'll get it cleared up quickly.

codybond ( 2023-08-10 13:06:21 -0500 )edit

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answered 2021-06-02 11:17:10 -0500

Scott Crowner
41 ●3

Figured it out. I needed to hard-size the input and output air humidity ratios. Otherwise, the system makes a bad assumption and sets the cooling load to zero, as found in the error file.