What can be causing unmet cooling hours in IESVE, system 9c?

asked 2020-06-29 09:01:30 -0500

713 ●2 ●9

I'm having an issue with resolving unmet cooling hours on a proposed model. I've assigned all spaces individually to their own layer on multiplex of system 9c and autosized the system. I've checked cooling coil capacities, air flow rates, etc (per unmet hours guidance doc), and they are reflecting autosized values during unmet hours. However, the LAT from the cooling is not maintained during occupied hours soon after occupied hours begin. The LAT indeed drops to the specified 55F when the system first comes on at the beginning of the day, but then drifts up to 60F, as if the problematic coils were undersized. I verified that the weather data for load calcs was correctly loaded. Are there IESVE users that might offer suggestions on potential cause of this?

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answered 2020-06-29 09:49:21 -0500

crduggin

800 ●4 ●5

There are lots of potential issues. Here are a few unique to that system type though. 9c is the DOAS with WLHP system, so you have a system for ventilation and a system inside the multiplex for space heating/cooling. First make sure your DOAS doesn't have something set wrong that would cause it to provide too hot of ventilation air to your zones adding load that the WLHP were not sized to handle. You can check this through system parameters and on the component controls themselves and verify in VistaPro results. Next, it sounds like you are already looking at not being able to maintain LAT from the cooling coils, so go upstream and check out the settings for the heat transfer loop. If it isn't providing proper CW to the WLHP, they won't be able to do their job even at the right autos-sized capacity.

I would also just double check the setpoints on the heating and cooling coil and airflow controls, due to a previous version bug. Make sure the cooling ones are set to action with high input ON. Make sure heating is the opposite. Make sure cooling is linking the cooling setpoints and midbands correct and vice versa.

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For airflow controls, are you referring to controller MC4 should be set to On Continuously?

keb ( 2020-06-29 10:27:25 -0500 )edit

The max supply temp target in the HTL is 86 which appears to be met. I'm aware of that previous version bug. FP2 appears to have resolved that. The air coming off the DOAS cooling coil is 50F if humidity controls are active, and 60F if they aren't active (troubleshooting experiment). In either case, on one of the most problematic spaces, the air enters the WLHP at 74 or a bit higher, after mixing with return air. Then is only cooled to 65F instead of 55F.

keb ( 2020-06-29 11:45:59 -0500 )edit

I would dive a bit deeper into the WLHP component results to understand why the coil isn't able to achieve the LAT. I start by comparing the target LAT to the actual LAT to first confirm the ApHVAC coil control is telling it to do what i expect. Then is the capacity maxed out.

crduggin ( 2020-06-29 17:51:29 -0500 )edit

Yes I thought of that as well. Target and actual loop temp are nearly one line visualized on the time series plot, fluctuating between 86F and 88F. Coil is maxed out at a magnitude of ~ rated capacity * the oversize factor. Loop deltaT is between 8.6F and 10F.

keb ( 2020-06-30 07:09:40 -0500 )edit

I meant coil target LAT not CW loop temperature.

crduggin ( 2020-06-30 09:11:03 -0500 )edit

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What can be causing unmet cooling hours in IESVE, system 9c?

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What can be causing unmet cooling hours in IESVE, system 9c?