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VRF indoor unit sizing issues with DOAS?

asked 2021-09-03 09:39:37 -0500

Greg Estep's avatar

updated 2022-02-21 20:47:37 -0500

Greetings Community, Note, this is just a sizing issue and not an hourly simulation problem.

I am a Trace 3D+ user and have uncovered some problems with the sizing of VRF indoor units when connected to a Multi System DOAS (OA delivered to back of VRF unit and mixing with RA) and just wanted to see if anybody else has noticed this. Trane says that the issue is with the energy plus engine and not their reporting.
The issues show themselves in the Trace 3D+ system component summary report in the airflow conditions at design table. For the cooling coil, the entering coil temperatures and humidity are a product of the mixed air (OA and RA), where the OA (delivered from the DOAS system) is the DOAS supply air dry bulb temperature used for sizing (53F in my case) and not the actual conditions I control to, which is a dehumidify and reheat control. This is causing under sizing. I am dehumidifying to 0.0087lb/lb humidity ratio and reheating to 65F. The energy simulation is working correctly, but the sizing is not.

For the heating coil sizing of the VRF indoor unit, the problem is also with the airflow conditions at design table. Here the coil is sized using the RA (room temp) and not showing any mixing with OA at all. This is also causing under sizing.

Has anybody else noticed this? I don't think this is too impactful for energy analysis, but it does impact the capacity of the equipment being used to meet all loads and it's corresponding energy efficiency.

I also think the same problem is occurring with zone-level WSHPs connected to Multi Sys DOAS. Best, Greg Estep

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I suspect the VRF TU is not accounting for DOAS air when sizing the coil. A work-around would be to look at the eio and hardsize the coil in question to overcome the sizing issue. I will look at the VRF TU coil sizing and should know more soon.

rraustad's avatar rraustad  ( 2021-09-03 10:50:54 -0500 )edit

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answered 2021-09-03 15:59:06 -0500

The VRF coils do appear to be sizing correctly as far as using the correct inlet temperatures. For the heating coil, the coil inlet temp/humrat are not reported correctly to the coil sizing summary. Regarding the undersized coil issue, do you have a plot of zone temperature and heating coil part-load ratio to show where the coil is undersized?

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I have a zone cooling peak with outside conditions of 83F and 83.7 gr/lb. My dehum and reheat DOAS should dehumidify to 61 gr/lb and reheat to 70F. The entering coil airflow conditions for this zone are 62.4F and 57.69 gr/lb (OA% = 66%). It just seems like the sizing calculation is not including the reheat. Perhaps I don't understand the "cool reheat" dehumidification control type and it is only reheating back to the DOAS cooling coil sizing drybulb supply air temperature (which is set for 53F).

Greg Estep's avatar Greg Estep  ( 2021-09-07 07:56:56 -0500 )edit

The reheat coil will only heat back to the heating set point temperature to avoid over heating. Is this what you are seeing? If not, the input file showing the problem will be needed for further evaluation.

rraustad's avatar rraustad  ( 2021-09-08 09:22:14 -0500 )edit

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Asked: 2021-09-03 09:39:37 -0500

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Last updated: Sep 03 '21