Modelling enclosure losses from a packaged or built-up RTU
I need to model the impact of increased enclosure insulation on the energy used by a roof top unit. I was planning making a small zone with the an exterior boundary condition wall having UA equal to that of the purposed RTU and using that space as a supply plenum. Depending on the size of unit and the location of the coils, I would also perhaps do the same with a return plenum to capture any gain/losses driven by the return air delta T.
Assuming I can come up with a reasonable UA approximation, would this work? Is there a better way.
Concerns I have about this approach
- Appropriately modelling the inside convective heat transfer
- Accounting for the variation of air temperature as it move though the RTU / estimating the area of enclosure that is subject to the return or supply air temperatures (or even something in between like the air between the fan and heating coil on a blow through unit).
UPDATE
Applying the above approach to enclosure losses before the coils (via dummy return air plenum) fails to account for the mixed air temperature after outdoor air has been introduced. For this I'm hoping that i can use an EMS program to calculate the mixed air temperature accounting for jacket losses and then reset the air temperature at the coil inlet node to this calculated value.
Concerns I have about this approach
- accurately calculating the losses/gains at the mixing box
- not sure if it is possible to just reset the air temperature at the coil inlet node using EMS
Using EnergyPlus, currently 9.0 but could update if necessary.
Any advice on how best to model this would be appreciated.
I would also be open to modelling losses external to EnergyPlus, exporting variables that would be key drivers of enclosure losses and calculating that way, but keeping it all internal to EnergyPlus would be preferably.