#1424 CBECC-Com Issue - WSHP and FPFC

[Felicia Lee]

Version = 2013_3c_760

Category = HVAC

Issue Summary: The WSHP and 4-pipe fan coil unit discharge air temperature control inputs in CBECC do not affect the actual DAT in the energyplus models.

List the steps taken to produce the error. For example:

  1. step 1 - open the file
  2. step 2 - enter 85F in the Heating SAT
  3. step 3 – When reviewing the hourly reports, the DAT at the WSHP goes up to 100F. For the FPFC model (and in the baseline) we have DAT that typically go up to 85F, which results in unexpected fan savings for the WSHP system.

Is there a way to adjust the DAT in the WSHP and FPFC models?

[Nikhil Kapur]

9/30 DR - The user's specification of the design supply air temperatures has little impact on the model since all proposed components are hard-sized. It is mostly for reference; it does not currently impact the control of zone system supply air temperature.

For simple zone systems (FPFC, WSHP, etc), the DAT is a function of coils sizes and the air flow across them; E+ calculate the resulting DAT in each simulation step based on how those components are internally controlled to meet the zone demand.

The design supply air temp is the value one would expect off the coil at the design condition, not necessarily the off-design conditions that the annual simulation accounts for. Is the plot below for the design day, or for 7/27 in the annual simulation?

[Nikhil Kapur]

9/30 User - The plot I sent was from the hourly reports of the annual simulation.
The main problem I have is that the WSHP seems to show a lot more fan energy savings than it should.
By undersizing the heating coil in the WSHP I was able to get lower DAT and higher fan power, which makes sense based on what you mentioned below. But it still seems to be an unfair comparison between the WSHP and baseline FPFC system as the discharge air temperature for the FPFC should be able to be similar to that of the WSHP system. Any other thoughts?

[Nikhil Kapur]

10/2 User - I still don’t quite understand why there is such a big difference in the discharge air temperatures between the different systems. By undersizing the heating coil in the WSHP in the idf file, I was able to artificially reduce the discharge air temperature in heating, but it still seems like the baseline FPFC typically has a heating DAT that peaks around 82F, causing higher fan power (and the same thing in cooling with higher DAT than for the WSHP). Any insight on this?

I updated the plots to show a heating day with the detailed output as you suggested:

[Nikhil Kapur]

10/5 User - We have been trying to use the SZVAVAC system but it seems to have very limited control options, with a fixed heating airflow setpoint (which by default gets set to the fan minimum airflow) and what seems to be a “temperature first” reset.
Are there currently no other control options for SZVAV systems in CBECC-COM?

Currently when using the SZVAVAC with the uncontrolled terminal unit in CBECC-Com, it gets translated with the “AirTerminal:SingleDuct:VAV:NoReheat” component which does not have the capacity to reset the heating airflow.
Couldn’t the air terminal be “AirTerminal:SingleDuct:HeatandCool:NoReheat” to be able to have a reset on the heating side?

Instead of using the SZVAVAC system, we have also been using the standard VAV system with a more aggressive SAT reset to avoid reheating, but it also doesn’t quite model a proper SZVAV system.

10/15 DR - We are aware that the heating airflow does not reset up per the control scheme described in the NACM. We had done some limited evaluations of different control strategies (see issue 677). I tested a few combinations of controllers/terminal types, and found that the current combination we are using (VAV:NoReheat + SPM:SZNoReheat with limits) was the closest I could get to matching the control logic described in the NACM.

When I was testing this, I did briefly experiment with AirTerminal:SingleDuct:HeatandCool:NoReheat terminal, but found that configuration didn't work. I can't find the output data from testing this combination, so if you happen to try it again yourself and find it is a better representation of the SZVAV control strategy, we'd welcome that feedback.

With regards to the reset of air flow during max heating? Do you have an application where this is necessary to meeting annual heating loads?

[David Reddy]

This issue was not resolved, though it may ultimately be a limitation of E+. I need more time to test to confirm.

[Nikhil Kapur]

Closing Issue - see new Issue 1970