Avail. Manager Hybrid Ventilation - High Fan Energy Use

asked 2021-11-05 14:23:06 -0500

Elisa Matar's avatar

updated 2022-02-23 14:11:19 -0500

Hi everyone!

I'm using Energy Plus to simulate a large office building with hybrid ventilation. For this, I'm using the object Availability Manager: Hybrid Ventilation. For modelling natural ventialtion I'm using AirFlow Network and my HVAC systems are ASHRAE 90.1 System Type 8 (VAV with parallel fan-powered boxes and reheat).

Before implementing the hybrid system, I've tested the building with natural ventilation only and during most of the occupied periods along the year, the office spaces manage to fulfill ASHRAE 55 adaptive comfort requirements with natural ventilation only (the building is located in the south of Brazil).

However, after mixing natural ventilation and HVAC, I'm getting pretty much the same energy use as before: even though I decrease my cooling energy use (and a small decrease, less than I expected from that previous natural-ventilation-only simulation), I'm getting higher fan energy use and I started to wonder if this is due to constantly turning the fans on and off with the Availability Manager somehow.

I am using control type number 5 for hybrid ventilation ("Operative temperature control using adaptive comfort 80% acceptability limits"). I used the output "Availability Manager Hybrid Ventilation Control Status" and it was set as 1 (natural ventilation allowed) during those periods I had identified as comfortable, so I guess the adaptive comfort criteria is working well.

I have also tested control type 1 and also got higher fan energy use.

When plotting the control status output for both cases I get something like this, which shows a very unstable change from natural ventialtion to HVAC.

*Blue = 0 (availability manager off, during unoocupied periods, when I only use cycling HVAC), Yellow = 1 (natural ventilation on) and Red = 2 (HVAC on)

image description

I would like to know if there are any settings that I'm missing and that might be causing this.

Thank you!

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I was checking the annual chart for my fan energy results for some specific zone fans and I realized that even thought the fans were indeed turning off when natural ventilation was on, the peak values when they turn on, for shorter periods of time, is higher, just as I thought. I'm still wandering why is this happening and if there are any settings to prevent this.

Elisa Matar's avatar Elisa Matar  ( 2021-11-08 11:12:51 -0500 )edit