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Fan Energy Consumption too high

asked 2015-01-23 02:47:37 -0500

MattStewart gravatar image

updated 2015-01-25 19:44:20 -0500

I have been working on smaller buildings that do not require Outdoor Air and HVAC systems do not run continually.

After setting thermostats and applying a fractional schedule to OA I am still getting Fan consumption at 1/3 of my building energy consumption.

This consumption remains fairly steady throughout the year. In the shoulder seasons there is almost zero heating or cooling so why is the fan running? Should it not follow the HVAC equipment? How do you suggest I debug?

I believe I will forgo the template PTAC and build an air loop with no OA.

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answered 2015-01-23 10:27:35 -0500

updated 2015-01-24 08:50:21 -0500

Short Answer:

PTAC that cycles the fan on during heating/cooling load, and does not provide OA:

  1. Use a PTAC (ZoneHVAC:PackagedTerminalAirConditioner in EnergyPlus) with a Fan:OnOff so that the fan can cycle.
  2. Zero-out the fields Outdoor Air Flow Rate During Cooling Operation, Outdoor Air Flow Rate During Heating Operation, and Outdoor Air Flow Rate When No Cooling or Heating is Needed so that the PTAC will never provide OA.

  3. In the field Supply Air Fan Operating Mode Schedule, put a schedule that with a value of 0 (zero) for the whole year to tell the fan to cycle on only when heating or cooling is needed.

Background:

The EnergyPlus documentation for the ZoneHVAC:PackagedTerminalAirConditioner can be found here. In particular, these two fields are the key:

Field: Supply Air Fan Object Type This alpha field defines the type of fan used by this PTAC. The only valid choices are Fan:OnOff and Fan:ConstantVolume. A fan of type Fan:OnOff is used with cycling fan operating mode, and a fan of type> Fan:ConstantVolume is used with continuous fan operating mode (see Supply Air Fan Operating Mode Schedule field below). The input requirements for these fan objects are described elsewhere in this document.

and

Field: Supply Air Fan Operating Mode Schedule Name This alpha field specifies the name of the supply air fan operating mode schedule. The supply air fan operating mode may vary during the simulation based on time-of-day or with a change of season. Schedule values of 0 denote that the supply air fan and the heating or cooling coil cycle on and off together to meet the heating or cooling load (a.k.a. AUTO fan). Schedule values other than 0 denote that the supply fan runs continuously while the heating or cooling coil cycles to meet the load.

In OpenStudio:

The PTAC currently in the OpenStudio library has a Fan:ConstantVolume in it, so it will not cycle. I have filed an issue so that we will add a cycling PTAC to the library for 1.7.0. In the meantime, as a workaround, you can manually add it to your library like this:

  1. Close OpenStudio
  2. Go to C:\Program Files\OpenStudio 1.6.0\share\openstudio-1.6.0\OSApp\hvaclibrary' and make a backup copy of 'hvac_library.osm in case you mess it up in the next step.
  3. Open hvac_library.osm in a text editor (like Notepad, etc.)
  4. Copy/paste the following text into the bottom of hvac_library.osm, then save the file.
    • Make sure to only copy/paste the text once.
    • In 1.5.0, the resulting file should be 7106 lines long, in 1.6.0 it should be 7135 lines long.
    • Since the file lives in /Program Files/, you may need to have admin rights to save changes.
  5. When you re-open OpenStudio, you should see a PTAC called "Cycling PTAC..." in the Library. It will be setup to cycle. (You'll still need to zero-out the OA fields if you ...
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answered 2015-01-23 07:58:15 -0500

OA schedules do not apply to PTACs in EnergyPlus, they have three inputs for a fixed OA flow rate during heating, cooling, and no-load operation. The PTAC templates should work just fine. Check the Supply Fan Operating Mode Schedule on the PTAC units. This schedule controls the fan operation: 0 = cycling fan, 1 = continuous fan.

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Comments

See aparker's answer for the OpenStudio explanation.

MJWitte gravatar image MJWitte  ( 2015-01-23 13:06:57 -0500 )edit

The final answer is the in.idf file since this is what EnergyPlus reads. In that file, see ZoneHVAC:PackagedTerminalAirConditioner, "Supply Air Fan Operating Mode Schedule Name", then find the Schedule:* object that defines that schedule. See other answers to this question for the OpenStudio details.

MJWitte gravatar image MJWitte  ( 2015-01-25 19:47:22 -0500 )edit

Mike, the in.idf file does not contain the object ZoneHVAC:PackagedTerminalAirConditioner. Please confirm as these objects were clearly selected in the OS GUI and should be present. The model seems to run fine and I have set parameters such as COP and fan efficiency. I only see cooling coils and heating coils. Is it a user fault (me) or a OS quirk: link textlink text

MattStewart gravatar image MattStewart  ( 2015-01-25 22:13:56 -0500 )edit

Are you able to open the linked files? Thank you,

Program flow must have changed. ZoneHVAC:PackagedTerminalAirConditioner, Supply Air Fan Operating Mode Schedule Name, Fan:OnOff do not exist in the in.idf. They are located in OSApp\hvaclibrary\hvac_library.osm.
OS:Fan:OnOff, {767b3df3-668f-4617-a401-010ddc2aa5b2}, !- Handle Cycling PTAC Fan, !- Name {9f54092d-a4a8-41b8-a381-c4c332ecb843}, !- Availability Schedule Name 0.6, !- Fan Efficiency 250, !- Pressure Rise {Pa} autosize,

MattStewart gravatar image MattStewart  ( 2015-01-25 22:14:33 -0500 )edit

@MattStewart That idf file has a built-up AirloopHVAC system in it. It will never cycle the fan with those components. You need a ZoneHVAC PTAC or PTHP, or an AirloopHVAC:Unitary.... equipment to get the cycling fan option. Simply adding a Fan:OnOff to this will not help.

MJWitte gravatar image MJWitte  ( 2015-01-27 14:18:40 -0500 )edit

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Asked: 2015-01-23 02:47:37 -0500

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Last updated: Jan 25 '15