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Fan electricity postprocessing Ideal Loads Air System results

asked 2020-05-17 06:26:47 -0600

EmanueleL's avatar

How to determine the total Fan energy through postprocessing the Outdoor and Supply Air Standard Density Flow Rates in output from a zone simulation with Ideal Air Loads?

The outputs that I'm referring to are:

  • HVAC,Average,Zone Ideal Loads Outdoor Air Standard Density Volume Flow Rate [m3/s]
  • HVAC,Average,Zone Ideal Loads Supply Air Standard Density Volume Flow Rate [m3/s]

I thought that running the simulation to get hourly values for the flow rate I could then input these values into a simple equation to obtain the fan power: fanPressureRise*fanFlowRate/fanEfficiency

(fanPressureRise and fanEfficiency being constant values)

I'm working with Grasshopper Honeybee to run the EnergyPlus simulation. I’ve done a quick test, but I’m not sure my assumptions are correct. Would the total fan power be: FanSupply + FanOutdoor + FanExtract


  • FanSupply = Fan energy obtained postprocessing the Ideal Loads Supply Air Standard Density Volume Flow Rate
  • FanOutdoor = Fan energy obtained postprocessing the Ideal Loads Outdoor Air Standard Density Volume Flow Rate
  • FanExtract = same values of FanOutdoor

This forum doesn't allow me to upload files yet, but I've posted screenshots and Honeybee script of the test in the Ladybug forum as well, here the link: link text

I've been advised to post the question in Unmet, hope you can help me. Many thanks.

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answered 2020-05-19 18:01:55 -0600

Hi, in general your assumptions are right. May you want remember that in Detailed energyplus simulations Fan consumption refers to actual air density, so the equation fanPressureRise*fanFlowRate/fanEfficiency using Standard Air Densiry will give you not the exact fan consumption, but a very close value (depending on the climate). But what I'm asking is (that could affect the results more than the previous consideration):

  • Supply and Outdoor air are via two diffeernt systems (e.g. FCU + DOAS), or could be in a single AHU?
  • Both cases have impact on DeltaPressure hypotesis, that directly affect the resuslts

Regards, Alessandro

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Hi Alessandro,

Thanks for the reply. What I'm trying to determine is a rough fan electricity number to be added up to the total energy use intensity, at concept stage. That's why I was looking at simulate the model using ideal air loads instead of modelling a specific hvac system. The only downside is that ideal air loads doesn't have fan electricity as output, so I was looking at post-processing the airflow results, assigning constant values to fan Pressure and fanEfficiency.

Do you know what typical pressure and efficiency values could be used for a residential setup e.g. with MHRV?

EmanueleL's avatar EmanueleL  ( 2020-05-20 04:58:59 -0600 )edit

If the Outdoor Air System instead was a separate system, without heating and cooling included, what would a typical design pressure and efficiency be?

EmanueleL's avatar EmanueleL  ( 2020-05-20 05:11:17 -0600 )edit

Usually MHRV cannot meet the total Clg/Htg loads of a dwelling ; it works only on the fresh air load contribution. But obviously it depends on your envelope building (WWR/orientation/U-values, ..); in a nZEB the remaining loads could be effectively a residual part.

Anyway reference values could be:

  • for MHRV: fan_eff 0.65; DP 1100 Pa in order to have in between 40-50 W/(m3/h)

  • for FCU: fan_eff 0.65, DP 300 Pa, in order to have around 0,15 W/(m3/h)


Ag's avatar Ag  ( 2020-05-20 12:35:04 -0600 )edit

Ok thanks, So, because the airflow in output from the simulation is a list of variable airflows I think I would have to determine the pressure of the system at each point. Reading this link: link text I understood that the relationship between DP and airflow(Q) can be determined as DP = R * Q2, where R is a constant for the ventilation system setup ...Following this I could then determine the R of a typical MHRV and an FCU using the values that you gave me, and use this to determine the power (P) of the system with: P = (R * Q2) * Q / effic

EmanueleL's avatar EmanueleL  ( 2020-05-21 04:31:26 -0600 )edit

Does this makes sense? What I've also read is that the efficiency of the system varies with a variable airflow, so a constant efficiency would be an approximation again, correct?

EmanueleL's avatar EmanueleL  ( 2020-05-21 04:39:38 -0600 )edit

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Last updated: May 19 '20