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ASHRAE 90.1 2007 peak cooling load

asked 2016-01-13 21:53:57 -0500

Chandan Sharma gravatar image

updated 2016-01-14 09:04:37 -0500

For a recent study, we were asked to calculate peak hourly cooling and heating loads for each of the 272 ASHRAE 90.1 2007 reference files (download from here). Peak cooling load square feet/Ton values reported by E+ were significantly higher than ASHRAE prescribed limits.

The three different approaches which were attempted were as follows:

a) First, the following output variable was added to each of the 272 files.

Output:Variable,*,Zone Predicted Sensible Load to Setpoint Heat Transfer Rate,hourly; !- HVAC Average [W]

Then based on whether the above output variable was positive or negative in eplusout.csv file for each zone, they were added accordingly to calculate total cooling load and total heating load. This gave peak cooling SF/Ton values which very off from ASHRAE limits. In the attachment, E+ reported values are in yellow cells. (Climate Zone 5A: Chicago) C:\fakepath\BuilsingEngChecks.png

b) Then the following two variables were considered but they also did not make much of difference and didn’t bring the results closer to ASHRAE limits.

Output:Variable,*,Zone List Sensible Heating Rate,hourly; !- HVAC Average [W]   
Output:Variable,*,Zone List Sensible Cooling Rate,hourly; !- HVAC Average [W]

c) Third and final approach was to report the following variables for and add them.

Output:Variable,*,Cooling Coil Total Cooling Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Heating Coil Air Heating Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Heating Coil Heating Rate,hourly; !- HVAC Average [W]

This significantly improved the results but still not all the files were within ASHRAE limits.

Also, we tried to add IdealLoad HVAC in a few files which made some more improvement to approach c) above but then decided not to take this route as this takes away the reference nature of the files. Since these were ASHRAE 90.1 reference files, it is puzzling as to why the results are off? Are we missing something here ?

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@Chandan Sharma Can you provide a reference for the ASHRAE checks? Why is the low check higher than the high check?

Neal Kruis gravatar imageNeal Kruis ( 2016-01-14 10:41:52 -0500 )edit

Interesting. I would check how setup/setback affect the results (i.e., use constant thermostat set point temperatures). I don't know how the ASHRAE low/high limits were identified. Also, be very careful about using data when the zone temperature is not maintained, the load then would include what is required to bring the space back to set point (i.e., try summing data only when the zone is at set point temp +- some offset).

rraustad gravatar imagerraustad ( 2016-01-14 11:12:06 -0500 )edit

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answered 2016-01-15 10:30:56 -0500

Archmage gravatar image

I would guess you want to compare the installed capacities, not the loads occurring during operation. Sum up the primary cooling and heating equipment size results instead of those output variables.

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Thank you all for the help. We are looking into some of these pointers and will get back with the update.

Chandan Sharma gravatar imageChandan Sharma ( 2016-01-15 11:37:05 -0500 )edit

He's not guessing.

rraustad gravatar imagerraustad ( 2016-01-15 21:35:12 -0500 )edit

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Asked: 2016-01-13 21:53:57 -0500

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Last updated: Jan 15 '16