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Unitary system not meeting cooling load

asked 2015-09-08 12:47:06 -0500

dpud12's avatar

updated 2017-08-05 13:29:20 -0500

I have used an OpenStudio measure that I wrote to set the hvac system for corridor zones in a multifamily apartment building. The measure sets these zones to be conditioned by an air loop with a unitary system that has a water source heat pump and electric resistance backup. As shown in the picture attached, the system is able to meet the heating setpoint of 21C throughout the winter, but is not able to maintain the cooling setpoint of 24C in the summer, even though I have the availability set to On constantly. When I change the hvac system to the standard Packaged Rooftop Unit, the zone loads are met and the temperature is maintained within the proper window, so I know that it is an issue with the HVAC system. Any ideas why the cooling load is not met? C:\fakepath\tempgraph.JPG

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answered 2015-09-08 13:57:36 -0500

updated 2015-09-08 13:58:36 -0500

The figure does show that the cooling thermostat set point is maintained, albeit for only a few hours. This is indicative of insufficient cooling capacity to meet the load. Check this report variable to be sure the system has enough capacity. If the system coil cannot meet the load, the RTF will equal 1 when the zone thermostat temperature set point is exceeded.

Output:Variable,*,Cooling Coil Runtime Fraction,hourly; !- HVAC Average []
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Thanks @rraustad This was indeed the problem, as the sizing method for the system was Sensible, rather than Total, and there was a high ventilation load in the space. One thing to note, Cooling Coil Runtime Fraction is not currently in the list of OpenStudio output variables, however, Cooling Coil Part Load Ratio is.

dpud12's avatar dpud12  ( 2015-09-08 14:35:01 -0500 )edit

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Asked: 2015-09-08 12:47:06 -0500

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Last updated: Sep 08 '15