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Why using warmest sp manager undersize cooling capacity for VAV reheat system?

asked 2018-10-04 12:08:28 -0500

RRPENGE's avatar

updated 2018-10-15 19:11:52 -0500

Hi All,

I am a new user of openstudio. I am trying to model a 90.1 baseline for a hotel using a central VAV system with electric parallel fan box reheat. But I am getting too much over heating. I use the warmest setpoint to in my air loop. However when I plot the warmest zone, I see that the autosize capacity cannot lower the air temperature to setpoint level. I also checked that the reheat is not active as well. I thought using the warmest set point manager would resolve this. Why is the autosizing algorithm under sizing the cooling capacity? Thanks

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answered 2018-10-04 20:26:05 -0500

The set point manager of choice does not influence the size of the cooling (or heating) coil. I would check the information in the Sizing:Zone (used to size zone air flow) and Sizing:System (used to size capacity) objects. If reheat is not active in a zone I would check the zone temperature with respect to the zone thermostat temperature to see if reheat is actually needed. If you need help reporting this information then simply ask for that additional help.

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Thank you for your feedback. I should have been more clear about the reheat. I meant the reheat coils was not the cause of the high temperature. When I changed "Type of Load to Size on" tab in the loop detail into from "VentilationRequirement" to "Total" (guess it means latent and sensible heat), the unmet hours dropped by half for obvious reasons. But they are still in the thousands. I also change design day so it falls on the weekend (highest load), without much significant change in unmet hours.

RRPENGE's avatar RRPENGE  ( 2018-10-05 11:15:52 -0500 )edit

There could be several reasons why unmet hours occur. Is the HVAC system available when occupancy starts? or before occupancy starts? If the HVAC system is available is the system able to meet the thermostat set point? I would plot these to help diagnose what is happening:

Output:Variable,*,Zone Thermostat Air Temperature,hourly;
Output:Variable,*,Zone Thermostat Heating Setpoint Temperature,hourly;
Output:Variable,*,Zone Thermostat Cooling Setpoint Temperature,hourly;
Output:Variable,*,Zone Heating(Cooling) Setpoint Not Met Time,hourly;
rraustad's avatar rraustad  ( 2018-10-05 20:27:24 -0500 )edit

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Asked: 2018-10-04 12:08:28 -0500

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Last updated: Oct 04 '18