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zone air temperature in cooling mode is the same that heating

asked 2020-04-09 04:50:43 -0500

lorenzo.belussi's avatar

updated 2020-04-10 11:55:13 -0500

Good morning! I'm simulating a building with several thermal zones with the Simple HVAC options of DesignBuilder. The HVAC is the default CAV system of DB templates. In the activity Tab I set the heating and cooling setpoint and setback temperature: 20°C for heating setpoint and 16°C for heating setback; 26°C for cooling setpoint and 30°C for cooling setback. I also set the heating and cooling schedules: from october to april for heating and from may to september for cooling. However, when I display the Internal temperature in the Comfort results, its value in a typical summer week is much lower (about 22°C) than the cooling temperature set at 26°C. Can anyone help me with some suggestions? Thank you very much

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answered 2020-04-10 04:38:04 -0500

Dave's avatar

DesignBuilder licences include technical support, so I'd advise you to send your model to the Support Desk:

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answered 2020-04-11 07:19:24 -0500

lorenzo.belussi's avatar

Dear all. thank you very much for you suggestions and I apologize because in the first post I talked about VAV instead of CAV. In particular I have a single duct CAV with reheat. The building is located in Milan and has significant internal gains (lighting, equipment, occupancy…) and there aren't internal airflows between cooled zones or they are very low. I attach the results of the temperature in a typical summer week with daily outside temperature higher than 30°C.

As you can see the internal temperature is lower than 22°C. If I simulate a VAV system the internal temperature is 26°C according to the schedule. I have check all the input of the system and seem correct. It’s like the cooling air of the CAV system was too cold and was not controlled by the activity schedule.

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answered 2020-04-10 10:58:24 -0500

updated 2020-04-10 12:36:25 -0500

CAV systems use either constant or cycling fan mode. For cycling fan mode the system should maintain the zone thermostat temperature in the cooling and heating season. For constant fan mode, the zone temperature will be impacted by the operation of the fan when no cooling or heating is needed, and the outside air system, if used.

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answered 2020-04-10 10:49:41 -0500

updated 2020-04-10 10:50:28 -0500

Some points on wich you may want to think about:

  • Are you sure to have significant internal load (expecially if your building is very massive and with high solar refelectance for opaque structures and/or with high SHGC for openings)?
  • Do you have great ACH (where is located your building?) or internal airflows between cooled zones ?



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answered 2020-04-13 07:33:44 -0500

Filipe Sousa's avatar

DesignBuilder Simple HVAC model option uses the EnergyPlus ZoneHVAC:IdealLoadsAirSystem method. This supplies hot/cold air to meet heating and cooling loads.

I would suggest to look at the System Loads graph to confirm if the HVAC system is working and producing cooling during these periods below the heating setpoint. This check should be done at zone level since you may have zones without HVAC that will also count to the building level results average.

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Asked: 2020-04-09 04:50:43 -0500

Seen: 311 times

Last updated: Apr 13 '20