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[EnergyPlus_Support] COP of AirLoopHVAC:UnitaryHeatPump:AirToAir



I am modeling System 4 - PSZ-HP according to the definition of Annex G of ASHRAE 90.1-2007. In excel I calculated the COP as ratio between "DX Coil Total Heating Rate" and "DX Heating Coil Electric Power" and I plotted the graph (outdoor air temperature ; COP) that you can see here:
http://imageshack.us/f/442/copg.jpg/
(when the "DX Heating Coil Electric Power" is equal to 0 I imposed the COP equal to 0)
In the object AirLoopHVAC:UnitaryHeatPump:AirToAir the field "Maximum Outdoor Dry-Bulb Temperature for Supplemental Heater Operation" is equal to 4°C. I think that is the reason why the COP decreases between 3°C and 5°C: the electric heating coil switches off and the DX coil has to heat the supply air to a greater temperature.
The COP decreases also when the outside temperatures are greater than 10°C. I don't understand why.
Any comment, please?
Best Regards
Francesco Passerini



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