EIRDefrost function for air-to-water heat pump

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I haven't tried to create the EIRDefrost curves and there is very little information documenting HP defrost operation. I can say that there is a frost layer on the outdoor coil so it really doesn't matter what the outdoor condition is during defrost because that outdoor air temperature is not the outdoor coil exterior condition. The frost layer insulates the coil from the outdoor condition. However, the outdoor dry-bulb and wet-bulb temperature does affect how much of a frost layer is present when defrost starts. That said, I don't see how any variable other than indoor WBT would be relevant (an indication of suction pressure/temperature) to EIR since the outdoor operating condition is fixed (outdoor coil is inside a frost layer and compressor head pressure would also be fixed). I suspect that the EIRDefrost curve is relatively flat wrt outdoor conditions since when the HP switches to cooling mode the outdoor coil operating condition is fixed and indoor WB temperature would be naturally low in winter but do not have any measured data to confirm that suspicion.