I am trying to model an air-to-water heat pump for heating by using performance curves. I have found this useful link that shows standard performance curves for heat pumps:
http://www.comnet.org/mgp/content/676-heating-systems?purpose=0#heat-pump-capacity-adjustment-curves-(CAP-FT)
If you look at the section "Electric Heat Pump Heating Efficiency Adjustment Curve" you will see that the parameter EIR_FT is function of two variables: Toadb and Tdb. Toadb is the outdoor air dr-bulb temperature, but unfortunately I cannot understand what Tdb actually refers to. In the description it says Tdb: The entering coil dry-bulb temperature.