Efficiency curve

The efficiency of the boiler that I’m modeling has to respect Table 6.8.1F of ASHRAE 90.1:2007. More precisely, the thermal efficiency Et shall be equal to 80% according to the test procedure 10 CGR Part 431. The boiler of the baseline model doesn't correspond to a real boiler, but to a reference boiler, therefore I’m wondering whether modeling the thermal efficiency of the baseline model as constant would be acceptable. If I have to use an efficiency curve, how should that curve be? I mean, should the thermal efficiency increase with the boiler part-load ratio? Should the curve be function of the water temperature? With Regards