I am currently working to create an energy model for an addition to be made to a hospital and am unsure how to go about modelling the heating system. New air handling units provide ventilation to the addition, however the boiler is outside of the LEED system boundary. Is it acceptable to exclude the existing building from an energy model of the addition? If so, what is the method of approximating the efficiency of such a system? If not, is the boiler to be modeled as a scaled down version of the existing boiler or a full sized model with an estimated "dummy load" applied to approximate the existing building?
A full building model may be required due to the boiler's use in both the existing building and the addition, although the addition will use new air handling units. I am unsure if this constitutes a failure to satisfy condition b of ASHRAE 90.1 G3.1. Also, the local electric utility uses a declining block power rate, not meeting condition d of ASHRAE 90.1 G3.1.
The addition in question only represents 15-20% of the hospital's floor area, meaning if the complete building must be modeled the opportunity for energy savings is limited.