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I'm currently working on an EnergyPlus model that simulates run-around loop heat recovery in a laboratory building. The system recovers heat from the lab exhaust stream, which is assumed to run at constant flow. I've created a plant loop (with propylene glycol solution fluid properties), served by district cooling and heating energy sources and connected to cold and hot water coils on the AHU. To adjust the available capacity at varying outside conditions, I'm controlling the loop setpoint temperature with an EMS routine that sets the fluid temperature based on outdoor air temp along a curve calculated from the design effectiveness (using the same equation as air-to-air hx sensible effectiveness). The routine also enables or disables the pump above/below OAT setpoints. Zone exhaust fans account for the exhaust energy, the loop pump accounts for the pumping energy, and the 'district' energy will be neglected in post-processing as recovered.

I'm not sure if this is the 'best' method, but in theory it seems to capture the available capacity between the exhaust air temperature and outdoor air temperature. It's still a work in progress, but I will update this answer as I ensure that everything is working properly.