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Hydronic systems are treated nearly identical to standard systems. The difference is that a hydronic system does not have a furnace, so you will not be able to use the heating output to calculate the nominal system airflow in order to establish a duct leakage target.
If it is a hydronic heat-only system then you will have to use "Measured Airflow" for the duct leakage calculations.
If air conditioning is present, you could also use the nominal cooling airflow (based on A/C condenser capacity).
See article on "Measured vs. Nominal"