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The coefficients for the curve define the curve completely so I'm not sure what the nominal thermal efficiency field is used for.

From the I/O Reference, the 'Normalized Boiler Efficiency Curve' describes the normalized heating efficiency (as a fraction of nominal thermal efficiency) of the boiler’s burner.

The nominal thermal efficiency is the equipment efficiency at full load and design outlet temperature. The output of the efficiency curve modifies the nominal efficiency based on the runtime (part load, temperature) conditions.

My advice would be: in lieu of more complete data about your system, use the curve that best fits the data that you have.

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The coefficients for the curve define the curve completely so I'm not sure what the nominal thermal efficiency field is used for.

From the I/O Reference, the I/O Reference, the 'Normalized Boiler Efficiency Curve' Curve' describes the normalized heating efficiency (as a fraction of nominal thermal efficiency) of the boiler’s burner.

The nominal thermal efficiency is the equipment efficiency at full load and design outlet temperature. The output of the efficiency curve modifies the nominal efficiency based on the runtime (part load, temperature) conditions.

My advice would be: in lieu of more complete data about your system, use the curve that best fits the data that you have.