A combustion process generating can not be used. A combustion process generating would be 25%  and that generating would be 33% efficient according to eq.3

GAH represents the heat available at the critical process temperature; it may not represent heat available to perform a given function due to the various types of losses. GAH may be used as a criterion for comparing different fuel-combustion systems.

Once the furnace is designed and built, the heat losses are not within the control of the operator; it is governed by the process critical temperature, refractory lining thickness and thermal conductivity of the refractory. Defining net available heat (NAH) as

NAH can be used as a criterion for comparing the smelting/melting/heating efficieny of different furnaces.