## 4 Discussion  
The authors draw important conclusions from the detailed analysis of the data presented in comparative Tables 4 and 5. The gaseous fuel consumption must increase as a consequence of constant efficiency and calorific value decrease. This fact may be observed in the above-mentioned tables. The fuel flow increases by approx. 16% over the entire interval in order to maintain useful thermal power. The useful condensing thermal load increases along with the H\({}_{2}\) concentration; the total effect being around 5%. In spite of this, given that the condensing thermal load (latent thermal load) is less than 10% of the convective thermal load (sensible thermal load), the result is that a general influence over the total thermal load is less than 0.5%, which is negligible. As a consequence, the boiler total thermal load remains practically constant over the H\({}_{2}\) variation range. This trend also applies to the overall boiler efficiency due to the calculation method, which reports the total useful thermal load (including condensation) to the energy flow brought by the combustible flow (without condensation latent heat flow). As expected, the inlet water vapor flow and condensed water flow produced by the boilers increase. The condensation efficiency of the boilers remains practically constant, averaging 60%. CO\({}_{2}\) flow, the main target of the study, decreases by around 7%, and when H\({}_{2}\) is in mixture with CH\({}_{4}\), it reaches 20%. This is in accordance with studies from [3; 6; 7; 8; 9; 10], which also highlighted the lowering of CO\({}_{2}\) in flue gases. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Parameter** & **Boiler with Finned/Simple Pipes (Thermal Load with No Condensation 24.0 kW)** & **Parameter Variation (*) (\%)** \\ \hline H\({}_{2}\) Participation into the & 0 & 10 & 20 & +20 \\ Combustible [\%] & 0 & 10 & 20 & +20 \\ \hline Flue gas temperature at & & & & \\ chimney inlet section [\({}^{\circ}\)C] & 64.8 & 63.8 & 63.6 & 0 \\ \hline Boiler efficiency when not & & & & \\ in condensation & 97.7 & 97.7 & 97.7 & 0 \\ regime [\%] & & & & \\ \hline Gaseous fuel consumption & 2.499 & 2.686 & 2.903 & +16.2 \\ (hourly value) [Nm\({}^{2}\)/h] & & & & \\ \hline Thermal load with no & & & & \\ condensation [kW] (purely convective thermal load) & 24.00 & 24.00 & 24.00 & 0 \\ \hline Condensing (useful) & 1.746 & 1.792 & 1.845 & +5.67 \\ thermal load [kW] & 25.75 & 25.79 & 25.85 & -0 \\ \hline Boiler efficiency in & 103.6 & 103.8 & 104.1 & -0 \\ condensation regime [\%] & 4.32 & 4.41 & 4.50 & +4.2 \\ with the flue gases [kg/h] & & & & \\ \hline Useful debit of & 2.65 & 2.73 & 2.82 & +6.4 \\ condensate [kg/h] & & & & \\ \hline Condensing efficiency [\%] & & & & \\ (condensate from total & 61.6 & 62.1 & 62.6 & +1.6 \\ water vapor inlet) & & & & \\ \hline CO\({}_{2}\) mass flow in the flue gases [kg/h] & 4.91 & 4.75 & 4.56 & \(-7.07\) \\ \hline Annual CO\({}_{2}\) emissions for & & & & \\ standard functioning of & & & & \\ the boiler [ions] (estimated equivalent of 2800 h/year at nominal output) & & & & \\ \hline \hline \end{tabular} (*) calculated for the entire domain of H\({}_{2}\) concentration and referred to the initial value (Corresponding to H\({}_{2}\) = 0%); (**) variation in absolute value [kg/year].  
\end{table} 
Table 5: Functioning parameters of finned/simple pipes condensing boiler. The CO\({}_{2}\) emission reduction for the modelled boilers was calculated considering an average operation of the boiler for one year. The operating conditions were determined over a set (historical) functioning diagram describing the way the thermal load of the boiler (in percentage from nominal thermal load) is required over a heating season. We considered that the partial loads were in fact generated by partial time boiler's operation at a nominal thermal load. This fact is obviously not true, especially for a condensing boiler with a premix burner (achieving down to 15% of the nominal thermal load modulation), but there is no difference for the purpose of the estimation.