## 2 Materials and Methods  
### Experimental Stand and Measurements  
The experimental stand--Figure 1 is located in the Centre of the Department of Thermal Sciences, part of the Technical University of Civil Engineering Bucharest, Romania.  
The stand consists of a programmable and completely automated test rig with integrated software, which can assure real-time data acquisition and parameters calculation, measurements and evaluations of all operational parameters, energy and mass flows, environmental impact, and complete flue gases analysis.  
In order to monitor the flue gases temperature, the quality of combustion, and also air temperatures and humidity in the endowment of the laboratory, a portable gas analyzer and portable thermo-anemometers (temperature range \(-20\)-\(60\)\({}^{\circ}\)C, precision \(\pm 0.1\)\({}^{\circ}\)C) are used. The water volumetric flow is measured with propeller flow meters (precision \(\pm 1\)% of the measured value)--Figure 2.  
For the current investigation, two water-tube boiler constructive solutions were chosen, one with corrugated stainless-steel pipes and the other with aluminum finned pipes.  
The two solutions were chosen based on the fact that they meet all the necessary criteria for carrying out this study:  
* Are representative of a wide range of market-available condensing boilers constructive solutions for the household [14; 15; 16], having a thermal power range between 10 and 35 kW as stand-alone units, or up to 300 kW in packings. It is important to mention that the derived solutions from the studied main ones are very likely to behave the same as the ones analyzed. The only constructive solution that was not approached in the article was the fire-tube boiler. The reason is that it is rarely used in small thermal  
Figure 1: Experimental stand.  
Figure 2: Main sensors and measuring device part of the experimental stand.  
power condensing boilers, being mostly a solution employed for separate condensing units or medium and high thermal loads condensing boilers.
* The computational programs used for the numerical modelling are already validated [1; 16] and so there is no danger of inappropriate modelling of heat and mass transfer phenomena; it is well known that the condensation process of water vapor from the flue gases to the pipe wall is very complex to model, especially when the heat transfer occurs at the same time (mass transfer with temperature variation in the limit layer, and species variation along the process); the temperature differences between the limit layer dew point and condensing surfaces tend to an average of 1 to 5 \({}^{\circ}\)C in the last condensing sections of the flue gases, just before exiting the boiler.
* The fact that this equipment is already present in most households in Romania makes the modification of gaseous fuel by H\({}_{2}\) mixing in the CH\({}_{4}\) more important, because the changes in flue gas content and thermal performance can then be extrapolated to obtain an important assessment at a national level.  
The testing procedure was carried out according to European Standard 15502-1:2021 for which the Laboratory is accredited. In these types of testing, according to the Standard in question, the data volume is not important. What matters is to perform the measurements in a steady-state regime. Moreover, upon reaching the steady-state operation regime, 2 data sets have to be recorded. Each data set is recorded for a fixed interval of 10 min, during which time the monitored parameters (water and flue gas temperatures and flow, and flue gas concentration) must not fluctuate by more than \(\pm\)0.5%. Considering the fact that sampling frequency of the measured data is 1 s, a total of 1200 measurement data pairs for each one of the boilers was recorded. The data used in this paper are the average of every measured parameter.