Amongst the various alternatives for hot water production for domestic use, the instantaneous variety is still widely used in many markets such as the Portuguese market. In this, a gas boiler converts the chemical energy of the gas (LPG, NG) to a water stream, as it is used. The complexity of such devices range from those with a natural convection to those with fully pre-mixing of the air-fuel. The tightening of the legislation about these appliances is promoting an increase on efficiency, pollutant emission reduction and an increase on the safety features. One possible alternative is to couple the widely tested and low cost venture like partial pre-mixers with a ventilator and calibrated orifices to better control de air-fuel mixture over a wide range of operating loads.

The purpose of this work was to test the thermal performance of a water heater prototype with 22 kW of nominal heat output, running on Propane. Changes were been made to the plate with orifices that limit the air supply to the burner flutes, where the pre-mixture with the fuel is partially made. Four different plates with different orifice diameters were built and tested in real case scenarios, taking into special consideration the pollution emission and the fuel consumption verified.

The experiments were carried out using a gas analyzer and a data acquisition system to monitor and collect the information, in order to verify the compliance of the 26:2000 standard, with regard to the efficiency and carbon monoxide emission (CO). For each plate, working exclusively with the appliance fan, the installation characteristic curve in three distinct scenarios was traced: i) with primary air ducts blocked; ii) with secondary air duct blocked; iii) free airflow.

From the results it was concluded that the best configuration in terms of efficiency is the original one, followed by the “-0.5 mm” and “+1 mm” plates. On the other hand, the best plate in terms of CO emission was the “-1 mm” plate. Concerning the plates with diameters superior to the manufacturer’s original configuration, flame instability was verified as a result of the greater primary airflow. Under the same test conditions it was noted that the introduction of a nozzle into the fan inlet conducted to the suction of a larger amount of air. Finally it was also concluded that the reduction of the orifice diameters of the plates, reduces the split of primary air, resulting in an increased pressure drop in the flutes and on the global pressure drop of the system. These results were validated analytically by the dynamic balance method.