Improving the thermal efficiency of balanced flue gas space heaters currently marketed in Argentina

Abstract

The most widely used home heating appliances in Argentina are the balanced flue gas space heaters. The reasons for this choice may be found in their affordable cost and in the comfortable and secure indoor environment generated (i.e., indoor air is not employed for combustion while flue gas is discharged outside). However, the units that are currently marketed have an important disadvantage: their relatively low thermal efficiency. Measurements made in our laboratory for a balanced flue gas space heater with independent inlet (for the fresh air) and outlet (for the flue gas) ducts, yielded values of around 73% for the maximum power (2.9 kW), while for the minimum power (1 kW) are reduced to 61%. Those values are directly related to the outlet temperatures of the flue gas stream of about 400 °C and 240 °C for maximum and minimum powers, respectively. In this context, it is of major interest to develop and implement alternatives to improve the thermal efficiency of such type of devices. In a previous contribution we proposed to employ a heat exchanger (designed ad-hoc) to take advantage of the relatively high enthalpy of the flue gas to preheat the fresh inlet air. This alternative was implemented in a balanced flue gas heater and encouraging results were reached, as the discharge temperature of the flue gas stream could be considerable reduced, but certain limitations arose, which are revisited in this article. The aim of this contribution is to present the advances in the design and manufacture of a new heat exchanger applicable to a different type of balanced flue gas heaters. The results obtained in the test-runs are reported and analyzed. It is anticipated that a remarkable improvement in the thermal efficiency was achieved.