Thermal efficiency of natural gas balanced-flue space heaters: Measurements for commercial devices

The performance of the most common gas space heater used in residences in Argentina was experimentally investigated. The furnace is a simple device with a burner (ranging from 2 to 7 kW according to the model) set into a metal chamber, and this separated from the ambient by an enclosing cabinet. This type of individual gas heater is installed indoors with both inlet and outgases chimneys connected to the outside. For the same furnace, two chimney configurations are regularly provided: a vertical one that intakes and exhausts through the roof, and a horizontal one with short concentric tubes connected to outdoors through the wall. To assess the thermal efficiency of the equipment, we have measured several running parameters, like temperatures, airflows, and gas consumption. Thermal efficiencies for indoor heating in the range of 40–60% were found, for vertical and horizontal chimneys, respectively. The main design characteristics leading to the poor performance were identified: low emissivity of the combustion chamber surface (leading to poor radiation transfer), tight enclosing cabinet (affecting radiation and convection transfer), and no chimney flow control (leading to large outgas losses). The findings are in agreement with previous works on the excessive amount of natural gas consumed in residences in the area studied.