Biomass combustion systems contribute to the use of renewables, whereby wood log stoves have a high share for domestic heating. Promotion of renewables and efforts for lower air pollution require an improvement of technology. Short-term effects can be achieved with the help of various retrofit systems. Apart from a survey of available systems, two measurements have been analyzed: additional heat exchange and emission reduction by catalysis. During normal operation of three low-cost stoves, high levels of carbon monoxide (CO), volatile organic compounds (VOCs), and particulate matter (PM) concentration have been observed, having a major influence of the operator's behavior. The best case scenario with the retrofit systems resulted in a significant efficiency increase and as well a reduction of CO, VOCs, and PM.