Sentence:  One of the major factors contributing to high particulate emissions from domestic burners is on-off cycling. ... <WSD>Underfiring</WSD> consists of using a smaller (undersized) unit which has to stay on longer and, therefore, cycles less. However, no data are available on the effectiveness, cost, and public acceptability (more complex system or inability to heat house as comfortably during very cold periods) of these proposals. 
Meaning of the word marked with <WSD> tags:  To intentionally operate a boiler, furnace, oven, "etc.", at a low level.