Sentence:  Damaged or clogged oil piping between the tank and the furnace will result in reduced fuel flow to the burner. This can cause burner <WSD>underfiring</WSD>, which is identified by: / Reduced heating capacity / High oxygen readings during combustion test / Low stack or flue temperature / Low temperature rise (temperature differential) across the heat exchanger ... 
Meaning of the word marked with <WSD> tags:  The state of not burning fuel at the desired level, and thus not providing heat efficiently.