Complex sentence: Reaching such level of efficiency, however, would require a lower flue gas temperature than most oil burners can produce, causing condensation that most oil-fired furnaces cannot handle without damage to the heat exchanger, venting pipes or outside casing of the appliance.
Split into simpler sentences: Reaching such level of efficiency, however, would require a lower flue gas temperature than most oil burners can produce. Therefore, causing condensation that most oil-fired furnaces cannot handle without damage to the heat exchanger, venting pipes or outside casing of the appliance.