What will happen if there is a blockage in the chimney or flue passage?

When there is a blockage in the chimney or flue passage, the correct outcome is that the boiler room will become filled with smoke. This occurs because the flue is responsible for venting the exhaust gases produced during the combustion process away from the boiler. If there's an obstruction, the exhaust gases cannot escape properly and instead back up into the boiler room. This not only creates a hazardous environment due to the accumulation of smoke and potentially carbon monoxide but also indicates a serious issue that needs immediate attention to ensure safe operation.

The other choices do not accurately reflect the consequences of a blockage. Increased efficiency contradicts the nature of a blockage, which typically causes inefficient operation as the system struggles against the obstruction. Higher combustion temperatures are also unlikely, as poor venting would generally lead to incomplete combustion and potentially cooler overall temperatures in the combustion chamber. Lastly, less noise production does not follow from a blockage; instead, the inability of gases to escape can lead to increased noise from the boiler as it operates under distressing conditions.