Hi,

I have visited a house recently. The house was built in 1963. The house use a oil furnace with forced air. I've noticed that the furnace has two exhaust pipes that are exiting through the basement wall. Around one of the exhaust pipe there is a fair amount of efflorescence.

Do you have some idea what could be the cause of this? I didn't find any other efflorescence except at this specific spot.

Thanks for your help!

I assume your talking about the wall not the pipe. If so the reason for this is moisture. Are you sure its an oil unit. As they must be vented out through a steel or masonry chimney do the high exhaust temps they create I know of no oil units that utilize a side vent and makeup air system.
If its a gas furnace two pipes indicate a Condensing furnace which does just that, condense the water vapor out of the flue gasses. If the condensate is dripping onto or within the wall cavity its causing the salts within the wall to come to the surface thus the deposits your looking at. I would check to see that the installer of the furnace did not join the PVC pipes within the wall cavity as they may be leaking. If the condensate is blowing back onto the wall from the outside then they did not properly terminate on the outside. This simply should not happen. It needs to be corrected regardless of what the reason for the condensate getting onto or within the wall is. Condensate is corrosive. It contains an acid that can corrode lots of stuff. Thus the reason for the efflorescence on the wall.
Does the unit have a fresh air makeup system on it. Some energy efficient homes utilize these air to air makeup systems. If its malfunctioning it to can cause condensation to develop on the pipe and again the moisture dissolves the salts on the wall causing the effect your seeing.