chimney was rebuilt 5 years ago. There is efflorescence on the outside and the plaster will not hold on the inside. Took the plaster off the wall to watch it and now there is efflorescence on the inside wall. The house is 96 years old. This flue is used for the boiler and hot water tank. It is fall and the heat has not been turned on. The cast iron cleanout is rusted through on this flue. The flue for the woodburner doesn't have problems and the clean out is not rusted. Thank you.
this sounds like a water problem wither out side rain/snow or maybe from a blocked or over sized GAS heating/hot water flue (if you have that). I would have a local Certified Chimney Sweep www.csia.org, (there you can look up by zip code to find one near you) take a first hand look at what you have they should be able to help.