it's a single pass /steel boiler, i am sure the "inside", water area, of the heat exchanger is getting corroded,, no matter how well it's clean /adjusted its always on the high side
stoker-man wrote:That's the problem with cast iron. It's greatest efficiency is when it's new. After the first and subsequent firings, the pores fill with soot and it's never as efficient again. You can clean the passageways, but not the pores.
You're a little high on the stack temp, but 350 is considered good for an unlined chimney.