"Stubby" wrote

I was quoted $3000 to replace a 100,000 BTU Burnham boiler. But the
plumber advised me that boilers don't wear out with use. Nor does the
efficiency go down. The fuel savings will never pay for itself in my
lifetime.

That is partially true. Boilers can crack and leak, fireboxes can collapse,
but most do not "wear out". If the one you have is working and not
nickle-and-diming you to death. Let 'er go til she IS broke....