The original: a 40 (or so) year old Bryant gas steam boiler rated near 390,000 BTU input. This poor thing had a leak inside causing steam to spray out. We could hear it and saw the steam come out the chimney. The customer let it go until the leak got so bad that water would drip on the burners and cause the pilot to go out. The soot on the front is from CO rolling out due to the crud on the burners. Finally the homeowners realized the boiler was unsafe and let us replace it.
The efficient replacement rated at over 80% in annual efficiency. It took 2 guys most of a day to break up and remove the old boiler from the basement. 5 of us hauled this one down in 1 piece. It's wider but shorter than the old one. Two installers spent 2 days doing all of the piping including gas then the last day was wiring, filling, flushing & refilling and getting everything perfect and all safeties tested. At the end of the 4th day, all radiation was hot and the boiler was whispering away.
We installed a 245,000 BTU input boiler while others quoted much larger boilers basing their choice only on the size of the old boiler. Our choice was to properly size the output of the boiler to match the radiation & piping load which required measuring all radiators & baseboards and use charts to determine the effective output. Oversizing a steam boiler, like any other HVAC product, is bad causing short cycles and higher fuel bills.