Steam boilers are hard to kill- you could probably limp along with this one indefinitely. Changing the boiler won't reduce the amount of sediment, which is all iron oxides that come back with the return water from the inside of the radiators and plumbing.
Steam boilers need to be sized for the radiation for the system to work properly. Any time a replacement is contemplated it's worth it to
re-calculate the effective direct radiation (EDR) square footage of the entire system to avoid going too far astray on either the oversizing or undersizing.
A 30 year old steam boiler that was probably 78% AFUE on day 1 is probably still going to be over 70% today. You won't get better than 82% out of any residential steam boiler. If your current boiler is running 72% efficiency and you're spending $1800/year on gas for heating, replacing it with an 82% boiler means you'd then be spending 72/82 x $1800= $1580/year, a savings of $220/year. If you're currently spending less, your savings will be less. Unless the boiler is becoming unreliable, it's almost never worth replacing it- the money is usually better spent on tightening up the place with improved air sealing, insulation, and in some cases, better windows (or low-E storm windows). Even if the weatherization only yields the same fuel savings for the same expenditure, it provides a higher comfort factor, and has a longer lifecycle.
