Oftentimes people don't want to get involved with replacing a boiler because it seems like such a tremendous effort and amount of money.They're right it does take a good amount of effort and it does take a good amount of money but it might make fiscal sense in the long run. Thankfully there's a fairly easy way to determine if replacing your boiler is a wise decision. First we need to find your fuel consumption for the year. Most average residential homes will burn through 2-3 tanks of oil in the winter and maybe another tank in the summer if the boiler has a hot water coil or an indirect water heater. So 4 tanks of oil x 275 gallons equals 1100 gallons of oil consumed. Next multiply the annual consumption by the average price of oil in your area. Oil in my area is at about $2.15 gallon at the moment. 1100 gallons of oil multiplied by $2.15 a gallon = $2365 annually to heat my house.  Your house may use less or more than this but try to get a general idea of your annual consumption. Next we have to find the Efficiency difference between the old boiler and the new boiler. The really old converted coal boilers will usually run in the low 60's in terms of efficiency. These boilers are easy to pick out because they're generally covered in asbestos and they usually have 2 massive doors on the front of the boiler. The poor efficiency is mostly due to the fact that the boiler absorbs heat very poorly and were generally over-sized for the house. Newer boilers sometimes have 1-3 passes before the gases exit the chimney and have a large number of protrusions that enable better heat transfer. Unfortunately these protrusions are also great at grabbing soot which is why it's important to clean your boiler regularly. Boilers installed from 1960-1980 will  usually run in the low to mid 70's depending on the boiler and condition. New oil boilers come in at about 84-85% efficient and some new gas boilers can top 98% efficiency. For our example we'll say we have a old converted coal boiler and are upgrading to a new 85% efficient oil boiler. So 85% minus roughly 62% efficiency gives us an increase in efficiency of 23%. To get out dollar savings a year we multiply out annual cost; $2365 by .23 and we get a savings of $543.95 A YEAR. Over the 30 year life of a boiler you'd be saving almost $15,000! If the cost of oil or gas goes up which is quite probable your savings goes up even more. 

Even if you do the math out and you're not saving a ton of money there are several other factors that can should be considered before making a decision. Older boilers often have a number of code violations that might have to be fixed before selling a home and sometimes it's more worthwhile to just replace the whole boiler. Sometimes boilers, like cars, can get to the point where the repairs are becoming too numerous and too costly and it just makes more fiscal sense to replace it. There are some other costs that can be associated with replacing a boiler that can run into some money. The two big ones are replacing an aging oil tank and lining the chimney. It's never a good idea to feed sludgy oil into a new boiler and if your chimney isn't lined when installing a new boiler a liner is required by law. Use the information I've given you and you should be able to easily make an informed decision about keeping or replacing your old heating system.