There are many advantages to an indirectly heated boiler, but other than the "pot in a pot" technique, they are difficult and expensive to build. You might be able to find a jacketed cooking vessel at a used restaurant supply store, and that would be the best way to create a true indirect heating system. Vessels with heating coils are generally too large to consider for a small-scale boiler.
This method is used extensively in industry, because a single boiler can heat many pieces of equipment. Industrial applications usually use steam as the heat transfer medium, because it is very efficient. Steam can be heated to very high temperatures and its own pressure will move it around the system, while hot water systems require a separate pump.
Whatever the advantages, steam heating should NEVER be attempted by the amateur! STEAM IS DANGEROUS. Because of the dangers of steam, pressurized boilers require licensing and regular inspections, and still manage to explode and kill people on a regular basis.
If you want to try this technique - and it can offer significant safety advantages if you are heating with a flame - then use hot water for the heat transfer medium. NEVER operate a pressurized system.
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