From Distillers Wiki
A boiler is a container in which a wash is heated to its boiling boiling point. The vapour then travels to the column or lyne arm. The characteristics of a good boiler is that it made out of a material that is strong and will not corrode(such as copper or stainless steel), it has a fairly good height to width ratio, and if external flame heating is used, the bottom of the boiler is thick enough to prevent scalding of any solid material in the wash.
Common items used as boilers include beer kegs, milk cans, water heaters / hot water cylinders, and stockpots with bowls clamped on top.
In order to make the rate of boiling more constant, boiling chips are often added to the boiler.
A boiler is also another name for a steam generator which is used to pipe steam through steam coils in order to heat a steam jacketed lauter tun or still - this is more common in commercial scale stills, however.
General Information and Essential Characteristics
The boiler is a sealed container that is filled with mash, beer, wash or low wines containing low quantities of alcohol by volume (5-40%). The boiler is heated, and the alcohols evaporate before other components of the wash. These vaporized alcohols rise up the column as vapors, which are then condensed and collected.
The essential characteristics of a good boiler are ruggedness and nonreactivity, because it is sometimes subjected to open flame, corrosive beer, and heavy charges. For those reasons selection of the materials and capacity for this component is very important. stainless steel and copper are the best materials to choose for boilers.
Boilers can be constructed by converting used large restaurant pots, stainless steel wash pails, bakers dough pans, used soda and beer kegs, old swimming pool filters, old copper hot water cylinders and a few other such things into a boiler. All are good candidates for the purpose, but some will require considerable modification and specialized welding in order to provide proper connections to the column, or a way to disassemble the apparatus for cleaning if you think it will also be used for running thick mashes (this is only likely to be a requirement for those wanting to run brandy or whisky "on the grain/pulp").
The boiler also needs to have enough room to hold the batches you intend to distill plus headroom to allow for foaming (about 20% for thin sugar washes, as much as 40% for foaming prone washes like rum or malt whisky), must be sufficiently durable to withstand the boiling and charging processes, and must not taint the distillate with any objectionable flavors or impurities. Stainless steel is an ideal material for use in a still boiler because it cleans easily, looks nice, and has great resistance to the effects of boiling corrosive liquids.
The most common boilers are made from Beer Kegs. Stainless steel beer kegs are more readily available than milk cans and water heaters, have handles, and come in various sizes, usually ranging from 7 to 15.5 gallons (25 - 55 litres), although some larger and smaller kegs may be available in your areas. Beer kegs have the disadvantages of needing modification if fill ports and drains are desired (to fill or empty the boiler without removing the column, useful or even required if you are not physically able to move your boiler around but not essential for most distillers), or if heavy cleaning and internal inspection is anticipated (due to anticipating running thick washes). They are also rather conspicuous when sitting around in the yard, but so are most large boilers, and they are frequently used by beer brewers. For unusual column sizes, especially sub 2" diameter stills where the column would have to be expanded to fit the standard keg flange, beer kegs can require more complicated means of column attachment. For the common 2" (50mm) build, however, they are extremely easy to adapt as a boiler, as an easy flange  or 2" triclamp ferrule and clamp can mate with the standard Sankey flange and seal with a PTFE triclamp gasket with one sides "rib" shaved down with a knife. Similary, larger columns can be reduced down to 2" to use this same method at the additional cost of a reducer. This will not impact on production speed unless you allow packing to fall into the reducer.
Stainless steel milk cans are also appropriate boilers. These are semi-antiques that have a tight-fitting sealed top making them easy to clean and charge, they have a flat lid making column connection easy, they have handles making them easy to move, they can be large: holding up to 10 gallons (38 liters), and they are fairly beautiful. Unfortunately they can be quite hard to source.
In some countries it may be possible to source old copper hot water cylinders. These will require some modifications for duty as a boiler, but they can make beautiful, practical units if the owner is prepared to put the effort in to modify them. Generally speaking, they need a column connection flange soldered or welded on, and may need to have some output lines sealed up. The materials should be checked for suitability for use in contact with ethanol (solder, seals, etc), and the standard thermostat will need to be removed in favour of a more suitable power controller.
Aside from ruggedness, the other critical consideration in boiler choice is size. Consider the batch size you'd like to be making, and work from there. A larger boiler will take you longer to run and will require more power for a fast heat up, and depending on your physical ability, could be difficult to move around, charge (fill), or drain. On the other hand, a small boiler will limit your batch size, make cuts harder, be time inefficent and generally frustrate you. In the extreme, a column still can be too big for the boiler and be "starved" of ethanol. Most regard around 5 gallons (~20L) as a minimum practical size, and 15.5 gallons (~55L) as a upper size for a hobby scale still - larger stills will generally be met with suspicion of an illegal commercial operation.
Main Article: Heat Source
The main two heat sources used are Propane / LPG burners, and internal electric elements. Stovetops and hot plates may be used for smaller boilers, but generally speaking their low power ratings and slow heat cycling limits their usefulness. Traditional inputs like wood / coal fires may also be used, but are uncommon.
Connection to column
Main Article: Column Connection Options
As noted above, as well as a column connection, the boiler will need access for charging (filling), emptying, and cleaning out. If a lidded pot or milk can is being used, the sealable opening is already present and provides no obstacles as long a good seal can be made. If you are using some other type of container it may need to be modified to provide proper access. Some distillers have cut the top of beer kegs for boilers , however the current consensus is that the standard 2" sankey attachment is sufficient for boiler access, and unless you have exceptional needs such as running whiskey on the grain, to leave the kegs unmodified . Many distillers with uncut kegs simply put a handful of chain or gravel inside the boiler and swirl it around with hot water in the very rare case of scrubbing being required - for the most part, a simple rise out with water after a run will suffice. Scrubbing is normally only needed if a thick mash is run and material is burnt onto the inside of the boiler. Reflux columns are normally run with thin sugar mashes for neutral spirits, so this won't be a large problem for reflux users.
If you do choose to make a hole in your boiler, it is important that it be properly sealed with a gasket. Gaskets will likely come into contact with vapors or high temperature liquid alcohol, and may impart unwanted flavors or contaminents. Common and safe gasket materials that are used are 100% natural cork, flour paste, and PTFE / teflon.
Information on cutting and using cork and flour gaskets can be found here: . Ensure the cork is natural and doesn't use artificial glue binders, which can be found at musical instrument stores or craft stores. Don't be afraid to use a flour gasket. It works better than you might think it will and is both cheap and easy  . Pure cotton such as a plain white tee-shirt can be cut up and then soaked in thin flour paste for an effective gasket. PTFE envolope gaskets are readily available for triclamp connections, and PTFE sheet can be purchased to cut gaskets for more unusual sealing requirements. Be aware that PTFE sheet is quite firm, so you'll need a good flush seal to use it well.
Electricly fired boilers have a huge advantage here, in that there is no concern about fire safety. Foil backed foam (e.g. Reflectix) or bubble wrap building insulation is very effective, and cheap.
Gas fired boilers are harder to insulate due to fire safety. Some distillers have had great success building metal shrouds, effectively using trapped air as their insulation. Fiberglass, fire cement, and fireproof expanding foam has also been used .
Your boiler should be made out of and sealed with safe materials, such as copper or stainless steel for the boiler, and flour paste, cork, or PTFE for the seals.
You should consider the weight of the unit, and how that weight will change as the run progresses. It's no good having your rig nicely balanced at the start of a run just to have it tip over once 20L of ethanol has been removed from the boiler!
Be aware of the heat involved, both in firing the still (insulation fire safety, especially for gas) and in draining the boiler (splashing yourself with boiling water is never fun). Be aware of the wrath of SWMBO if you empty boiling liquid on her plants. Hot backset will kill a garden pretty effectively!