A once popular but now discontinued option.
Originally AGA Cookers were designed to be on and up to temperature all the time so it wasn't long before someone realised that putting a small boiler in the cooker to heat water would be a good idea; after all, the AGA would be on all year round so doing the water without having to put a large boiler on just to do the much smaller job of heating the cylinder in the summer (when the radiators weren't needed) made sense.
So a small 'water jacket' type boiler was added to absorb some heat from the AGA's firebarrel, this was then circulated 'by gravity' to the cylinder. (The pipes laid to a gradient, hot water rises up the 'flow' pipe, heat is transferred to the water in the cylinder and the now cooler water flows back down the 'return' pipe to the boiler to be re-heated.)
This produced a constant low-output of hot water that continually heated the cylinder - a large cylinder was fitted to act as a 'buffer' absorbing the heat when more was produced than being used (overnight normally) and providing a store for when demand outstripped supply (bathtime!)
Since there is no thermostat for the hot water in the cooker a heat-leak radiator was often fitted to help moderate the hot-water temperature. (Hot water is 'made' purely secondary to the cooker being maintained at the correct cooking temperature; the thermostat sensor is in the top oven - when water useage was low, the water might boil - a heat-leak radiator [or towel rail] would 'use' this excess heat to give a degree of control over the amount of hot water the AGA made.)
The extra fuel the cooker burns to heat the water (that's right, it's not 'free'!) was, at the time, less than the boiler would burn to do the same job so it was more efficient and therefore cheaper to run, particularly in summer when the boiler would running at its least efficient (in 'hot-water only' mode without the radiators).
Nowadays though modern systems (super-insulated cylinders, unvented [mains-pressure] cylinders, fully pumped systems [none of which are compatible with the AGA] and non-storage 'combi-boilers') are much more efficient, more controllable and more suited to modern life than the AGA system. As a result AGA hot-water has fallen from favour and has been discontinued (in late 2014) as an option.
Types of AGA Hot Water System
Also referred to as 'single feed' system. Only one tank is required, usually in the loft or the cupboard above the cylinder - the same water fills the hot water cylinder and AGA's boiler.
It seems simple, and is, but since the water running through the boiler is constantly refreshed, all the limescale and corrosive attributes of the water are refreshed too. Deposits build up quickly in the boiler, or if the water is of a more corrosive nature, the boiler and fittings are corroded quicker (the boiler in the cooker is usually first to fail - it's the hottest part of the system and the heat aids and speeds the corrosion).
It's not possible to 'dose' this type of system with a corrosion inhibitor so the boiler, cylinder and associated pipework corrode (or scale up) relatively quickly.
Since the same water runs to the hot tap it's important that all the components in a 'direct' system are compatible; only relatively inert pipe and fittings unlikely to be affected by water such as copper, brass or stainless steel can be used; steel radiators and fittings must not be used as they'll corrode very quickly and colour the water a rusty brown colour as they rot from inside!
For the same reason it's also necessary to fit a special enamel or glass-lined boiler to the AGA (some old ones used a copper boiler) rather than a plain cast-iron one.
Note: Four oven AGAs use a different type of boiler - instead of wrapping round the outside of the cooker's firebarrel a smaller 'horseshoe shaped' boiler is used inside the firebarrel. These are cast in one-piece and can't be 'glass-lined' so as a result it's not possible to use these on a direct system.
(If you're an AGA uber-geek you may know that there was, for a short time, a copper coil type boiler in some four oven models which could be used on a direct system; these haven't been made for many years now and aren't available as spares.)
The modern way (it's recommended that an old direct system is upgraded to 'indirect' if the cylinder is ever replaced).
There's a coil of copper pipe in the cylinder that keeps the water running through the AGA's boiler separate from the fresh, clean water going to the taps.
This means the water running through the boiler can be dosed with corrosion inhibitor which, coupled with the fact that it's not constantly refreshed slows the corrosion and scaling in the system.
It also then allows other components such as radiators and towel rails to be made of steel.
It's rare to see these nowadays however quite a lot were fitted 'in the old days'!
Instead of there being a coil of copper tube in the cylinder a 'self-priming' (or 'primatic') heat-exchanger is fitted. This fills using the same water as the main part of the cylinder (so only one tank is needed in the loft) but once the system is full an air-bubble separates the water running through the boiler and heat-exchanger from the water that goes to the taps.
Its advantages are the single tank needed and the separation of the water means steel components can be used. It's a 'half-way-house' upgrade from a direct to indirect cylinder when installing the second cistern is difficult.
Disadvantages: you still can't dose with inhibitor so longevity isn't as good as having the coil heat-exchanger and the way the 'air-bubble' separates the two sides means there's a limit to the volume of water allowed in the boiler loop - too much can 'blow' the bubble out and cause the two sides to mix.
Schematic - AGA with Self Priming Cylinder.
In a larger property where the AGA's output isn't enough, or where a secondary source of heating such as a loop from the central heating boiler is also to be connected a 'twin-coil' cylinder was used.
The AGA would normally be connected to the lower coil so it constantly heated the whole of the cylinder, the secondary source is then connected to the upper coil to help out when needed.
It's important that any cylinder thermostat fitted to control the secondary source is mounted away from any of the connections to the cylinder and is about half-way up (they're normally fitted about a quarter of the way up but they need to be higher in this case so the thermostat shuts off the secondary source sooner and prevents the AGA's extra heat [which can't be controlled] from overheating or boiling the cylinder.