I think I may be missing something and would be grateful for clarification.
With regard to vented heating systems as I understand it, 'direct' means the water that ends up coming out of the hot tap goes through the boiler where it's heated up. It may than be stored in a cylinder before use.
Indirect means that the water in the tank is heated by a smaller volume of water which circulates through the boiler and tank but never comes out of the tap.
What I don't quite understand is how, or why, one would have an direct unvented system.
Direct of Indirect refers to how the water is heated.
In a Direct system, ( either vented or unvented ), the store of water for your hot taps is directly heated from the heat source, like an immersion heater.
In an Indirect system ( either vented or unvented ) the store of water is heated indirectly: the heat source ( boiler ) heats up the central heating system water, which flows through the radiators. There is basically a radiator inside an indirect cylinder which in turn heats the hot water for your hot taps. The heating loop water does not mix with the stored water for your taps ( DHW. )
I meant that you wouldn't have an unvented cylinder directly heated by e.g. a back-boiler, the way you used to with ancient gravity-fed open-vented systems.
For exactly the same reason you might want a direct venter cylinder.
So you can have stored hot water, where you can't or dont want to run the hot water from a gas or oil fired central heating boiler.
The fact it's un-vented it irrelevant to the direct or indirect method of heating it.
In most Alpine ski chalets I've stayed in, the hot water system comprises several large unvented cylinders all in parallel together, heated electrically. Often, they have several immersion heaters per cylinder. I guess it's more economic to get electricity up to resort level than other fuels, and in France perhaps it's even more so with their nuclear capacity.
In days of yore (like up to about the 1950's or even 60's) it was quite common to have a back boiler behind a coal fire, connected to a direct cylinder so that the water coming out of the hot taps did indeed go through the boiler. With a cast iron boiler and copper pipes and cylinder, corrosion isn't too much of a problem, and you can get away with a direct cylinder. Such systems usually had some sort of automatic air eliminator because, of course, dissolved air was introduced into the system with each new lot of water passing through.
As soon as you have a (solid fuel, gas or oil) boiler which heats pressed steel radiators as well as the domestic hot water, it's no longer acceptable to have constantly changing water passing through the system. The trapped air will cause horrible bleeding (interpreted any way you like ) problems with the radiators was well as rapid corrosion. Separating the system into primary and secondary circuits - with heat transfer occurring in a heat exchanger inside the cylinder - solves the problems since, once the air has come out there should be no more, and you can use corrosion inhibitor in the primary circuit - which you can't do with a direct system.
The nearest you'll get to direct systems today is those which use electric emmersion heaters inside the cylinder to heat the water, and don't have an external boiler.
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