Hi Aragorn,
Ah, right - no, I wasn't missing this. A well-designed tank will minimise churn, so that as hot water is drawn out of the top of the tank, and cold water flows into the bottom, the hot and cold layers will stay separate --- at least for a while: I agree they'll even out to lukewarm over the long term. This means that if you make a continuous draw on either a hot conventional gravity-fed immersion cylinder or an unvented cylinder, you'll get very hot water until you've drawn off almost all of the tank.
In contrast, with a heat store you're not drawing off a hot layer, you're running the incoming cold mains in a spiral through most of the tank, so after you've pulled half a tank's worth of water through the heat store, the whole store will be half way to cold, and it won't be able to raise the temperature of the mains water you're pulling through it by anywhere near the amount it could at the start of the exercise. The heat store can't benefit from the hot/cold layering in the unvented cylinder because it needs to run the incoming mains through a lot of its stored hot water in order to transfer enough heat to the mains water.
This is illustrated in this company's "why-we're-better" page:
http://www.systemdesigner.co.uk/documentation/Xcel vs Competitors.htm
... obviously you can't trust someone's own blurb about their own product, but the behaviour you should note is that the output temperature from the "normal" heat stores drops pretty quickly immediately you start using them. Their supposedly-superior product has an external pumped plate heat exchanger which improves usability a lot (and by some people's terminology makes it a heat bank), but adds complexity to the system.
I dare say that with a much larger heat store this might not be so much of an issue, but if we're talking about a small flat, the heat exchange spiral through the store runs through most of the tank from what I understand.
Edit: I should point out that I'm only linking to that graph to show the heat store's temperature behaviour. The graph is fundamentally misrepresentative for its intended purpose, because it's showing constant water flow, not constant heat flow: in the first minute, the 180l heat store will have released about 1.3kWh worth of hot water (18l at 75C), where their heat bank will have released less than 0.9 (18l at 55C). As a result you'd use significantly fewer litres per minute from the heat store than from the heat bank in order to get your 38C shower!
Conrad
The bit your missing is that as soon as you draw water from a hot water cylinder, be it vented or unvented, its immediately replaced with cold water. It is after all the incoming cold water that pushes the hot water out of the cylinder and down the pipes to your tap! Thus both systems will infact exhibit the same "gradual decay".
Ah, right - no, I wasn't missing this. A well-designed tank will minimise churn, so that as hot water is drawn out of the top of the tank, and cold water flows into the bottom, the hot and cold layers will stay separate --- at least for a while: I agree they'll even out to lukewarm over the long term. This means that if you make a continuous draw on either a hot conventional gravity-fed immersion cylinder or an unvented cylinder, you'll get very hot water until you've drawn off almost all of the tank.
In contrast, with a heat store you're not drawing off a hot layer, you're running the incoming cold mains in a spiral through most of the tank, so after you've pulled half a tank's worth of water through the heat store, the whole store will be half way to cold, and it won't be able to raise the temperature of the mains water you're pulling through it by anywhere near the amount it could at the start of the exercise. The heat store can't benefit from the hot/cold layering in the unvented cylinder because it needs to run the incoming mains through a lot of its stored hot water in order to transfer enough heat to the mains water.
This is illustrated in this company's "why-we're-better" page:
http://www.systemdesigner.co.uk/documentation/Xcel vs Competitors.htm
... obviously you can't trust someone's own blurb about their own product, but the behaviour you should note is that the output temperature from the "normal" heat stores drops pretty quickly immediately you start using them. Their supposedly-superior product has an external pumped plate heat exchanger which improves usability a lot (and by some people's terminology makes it a heat bank), but adds complexity to the system.
I dare say that with a much larger heat store this might not be so much of an issue, but if we're talking about a small flat, the heat exchange spiral through the store runs through most of the tank from what I understand.
Edit: I should point out that I'm only linking to that graph to show the heat store's temperature behaviour. The graph is fundamentally misrepresentative for its intended purpose, because it's showing constant water flow, not constant heat flow: in the first minute, the 180l heat store will have released about 1.3kWh worth of hot water (18l at 75C), where their heat bank will have released less than 0.9 (18l at 55C). As a result you'd use significantly fewer litres per minute from the heat store than from the heat bank in order to get your 38C shower!
Conrad