Cylinders run out of water.
An indirect cylinder will, but only because the indirect coil has limited capacity. A direct heated thermal store/heat bank won't run out as long as the boiler can keep up. Since many people seem content with the restricted output from a combi, you'd have to draw off quite a bit more to empty a thermal store unless the boiler is fairly small. Even if you have a smallish boiler, the recovery time when you can pull it's full output into the store, recovering top down, will be quite fast.
But then that's part of the system design - if you are supplying a large house and there's the potential for three people to run a bath at the same time then you need to size the store accordingly.
Thermal stores will be neglected. The CH coil eliminates sludge build up, so best have one in to be sure. It is not a great cost.
But there is a cost in system complexity by having a separate circuit. Looking at the relative sizes, if you have sufficient sludge buildup to cause significant problems in the bottom of a store, then it would have blocked all your rads. That's a LOT of sludge, and if you are getting that much then you do have a much more serious issue with your system.
Great install. I like the insulation box. A thread here recently stated that the new Collins Plumbing and Heating Guide states that DHW only thermal stores use 15% less energy.
How is your installation operating?
It's working fine, but I suspect I might need to add a motorised valve to the heating circuit to stop it thermo-syphoning. Because the store is downstairs, there is enough vertical rise for it to thermo syphon in the normal flow direction without the pump running. Not an issue now, but I suspect it might be annoying in summer.
Apart from keeping heat in, the box is there to a) make it tidy and hide all the pipes, and b) restrict access to make it harder for people to
adjust things.
In terms of efficiency, once the boiler gets changed (which will be an interesting exercise in itself) for a condensing one, then I reckon the store will really help. Under light heating load, the radiator return loop can be down into the low 20s in temperature. The result is that the store cools bottom-up (the DHW coil also has this effect) and so when the boiler fires up it can draw cool water from the bottom of the store - I anticipate it should rarely fail to be condensing if setup right.
With a
conventional system, to maintain the minimum flow required through the boiler, the cool return from the rads is mixed with the hot flow (via the bypass loop) - so I reckon it's probably rare for a boiler to be condensing while running the heating. So you get all the added complexity of a condensing boiler, but none of the claimed savings.
Since my rad loop flow is completely separate from the boiler flow, the modulating pump just sits there quietly supplying only what the TRVs demand - so the TRVs and pump are barely audible.
I suspect that on cost grounds alone, any savings wouldn't repay the investment in a thermal store - but what price do you put on comfort and convenience, not to mention still having heating & hot water when the boiler breaks down ? With a passive DHW coil thermal store, you even still have hot water (obviously limited in endurance) when the electric goes off.
Of course, all this probably isn't helping to answer the OPs question
