After reading all of this, I really want a thermal store
The first thing I notice is that several of the first responses must be from stereotypical plumbers - with the mantra "rip it out and fit a combi". Any plumber that says that to me has ended the conversation and better find the door before my boot finds his backside
I'm not surprised by the " there is a large estate up the road form here that had themal stores installed from new , they are gradually being taken out" comment - many plumbers don't understand a piece of pipe, and given that so many of them appear to be both lazy and ignorant of simple plumbing concepts, they fall back on the "rip it out and fit a combi" mantra, and the customer who foolishly thought the plumber might know what they are talking about goes along with it.
If you are insulted by that, then you are probably in the camp I describe. If you aren't then I suspect you have a similarly low opinion of the majority that give good plumbers a bad name. I speak as I find - I refuse to apologise to bad plumbers for the above, and don't think I have anything to apologise for in respect of competent plumbers.
Combi boilers have their place - mostly to get round an incompetent developer who could't find 2 sq foot of space for a cylinder. Other than that they have no redeeming features. First problem is that to get even a half decent DHW flow rate you need a MASSIVELY oversized boiler - in my case a 30kW boiler (min 10kW I think) for a single bed flat with a heating load of one or two kW and it still can't manage a good flow rate. Then of course, the dirty secret about condensing boilers is that due to the requirement for a minimum flow rate, you end up with a bypass valve somewhere so realistically in most installations they won't be condensing much, if at all, when running the heating - the return from the rads may be cool, but it isn't when mixed with the bypass hot flow.
And the biggest thing of all -
when they break down, you have no heating AND no hot water for as long as it takes someone to fix it (just hope the factory still have spares when it's 10 years old).
And that's supposed to be good ?
A thermal store allows you to run the boiler at it's optimum. As already pointed out, you should have no trouble getting it to condense most of the time. It doesn't have to short-cycle all the time you have the heating on as the store acts as a very effective buffer. If you go for a coil type then you can have (for a while) DHW without any electricity at all - useful if you are in an area prone to power cuts.
And for me, the other factor was the ability to have backup for both DHW and heating from an immersion heater. Yes I've tested it, and yes it works.
Trust me, tenants really don't like it when they are without heating and hot water.
So having looked at a number of options, I chose a Gledhill Torrent for the flat. It's simple, simple plumbing, simple wiring. The boiler now fires up and heats at full whack for a while and then turns off altogether - no short cycling, and it runs with a cooler return temp. The shower now runs at a uniform and stable temperature - while before it was all over the place as the thermostatic valve did it's best to cope with the varying DHW temp from the boiler.
I've fitted a fully modulating pump, and TRV'd the last of the rads - the return temp is now quite low and so the store cools 'bottom up'.
The comments about Gledhill's reliability are noted - I'll see how it goes.
But back to the original question.
From the research I did before fitting the store, it seems that you can get a significantly higher transfer rate from an external heat exchanger - basically you can fit as big a unit as you need/want and easily get an output in excess of 100kW if you need it. The downsides as I see it are :
1) You need a flow sensing switch in the DHW pipe and pump in the primary side - so potentially have a minimum flow rate. That may or may not be an issue, but as someone used to being able to run the taps at a trickle and blend hot/cold as required (scrubbing dirty hands, washing paint rollers, etc) I found the inability to do so in the flat with a combi irritating.
2) If you are trying to keep the store stratified (so as to have a cool inlet temp to the boiler, or take advantage of solar) then the pumped primary of the heat exchanger is a problem. Unless you draw off DHW at the maximum rate the hear exchanger is sized for, then the return water going back into the bottom of the cylinder will be hot - and the slower your draw off rate, the worse the problem. It could be dealt with by modulating the flow rate in the primary circuit (ideally you want to restrict the primary flow rate so that the return to the bottom of the tank is not hugely higher in temp than your cold water supply), but then you are introducing more controls.
With an internal coil, the natural action is that it cools the bottom of the store to a lower temperature since the DHW is heating up as it rises through the store. So you naturally end up with a cool bottom which is great for keeping a boiler in condensing mode.
