I have lived with a Range Flowmax fully vented thermal store in a 4 bedroomed house with 3 children for the last 10 years. A thermal store is great!
And now I am going to instal a tandem system of 2 thermal stores, this time from Neward Copper Cylinder Company that will do the same thing for a large 5 bedroomed house.
Why?
Well, if you get past the bit in the argument that you need to have a pressurised thermal store in order to have mains pressure hot water (which is just not true ... and why would you want so much hot water under pressure anyway in your home?) and let me explain that irrespective of boiler or heatsource type, a vented heatstore is simple and effective to use.
Mosty people have 1 source of heat - a boiler. Forget about condensing or not, a boiler is a unit that uses energy or fuel and creates hot water. If water is to be used in the home, then it is primarily for 2 reasons; Heating or Domestic Hot Water (DHW) and dependant on your space heating type, you may only need 35°C or as much as 65°C (underfloor versus radiators) and DHW at the point of use should be between 35°C and 42°C where it is used.
Now, you do not have to store lots of DHW in order to have mains pressure hot water. What you need is a heatstore that is capable of acting as a buffer between your boiler and your supply that feeds your hot taps. In the middle is an exchanger - either a plate heat exchanger or a coil in you thermal store. Any good system will supply 30 to 35 litres of DHW at 40°C per minute - dependant of course on the temperature of the heatstore.
So why have a heatstore at all? Well initially I bought one as a replacement to an indirect cylinder that had a seam fail on it after possibly 40 years. I liked the idea of an integral header tank (i.e. get rid of the system and DHW header tanks) and the notion with the Range product that through a PHE, the water would be at mains pressure. As I was not replacing the boiler, this was literally a 1 day DIY task and involved connecting 6 pipes to the existing circuit. Simple.
What I also noticed was that as I now had a buffer between my space heating radiators / DHW heat exchanger and my boiler, my boiler cycled less - i.e. when the heatstore called for heat and started the boiler circuit pump (the boiler is subservant to the heatstore) it came on for longer and had less startup/shutdown cycles. This gave me about a 7% improvement on fuel consumption.
Now, if you are considering multiple heat sources, the a heatstore is great at linking everything together - especially as a vented system just needs the odd thermostat and pump adding so that when the flue on a log burner reaches a high temperature (over 150°C), water can be passed to and from the heatstore, reducing your bills on the main fuel/energy type.
The arguements about condensing boilers and return temperatures are largely irelevant when (as other have noted) the tank is large enough and design well enough to stratify the layers or zones of heat within the heatstore. For me condensing boilers that are working optimally are fine, but this means that they should be sized a higher than required as it is the cycling of the boiler that often consumes more energy than the heating requirments. Every heating specialist knows that the bigger the mass or water (volume) within a system, the less the boiler will cycle.
A heatstore, unvented, with an integral header tank (sometimes referred to as a combination unit) does not require building regulations notification as there is no risk of overpressurising a large amount of water and they do really work.
Plumbers who insist that the boiler has to be replaced and a pressurised 'unvented' system is the only way to go are doing so purely because they want to make lots of money from you.... and make the installation simple from their part.
Save energy by servicing your boiler annually and keeping it going until it springs a leak or needs the burner replacing (in which case it may be chaper to fit a new boiler). The heatstore is purely a useful way of interfacing your space heating and DHW needs with a boiler - a boiler is simply that, it boils water.... kepp it that way and don't waste money on lots of technology that may give you 1 or 3 percent more efficiency - by the time they (may) have paid for themselves, they will be broken and due for replacement.
Keep it simple.