croydoncorgi said:
I also question ANYONE's design that includes a high-temperature (80+ degrees) heat store and a high-efficiency boiler.
You have come out with this misinformation a few times and few times it has been explained to you.
Thermal Stores/Heat Banks are far superior than unvented cylinders - and safer. Read below....
Advantages of Thermal Stores/Heat-Banks
- Instant high pressure hot water - When the store has reached temperature water is delivered instantly at the taps.
- High mains water pressures - At up to 10 bar, compared to 3.5 bar maximum with most unvented cylinders.
- Very high water flow-rate - The high-end heat-banks have a flow-rate up to 45 litres/min.
- Long efficient boiler burn - Eliminates boiler on-off cycling increasing efficiency and boiler longevity.
- Maintains a Low Return Temperature Promoting Condensing Effiviency - using high efficient plate heat exchangers the return temperaure is guaranteed to be very low.
- Maintains optimum boiler temperature range – Using a blending valve the flow/return of the boiler can be kept to optimum maintaining greater efficiencies. The boiler operates at optimum performance.
- May combine the output of the stored water and the boiler - The boiler and energy in the store may be combined to increase output. This in effect makes the cylinder larger.
- Cylinder may be smaller for a similar performance - smaller cylinders than unvented cylinders.
- Cylinder at low pressure - Unlike an unvented cylinder vented stores do not store water at high pressures.
- Fast cylinder recovery rate - When the boiler is connected directly to the heat-bank, and not via an indirect coil, the recovery rate is rapid. Although in some cases a boiler may heat the heat-bank via an indirect coil, reducing the recovery rate.
- Legionella bacteria virtually eliminated - The Legionella bacteria cannot survive in the high temperature sealed conditions of a heat-bank.
- No scale build-up in heat-bank – Containing primary and not secondary fresh water, there is no scale build-up inside the heat-bank cylinder.
- Cold water storage eliminated - No need for cold water storage tanks.
- No BBA certification to fit – Unvented cylinders require an approved fitter to install. Thermal stores/heat banks require none and can be DIYed.
- Solar heating storage - Water heated via solar panels may be stored in the heat-bank via a solar coil.
- Easy maintenance - If an external plate heat exchanger requires cleaning or replacing it is a matter of draining down the heat-bank, or closing isolating vales, and unscrewing the plate heat-exchanger. In some rare instances plate heat-exchangers are fitted directly inside the heat-bank preventing on-site maintenance.
- Easy to improve hot water flowrates – By simply adding additional plate heat-exchangers in parallel, hot water flow rates may be improved. Retrofit additions are possible if extra bathrooms or showers are installed. This is impossible with unvented cylinders.
- Stored water vessel need not be cylindrical – As no internal coil is used for hot water heat transfer the stored water vessel may be any shape, as opposed to a thermal store which has to be cylindrical for maximum
efficiency. This has advantages where space is limited.
- No Annual Service Charge – Unvented cylinders require an annual service of between £60-£100. Thermal stores and Heat Banks are service free.
- Thermostatic Radiator Valves can be on all Rads - No wall thermostat required and TRV s on all rads when the CH circuit is take off the store cylinder and an auto modulating speed Grundfos Alpha pump used.
- Cheaper, Smaller, Simpler Boilers May Be Used – The large buffer of stored water means that a boiler sized for average use, not peak use, can be fitted.
- Larger Boilers may Be Used Without Fear of Boiler Cycling – A larger boiler can be used to reduce the cylinder size or give rapid heat recovery rates or both.
- Full Electric Backup of CH and DHW – An immersion Heater(s) can be fitted in the cylinder that will give CH and DHW backup very cheaply.
- Vented Thermal Stores/Heat Bank Eliminate Explosion risk – Unvented cylinders have an explosion risk. This is not a problem with vented thermal stores/heat banks.
http://www.waterheaterblast.com
- Pressurised Thermal Stores Don't Need Annual Service - No BBA unvented certification for fitting or annual service is required if a thermal store is pressurised and uses a plate heat exchanger.
Disadvantages of Thermal Stores Heat-Banks
- The store needs be near fully temperature to supply baths - Before any hot water is drawn off, the store must be up to temperature. Many later versions use a blending valve on the return to the boiler to ensure only up to temperature water is pumped into the store by the boiler. This prevents agitation of the stored water, and aiding heat stratification within the store giving useful water at the top of the store within a short time. The water is heated only in one pass through the boiler.
- Lower water temperatures with fast flow-rates - As with Combi boilers, fast flow-rates through the plate heat-exchanger results in lower water temperatures. This is not so pronounced with heat-banks as with internal coil thermal-stores. This also applies to combi's. With a heat bank, larger or extra plate heat exchangers can be fitted to increase flow rates. Sized up properly this is not an issue in operation.
- Needs more inhibitor - A minor added cost.
Advantages overwhelmingly to thermal stores/heat banks. 
