A bit of controversy can be fun, but I'm actually serious. I'm specifically not talking about the uber-expensive foreign behemoths that rich people with lots of land can tuck away in part of their estate, but thermal stores of a scale that "ordinary" folks can fit in an airing cupboard in their ordinary home. About 250l is considered relatively large by the volume cylinder manufacturers in the UK, costing about £1k. I think they are a waste of time, and here's why:

1. They _need_ to be maintained at a high temperature (about 80degrees) according to manufacturers. This is bad news for heat loss, and makes the utilisation of condensing boiler problematic (if not pointless). It also belies the claimed reduced boiler cycling.

2. The operating range of temperatures for hot water is quite small(~20 degrees) which means they can't produce as much hot water as you might hope without frequent input from a boiler.

3. Your typical (say 250l) thermal store will not stratify - the water inside will mix through conduction and convection, particularly if the heat exchanger is internal. This is in contrast to an unvented tank, with fresh cold water coming in from the bottom.

4. Although they don't require an annual inspection, neither do unvented cylinders.

5. You can incorporate multiple heat sources with a thermal store, but you can do that with a sealed system as well.

6. There are often restrictions as to where you can put the store and radiators.

7. Many thermal store designs have complicated heat exchanger and pump arrangements which look to me to be a source of constant maintenance. These systems won't work in a power cut.

8. The flow rates for hot water aren't impressive when compared with an unvented cylinder.

I think the advertised benefits of your typical small (say up to 400l) store are entirely dubious if not utterly bogus! An unvented tank seems a much simpler and technically superior option.

T

Definition
Thermal Store - has an internal DHW take-off coil
Heat Bank - has a DHW plate heat exchanger and pump.

People get them mixed up.

An integrated heat bank, CH & DHW, is very different to an unvented cylinder. It is not a direct replacement, although there are DHW only heat banks/thermal stores. It offers so much more. In most cases it is foolish not to use an integrated heat bank/thermal store.

Thermal Stores/Heat Banks are far superior to unvented cylinders - and safer. Read below....

Advantages of Thermal Stores/Heat-Banks

- Instant high pressure hot water - When the store has reached temperature, or just the top of the store in a heat bank, water is delivered instantly at the taps.

- High mains water pressures - At up to 10 bar using a heat bank plate heat exchanger, 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 Efficiency - using high efficient plate heat exchangers the return temperature from the plate s guaranteed to be very low. The bottom of the cylinder get very cool while the top can be 70C to 75C. Even is the store is set to 80C, 80% plus of re-heat time the boiler is condensing.

- Boiler Operates in the Ideal Hydraulic Environment - Using a blending valve the flow/return of the boiler can be kept to the ideal heat exchanger temperature differential. This maintains greater efficiencies. The boiler operates at optimum performance in the ideal hydraulic environment.

- 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.

- Superb Neutral Point For Combining Different Heat Sources - Provides for linking a variety of directly connected heat sources providing a brilliant neutral point: boiler, wood stoves, electric immersion, solar panels, heat pumps, etc, can all be connected to the one cylinder.

- 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. Even pressurised heat banks operate at 1 bar cold, and rarely get above 2 bar.

- 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 G3 certification to fit - Unvented cylinders require an approved fitter to install. Heat banks, pressurised and vented, require none and can be DIYed. Also vented thermal stores required no G3.

- 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 TRVs on all rads when the CH circuit is taken 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
http://au.youtube.com/watch?v=pu3FwgIHsQA
http://au.youtube.com/watch?v=GF_Wrm-Ns0I

http://www.masterplumbers.com/plumbnews/2001/dec/avon.asp
http://seattlepi.nwsource.com/local/33094_boom28.shtml
http://www2.canada.com/edmonton/edmontonjournal/news/story.html?id=02acabcc-e7c4-43e4-8fe0-5c298527e2d5

http://www.abc15.com/news/local/story/Explosion-rips-through-north-Phoenix-home/7p8jpeMzcU664FSrKb_osA.cspx

- Pressurised Heat Banks Don't Need Annual Service - No G3 unvented certification for fitting or annual service is required if a heat bank is pressurised and uses a plate heat exchanger.

Disadvantages of Thermal Stores Heat-Banks

- The store needs be near full 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 hot 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 easily fitted to increase flow rates. Sized up properly this is not an issue in real world operation.

- Needs more inhibitor - A minor added cost.

Advantages overwhelmingly to thermal stores/heat banks.

OK, I'm up for this.

I am in the early stages of speccing up our self build and had intended to have a combination of wood fired Rayburn (kitchen), small wood burner (living room) and solar thermal panels (roof) all feeding a central heat store such as the ones you are discussing which then feeds UFH over 3 floors and DHW.

So, please explain how I could achieve all of this with a standard unvented tank - taking multiple heat sources and feeding the UFH and DHW.

If it's doable I'd be very interested as I too have been concerned about the complexity of these heat stores and their reliance on electricity.

Tom.

Posted By: GrizzlyHippoOK, I'm up for this.

I am in the early stages of speccing up our self build and had intended to have a combination of wood fired Rayburn (kitchen), small wood burner (living room) and solar thermal panels (roof) all feeding a central heat store such as the ones you are discussing which then feeds UFH over 3 floors and DHW.

So, please explain how I could achieve all of this with a standard unvented tank - taking multiple heat sources and feeding the UFH and DHW.

If it's doable I'd be very interested as I too have been concerned about the complexity of these heat stores and their reliance on electricity.

Tom.

If there is a power cut most heating systems die, so that is not real issue. Unless you have solid fuel. You can have an internal DHW take-off coil and an external plate heat X too for heavy DHW draw-offs. A hybrid.

If you are in a power cut prone area then have a small 3kW backup gennny that plugs into the essential services: CH, freezer, lights. Have a throw-over switch that brings in the genny and only supplies the essentials on one essential circuit CU. The genny can be one of those on wheels so used for other things. Just plug in, throw switch and start genny. Not expensive.

An unvented cylinder is not comparable with a full blown heat bank/thermal store. They are quite different. The last thing I want in my home is a potential bomb.

How about some real numbers to back up the statements?

i.e. 300L tank with 2m 12.5mm ID copper coil heat exchanger in the upper half of tank costs X.
With tank water at 70C, mains cold water in @10C & 2GPM flow hot water out >= ???.
Most showers are ~2GPM flow (older shower heads around 3GPM). Drawing a bath is much more; ~8-10GPM.

1. Manufacturers specify the high temperatures for reliable hot water production. High return temperatures prevent boilers from condensing. Frequent topping up, causes cycling.

2. Below 70degrees you don't get hot water - so you've got between 90ish and 70ish to play with. With an unvented cylinder you've got the whole volume at a useful temperature.

3. Stratification cannot occur if the water is stored for any length of time - the conductivity of water is too high. If you believe it possible, demonstrate it - otherwise it's just marketing drivel. This is where unvented cylinders win. They are sized so that the water us used, not stored. Stratification is a temporary and useful phenomenon in this case.

4. Unvented cylinders do not require an annual service - it depends on the manufacturer specifications. Get a quality cylinder!

5. Very efficient and straight forward. eg:
http://www.40percent.org.uk/40-percent-exemplars/case-study-1-an-18th-century-cottage/
I have a stove with boiler on a sealed system and unvented cylinder too! High temperature storage is inefficient!

6. An unvented cylinder can be placed anywhere. Thermal stores are often specified with minimum height difference between top of store and highest radiator. Also, in a typical domestic situation, they are almost invariably gravity fed if a stove is used.

7. Don't work in a power cut. Water softener can also be required. Some think plate heat exchangers are great, others don't. There is simply no benifit to be gained for this added complication.

8. I get my information from manufacturers. "Flow rates of up to 30l/min" is a typical claim for a store. Good, but significantly less than an unvented cylinder.

T

Posted By: tom.harriganminimum height difference between top of store and highest radiator.

Who needs radiators when you've got super insulation!...

Seriously though appreciate this thread very informative.

J

(PS Tom, are you sure you don't sell unvented cylinders...? )

Sorry Tom but 1 and 2 are simply incorrect. Please listen to the people who actually have stores. Did you read my post above? You're obviously a troll who doesn't listen and who has made his mind up without any practical experience of a store "I get my information from manufacturers" demonstrates this. So this discussion is fairly pointless. With regard to "Frequent topping up, causes cycling" have you heard of twin stats? The buffering of the store means you don't need frequent topping up.

If the highest radiator HAS to be above the header thank then don't use a fully vented store. It's not rocket science. If you have a high tank why not chose a vented cylinder anyway.

3. Clarify what you mean by "any length of time". It does not need to act for days on end with a small store anyway. I know that if my store is at 40 degress during the summer from solar input alone then I can run the boiler for less than 5 minutes to give two showers, some washing and all the other DHW I need in a day. That's the benefit of stratification that you don't seem to understand.

7. Again this is a red herring, if you have frequent power cuts, have some backup power. Having a shower during a power cut may not be your top priority anyway. Or simply use an internal coil heat exchanger instead of external.

8. More stupidity. Simply size the heat exchanger for the job. Another advantage of stores.

Posted By: Miked27148. More stupidity. Simply size the heat exchanger for the job. Another advantage of stores.

With heat banks you can add plate heat exchangers if say you have an extension. Try that with a coil. With a large heat bank supplying many bathrooms a number of plates are fitted - divide and rule.

Fitting an oversized plate heat exchanger means the store can be run at quite a low temperature. Stratification is then not so much a big issue - although aim to maximise stratification and it works even better.

Tom

Please read past posts regarding buffering / lag. Stored heat means that when there is a demand there is heat already there. Your statement demonstrates that you don't know how a store works. The store never cools enough: The situation you imply that there is heat demand but a lag in delivering that heat because of the store volume never arises.

OK we're agreed that if the store is heated to 90 degress then the boiler won't be in condensing mode. BUT YOU DON'T NEED TO HEAT YOUR STORE TO 90 DEGREES. That's the same as saying if you heat your unvented cylinder to 90 degrees then the boiler won't be in condensing mode.

What you forget, which Water Systems points out, is in your system, when they boiler is satisfying radiator heating demand, as soon as multiple TRVs switch off, the return temperature will rise, pushing the boiler out of condensing mode. This is exactly what doesn't happen with a store.

Can you explain what you mean by the operating temperature range? I think you are missing something here. The top of my store is at 70. The bottom at 55. Boiler pumps in hot water at 70, this moves the interface down the tank to the lower stat. Boiler switches off. Cooler heating water eventually moves the interface back up to the upper stat. Boiler switches on. Etc.

You say "some suggest setting the thermostat at 75degrees - it as soon as that temperature is hit (which is above condensing return temperature) the boiler switches on.". This demonstrates that you don't understand how a store works. Say the TOP of the store is at 75 degrees. The bottom of the store will be cooler. About 55 degrees in fact. Hey presto, below the temperature required for the boiler to remain in condensing mode full time. I have monitored boiler flow and return temps over the winter with my store and I can tell you for certain that the return temperature is always below 58 degrees when the boiler is running.

Gentleman, gentleman. I now have a headache after reading the above. I admire the demonstration of how big your brains are but for the uninitiated like me can you please sum up exactly what a thermal heat/bank/store/buffer is? Your challenge is to describe in simple terms exactly what it does and what the benefits are in one sentence.

Posted By: tom.harrigan"Water Systems" seems to be getting irate!

You are given explanations and then spout the same diatribe, clearly not taking a blind bit of notice.

You appear to be on about the outdated DHW coil thermal stores. Which benfits more from higher store temperature. Using a simple plate heat X and simple CH pump transforms a thermals store - into a heat bank.

Contact OSO about servicing. Don't believe the installer.

What OSO say:
"Cylinder should be SERVICED ANNUALLY and benchmark logbook should be updated in order to validate guarantee. Logbook and service records act as guarantee document. For terms of guarantee please see Benchmark logbook on page 14."

http://www.osohotwater.com/uk/pdf/Super_S_installation_manual.pdf

You say a store has to run at 80-90C. Look at Range who specify 75C.
http://www.rangecylinders.co.uk/products/flowmax/how/

You can get away with 70C if your DHW needs are not that great.

BTW, a normal storage cylinder has to maintain the water 60C minimum. ACV supply a blending valve and recommend storage at 65C. Their cylinder stat has a range of 60-90C. Not much difference to a modern heat bank.

I can't find a single technical argument to change my point of view that small thermal stores are pointless.

Well you can't read much then. What are you referring to? A small old fashioned DHW only thermal store with coiled DHW take off? These are old hat. Plate heat exchangers have largely replaced them.

This 20C temperature range you are on about. The newer coils tend to be split into two coils. One at the bottom as a preheat (this keeps the bottom cool for condensing efficiency too) and one at the top to take up to temperature. They operate over a wide temperature range.

Plate heat exchangers are so efficient you can have one hand on the return pipe to the cylinder as it will be around 20-25C and unable to touch the top pipe into the plate. They extract so much heat.

I have done tests on a bare copper cylinder using a plate. The cylinder was set to 70C. When the cyl' stat was satisfied the DHW was run. The return into the bottom was around 20-25C. There was a spreader in the bottom. The cylinder cooled from the bottom up in a line of cool water rising. It could be felt by hand. The line just rose up and the top was still around 70C. Only when the line was very near the top did the DHW temperature at the tap fall away. That operated at very wide temperature range, about 50-55C. A larger plate heat X may take that nearer to 60C.

One more thing: The CH buffering effect - it should be called the CH lag effect!

No. Is is CH buffering. Which DHW cylinders do not do. T Stores and unvented cyls are not the same thing. You must understand that.

It appears you are attempting to justify buying an expensive purchase, now you know a better solution was about.

I have a vented 300L store from heatweb/DPS. Works ok.

It was built to order and we had it fitted with TWO plate heat exchangers. One feeds the master bathroom with an high flow rate shower (not very green sorry). The other feeds the ensuite shower and rest of the house. I run the store at 50-55C (middle of the tank) and mix down the DHW to about 44C.

I've measured 30l/Min out of the hot bath tap. In theory that means the two plate heat exchangers are capable of 60L/min for a short time!...but I doubt the mains is actually capable of delivering that flow rate.

Heat is provided by an oil boiler which is a little too big. When first installed I did have problems with it cycling even with the dial right up. This I fixed myself by upgrading the boiler pump to a faster model to keep the flow temperature down below the boiler cut-off. No cycling at all now and the boiler is fully controlled by the store stat.

Just to pick up on the title of this thread.. I'd say there is little benifit having a store if you only have a gas boiler feeding rads. The benifits are greater if you have an oil boiler (as those can't modulate down) or multiple heat sources.

<<Plate heat exchangers are so efficient you can have one hand on the return pipe to the cylinder as it will be around 20-25C and unable to touch the top pipe into the plate. They extract so much heat.>>

This would seem to imply that the plate heat exchange can heat the DHW to a higher temperature than in the Thermal Store. How does it do this?

As far as I can see Tom has been perfectly polite, but because he has a different view from most (some of whom has a vested interest - not that that is a criticism) he is getting slagged-off, come on guys be nice, differing views are still valid.

In fact in the terms of < 250 litre TS's while accepting that they do have many advantages, nothing I have ever read would indicate that they are more energy efficient than a non-vented tank sized to support a comparative demand.

Brian

Thanks Mike,

I probably do have the wrong idea, but how can a heat exchanger with an input of 70C. heat water to 62 C wthout the return also being 62 C?

i am quite happy to admit that I am probably missing something

Brian