I think WS means a store will store solar energy for longer/hotter than an unvented. I reckon either would take the super-hot summer solar heat as well as each other, assuming everything else eg prv, temp relief valves etc are equal.
Normal running temps are what was discussed above – and the OP is mostly wrong on his various arguements.

So, "Is a thermal store pointless?" Damn right it is, if it’s badly designed. A 250 litre stumpy store, boiler coil heated, with a piddly hot water coil is next to useless.
Give me an unvented any day of the week.

Now something tall and thin, heatbank shaped, hooked to everything directly, with a plate heat exchanger for the hot tap – now that’s a different kettle of water.
I'm only answering because I have one (a heat bank, as well as a kettle), and did my research before leaping. Probably only about my second post on here in six months!
In turn, as OP stated ....

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.

High temps - Wrong. High temps are recommended if the hot water supply side is underdesigned. My store is 65-70 degC at the top, maximum, boiler directly linked to the store water with no coil. Gives a happy condensing boiler! Bung a condenser onto an unvented coil, and see which is better!
Hot water is great, even in current freezing temperatures. So there goes the argument that a store will lose more energy due to higher stored water temps. I’d counter that it’s the same as an unvented, or any similar temperature body of water, but it is likely a store may have more tappings, so assuming each pipe in/out loses a bit of heat, chances are a store may kill a few more polar bears than an unvented.

Cycling – Wrong. One reheat stat is useless – you need two, to stop the odd current of water switching the boiler momentarily. If one of my room stats calls for heat, chances are the boiler will not fire. Only when the store drops in temp by a good 20 degC does the boiler come on. Rads are hot, hot water is hot, and the boiler only fires when either rads or taps are close to underperforming – NOT firing the boiler for five minutes every time a door wafts cold air against a roomstat. Boiler buffering is a good idea. Best bit about the store I reckon (apart from the hot water).
On the flip side, run your hot tap for five minutes, and see if your boiler fires up – bet it does. Half the time mine won’t.

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.

Wrong. Not if you have a heatbank with a big heat exchanger. Under-spec’ing an internal coil will end in tears. If you have a small surface area hot water coil, you WILL need near boiling store temps to warm your mains water. I have a heat exchanger about the size of a playstation 2, a circ pump only set to speed 1, and the hot water is HOT.

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.

Not 100% sure on that one. Surely the unvented would have the conduction and convection thing going on too?? Heat rises, mine works ok, boiler gets fed the coldest water available. The coldest water (30 degC) is LEAVING my cylinder, not being fed INTO it (5 to 10 degC in winter?).
BIG problem is stratification if your store/bank/cylinder is underdesigned. If I shower with the heating on too, then have a second shower or a bath, the store gets a bit stirred up, and I’m left with a tank of tepid water. But the spec of the hot water supply bits means the hot water is still hot. Design it right, and stratification becomes largely irrelevant, as long as the boiler is getting fed a good slug of stone cold water to play with.

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

Ding – 1 point (please clarify as it may invalidate your home insurance…). BUT - if you were going to be worried about a cylinder going pop despite expansion vessels and relief valves, which would worry you the most – one which is set to about a bar and a half, or one that depends on the water board for it’s pressure control before you start heating it (close to 4 bar mains here, at night).

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

Ding – 1 point each. Sort of. Combining, as I plan to, wood burner, solar and gas boiler for all three to heat space AND water isn’t do-able without, as WS puts it elsewhere, a neutral point. How do you heat your radiators with a solar panel? Thermal store. Chuck on an ASHP and a plutonium drive and you’ll still need a ‘neutral’ point.

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

Correct – it would look daft halfway up the stairs.
My store is pressurised to about 1 bar, same as a sealed radiator system. Will go anywhere an unvented can. No header tank, just an external expansion vessel, which you’d have on a sealed rad system anyway.

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.

Fair point. 1 shunt pump, 1 heat exchanger, 1 flow switch. Oh, and an extra cylinder stat. Woooo scary! Now rip into a combi boiler and see what’s in there for the hot water section. I suppose the unvented wins. Ding.
But note I’ve just blown £95 on an ebay uninterruptible power supply that’ll run everything (rads/boiler/shunt pump/zone valves/stats) for a good couple or four hours or so during a power cut. Have to say, I can’t wait to test it :smug grin:

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

Wrong. Wrongity-wrong-wrong-wrong. What goes in is what comes out.
Get the mains flow up to spec, and a proper hot water design spec, and both perform fine. Carp in equals carp out.
Yours is an OSO? So you may have a duostream with an accumulator. I too have a 250l accumulator bearing down on my heat exchanger, and the hot water is silly to the point of painful. As would it be combined with an unvented cylinder. One point to note is when it’s very cold weather, hot water supply *may* reduce, as the incoming water temp reduces, same as with a cheap combi, but if it’s designed right, you won’t notice. We don’t.

I will concede a smallish, say, 200 l store hooked to 15 rads will not exactly buffer as well as a 1000 litre monster, but it does, at least with twin stats, stop the boiler kicking in ALL the time. Taking the rads off the middle of the store leaves the best half of the energy for hot water – from experience it’ll run a shower and keep the rads going at once, with the boiler kicking in after a couple of minutes, then keeping going until shower and rads are done and the store is recharged, ready for the next time the front door is opened and that damn roomstat kicks the heating on for 5 minutes again.

If we move house, I wouldn’t have a combi again, ever. Not if you paid to fit it for me. If you pay the gas bill as well, we’ll talk.
I wouldn’t have dumb shower pumps and open vented hot water again either.
I might consider an unvented cylinder but only with an accumulator unless mains water flow was perfect. And even then the boiler cycling would annoy me.
And if I wanted to use more than one heat source, I’d be sticking with a thermal store.
BUT if I couldn’t find a good plumber again, I guess I’d be back to the unvented idea.

And breathe…

Andy500

A couple of other things:

Unvented cylinders do _not_ require an annual inspection, unless demanded by the manufacturer. Mine doesn't!

You _can_ incorporate a gas, oil, woodburner, electric heat source on a sealed system, and attach an unvented cyclinder. Solar would heat the cylinder.

Mine is 300 litres, stainless advance appliances. If you have an airing cupboard, fill it to the top, that's what I say
Dunno about recommending or slating different manufacturers - I might have seemed quite vocal, it's just I spent so long reading up on stores before I actually 'jumped'. Water Systems speaks sense, perhaps not very diplomatically, but it does make sense.
Advance were very good, DPS come with all the kit and it costs, other than that Navitron (who make vented stores anyway) forums are a good place to start for recommendations.
The plumber I found, who fitted the lot, is Nuheat approved so he had experience of their sealed system store used with underfloor heating - which is basically the store I have, with an external plate X and a couple extra sensor pockets. For less ££ than Nuheat as well

Re the hot water - the only thing with it so far is the incoming temperature from the mains. My heat exchanger (I think) was spec'd to do 30 litres a minute at 45 deg temp rise assuming store is 75 deg. Compare that to a combi!
Normally, it's around 65 deg max - 6" below the top of the store, where the nearest gauge is. In the summer, we were fiddling and were getting good results at little more than 50-55 degrees store temp - but that's with probably 15-20 deg mains water. Now it's 4 degrees or thereabouts as it comes in - so either a hotter store, or less flow (as a combi) to get equal flow/temp.
Even if you have 4 degC water, add 45 degrees to that, and that's a hot shower (at 30 litres a minute, which is a lot) - so once it's blended thru a thermostatic mixer, the store is doing less work anyway.
Throughout, we haven't had a real problem - which suggests the performance estimate was conservative or OTT depending on your viewpoint.
With a big plate X, the heat transfer is brilliant, so store water literally 5 degrees above hot water requirement will still perform. Coils may be harder to get to the same level of efficiency.

Charging the store, I have two stats with a relay, one at the middle just below rads flow - set to 45 degC, plus one lower down - set to 60-65 degC.
So the boiler will come on once the central space heating part of the store is close to useless, then only go off once the lower stat gets a slug of 65 degC water past it.
Hot water section at the top is always ready for action - you need to have two long full-flow showers with the radiators on as well in the depths of winter to get to the point where you might catch the store out while reheating, and notice a cool end to the second shower.

Re the breathing, I felt like my fingers were going to drop off typing that lot

S'funny, opinions on thermal stores really DO seem to have NO middle ground - love 'em or (maybe irrationally) hate 'em.

I think Andy500's post is the most compelling pro TS argument so far, but non-the-less I can see no definitive "proof" that TS's are more energy efficient - which is my main interest.

I would really like to believe in TS's but by gut feeling think that, despite their many other advantages, they use more energy for a given demand in the UK environment.

Is anyone aware of any evidence-based research that demonstrates the superiority of TS's in Northern Climes? Or even before / after energy consumption figures for domestic TS? In the latter case I guess we would need to factor-in the gains provided by the solar installation that are normally included with TS.

Brian

Bowman,

I think your point is valid and is my big concern about TS's. What do you do when the only heating requirement is for a shower?

Posted By: tom.harriganAndy500

Unvented cylinders do _not_ require an annual inspection, unless demanded by the manufacturer. Mine doesn't!

This is a dangerous statement. He hasn't contacted the makers yet. All Oso instructions state that a service is required. I advise people to ignore this sort of drivel, as that is what it is.

You _can_ incorporate a gas, oil, woodburner, electric heat source on a sealed system, and attach an unvented cyclinder. Solar would heat the cylinder.

You can, but it will be a very poor system. Anyone who knows of thermal systems would never dream of doing anything so asinine.

Posted By: tom.harriganAndy500

30l/min is good, but it's not as good as an unvented cylinder.

Stop prattling tripe!!!! You have been told a number times. So, by putting an unvented cylinder in, the cold main miraculously improves its flowrate. Yep!!! You couldn't make this up.

Two showers in the winter before your water gets chilly may be acceptable to you, but I think it may be totally unacceptable to others. It's nowhere near the performance of an unvented cylinder of comparable size!

More trolling. Please re-read the thread. The heat bank will acts as combi when the cylinder is depleted of heat - it runs from what the boilers burner can give. It NEVER runs out of DHW.

You have reinforced my view that small thermal (size now raised to 300l from 250l) are utterly pointless. You have a worse hot water system than an unvented tank in return for complexity.

Tom

I get the impression you are just trolling (or a complete idiot) or have a vested interest in selling unvented cylinders. The admin should have a word with you.

Posted By: jerseymanI think Andy500's post is the most compelling pro TS argument so far, but non-the-less I can see no definitive "proof" that TS's are more energy efficient - which is my main interest.

Cranfield University did studies and concluded that 15% energy savings can be had by using thermal storage. A Google brings it up.

The lower the return temperature of a condensing boiler the greater the efficiency. If you look at a gas condensing boiler graph, the boiler starts to condense at around 53C return water temperature. That is the key point. Just above and below doesn't make that much difference in efficiency. One the return temperature drops to 30C the efficiency rises sharply.

At 65C the efficiency is 87%
At 53C (dew-point) the efficiency is 89%
At 30C the efficiency is 97%
At 25C the efficiency is 98-99%
At 20C the efficiency is near 100%

So 10% more efficient using lower temperatures, which a heat bank gives. A cylinder heated by a coil will not get near the condensing efficiency re-heat figures of heating a cylinder directly.

Look at this:
http://www.keston.co.uk/downloads/pdf/cel25-b.pdf
There is a graph here (the boiler is now discontinued)

A heat bank using a plate heat X will give temperatures at the bottom of the store at 5C above the incoming mains temperature (see Any's post to confirm this). That could be 10-15C in the bottom of the cylinder, while 65-70C at the top. When the boiler kicks in the boiler efficiency is approaching near 100% on most of the cylinder re-heat. Have the heat bank set to 65C and the efficiency is beyond question.

I don't think anyone is against TS per se.

Just don't much appreciate being told by a hard selling no compromising aggressive salesman that they are the bees knees for everyone.
We all realise that just is not the case.

Here is an interesting article about someone experienced in thermal stores giving an objective view of their real performance.
http://www.housebuildersupdate.co.uk/2008/01/thermal-stores-witherspoons-vision.html

Clearly higher storage temperatures are an issue as heat losses will be significantly higher.

Also if you are using one for heating as well there will be an additional pump.

Posted By: jerseyman
I cannot find the article from Cranfield University, but what I was hoping to find were specific evidence based articles that demonstrate that TS are more energy efficient than Unvented Cylinders despite TS's needing 10% more water (source this forum) held at 5 C higher temperature (best case).

I am not anti TS - I want to believe!

Brian

Have you read this thread, especially my posts? I doubt it. a thermal store/heat bank is not an equivalent to an unvented cylinder It is much, much more. Only simple DHW stores are comparable. Yet with a vented store you save a £60 min' service charge each year, so they are ahead in running costs. £60-100 buys a lot of gas, elec or oil.

Of course it will have more water!! It is a CH buffer as well!!! Please understand before you post. What do you not understand ...ask.

Posted By: nigelI don't think anyone is against TS per se.

Just don't much appreciate being told by a hard selling no compromising aggressive salesman that they are the bees knees for everyone.

Who might these salesmen be? The only salesmen I can see are those pushing explosive, complicated devices than require annual service charges. They have not come out with anything concrete to justify such devices - just quarter baked repetitive opinion.

We all realise that just is not the case.

Who are these we?

Here is an interesting article about someone experienced in thermal stores giving an objective view of their real performance.

Clearly higher storage temperatures are an issue as heat losses will be significantly higher.

The article was complimentary towards thermals stores. Chelmer to my knowledge have never used plate heat exchangers on DHW - may be wrong there.

The Chelmer store is like this ACV. An unvented cylinder (or vented) inside a cylinder. Tank-in-tank.
http://www.acv-uk.com/slme.htm

As you have read, a store only needs the store temperature 5C above DHW setpoint (tap temp).

The Chelmer store is designed to run specifically with a heat pump. Running one with a store is pretty dumb really. The inner fresh water cylinder has to be run at over 60C (more like 65C as ACV stipulate) for legionella purposes.

Also if you are using one for heating as well there will be an additional pump.

Which is no great thing as they are cheap. Use a CH Smart pump and TRVs all around and no nuisance central wall t/stat. The pump uses lees energy when the TRVs close up as it modulates down.

If the temperature only needs to be set at 5c over DHW setpoint why do gledhill advise that in order to deliver water at 55c their store temperature should be set at 70c. FACT - taken from their product literature.Not inconsistent with Chelmers view that the tank needs to be at 80c

Thats a difference of 15c - 3 times the figure Water Services claims.

Gledhill also recommend setting the pump from the boiler to the store to run as fast possible so as to induce a maximum temperature drop of 11c.
So store temp should be 70c, less temperature drop of 11c so the minimum return temperature is 59c which is above the level required for a boiler to run in condensing mode.
So if you follow their guidelines you will NEVER operate the boiler in condensing mode.

Pumps may be cheap - but thermal stores need an extra one when used to supply space heating- which incidentally uses energy which reduces efficiency - perhaps for the sake of balance Water Services should add that to his list of disadvantages.

Viessmann sell many unvented DHW cylinders and there is no requirement for an annual service.

Seems Water Services needs to update his one sided blurb to something a little less biased and then we might take him seriously.

As been said (pay attention) Chelmer do not use plate heat exchangers.

Gledhill thermals store have a self learning pcb. They operate from 75C to 82C. They learn the maximum temperature given by the boiler and also its heat up period. So it will run at 75C as the pcb sets the store setpoint to suit what the boiler outputs.

A store may have an extra CH pump to an inefficient directly coupled CH system, which can be a Smart pump and TRVs all around giving far superior comfort conditions and lower pump operating costs as the pump modulates down reducing energy consumption. But the boiler pump is on rarely due to the anti-cycle merits of a heat bank/t store, so not an issue. The boiler is also in the ideal hydraulic environment unlike a boiler in a directly coupled rad system.

Pay attention, this has been stated - the Gledhill stores are designed primarily to run with non condensing boilers (11C temp differential). They are old hat, but will take a condensing boiler and operate with store setpoint of 75C. Turn the max temp of the boiler to 75C and the store learns this and set the store setpoint to suit. Gledhill stores are also designed to pack in as much energy as possible, by upping the store setpoint, to reduce the size.

Viessmann sell many unvented DHW cylinders and there is no requirement for an annual service.

I will not believe that until I see it in writing from Viessmann. Like most here who are not that technical, you are confusing an unvented cylinder with a heat bank/t store. They are not the same, a heat bank is far superior and does far, far more. You must understand that before you post. Read this thread and my posts and then you will understand thermal stores more. Get back if you are stuck. Do not dictate how they operate, ask questions were you are stuck. It is easier that way.

Posted By: DantenzJust like to add my penny worth re: unvented cylinder servicing. Now i know that the manufacturers instructions are the overriding document and all that but, I am of the opinion that ALL G3 approved unvented cylinders require an annual inspection/service. The extent of the maintenance may be minimal however, it is a requirement and must only be carried by an unvented qualified/competent person.

The maintenance is to test all the devices. That means simulating they operate, which takes time. Few do it properly.

Posted By: nigelSo lets summarise where we are at; Water Systems claims:

1. Thermal Stores are great, just dont use the Gledhill one its old hat - but does not tell us of any good ones.

Gledhill are quite good and a condensing boiler can run with them at 75C. It is clever enough to learn the max output of the boiler.

2. The required storage temperature is higher than he states and he produces no independent or manufacturers specs to demonstrate his claim.

You made that up. Selectrive amnesia again. Re-read the thread. Range, Gledhill, etc all stipulate less than 80C.

The quotes regarding Cranfields research refer to boiler efficiency but is not specific to thermal stores.

It is. Cranfield worked with Gledhill on new projects. Gledhill are on tick-over because of the credit crunch.

3. Thermal Stores need an extra pump- he concedes that at last but then adds a load of blurb to suggest it does not matter.

I don't think you knew what a Smart pump was.

4. There is no regulation that requires unvented systems to be serviced annually - if there is point us to it.

All the makers instructions. I have not see one that does not stipulate a service.

Water Systems therefore needs to produce some references to his claims otherwise we will continue to treat them the with sceptisism they deserve.

The claims are the other way and this thread is aimed at that. You and the troll on here are stating that thermal store/heat banks are inefficient, when it has been proven the opposite, providing superior heating systems all around - even by user of thermal stores who counter all you say. Back up your ridiculous claims instead of inane babble. You claim, no service, inefficiency, etc. You and the troll please back your nonsense, and assertion and repetition does not count as proof. I have even pointed you to uni research which states the opposite.

Water Systems seems to think he is technically superior but I think he is just a rude, arrogant, blustering salesman.

Technically superior? It is clear I am. Salesman? No.

His claims lack independent examination and despite being eminently technically superior he has produced no technical information to support his claims.

More repetitive personal attacking mindless tripe as all is actually in this thread - and I am not being rude.