Green Building Forum - Are thermal stores pointless?

Are thermal stores pointless?

Bowman — Wed, 11 Feb 2009 14:18:14 +0000

Stratification: Hot water rises so (in theory) the top half of a thermal store could be at 80C while the lower half is at 20C, these layers (strata) have a well defined boundary. The advantage of this is that for any given amount of stored energy, that energy can be extracted at a higher (more useful) temperature.

Are thermal stores pointless?

ralphd — Wed, 11 Feb 2009 14:24:58 +0000

Posted By: Water Systems
Posted By: tom.harriganMeanwhile...
[...]
High temperatures = high losses - small thermal stores cannot be as efficient as an unvented tank.
Smaller usable water volume - small stores are less space efficient than unvented cylinders.
Lower flow rate - small stores cannot match an unvented cylinder.

You must be trolling, thick or having a laugh.

This forum is not for personal attacks.
I know it is a lot of work for the forum moderator to police. I think some form of member enforcement could work. I don't have a specific solution; just thinking out loud.

-Ralph

Are thermal stores pointless?

ludite — Wed, 11 Feb 2009 14:41:28 +0000

I don't need to know what 'trolling' means, (could someone please whisper what it means?) to know that it is an unpleasant turn of phrase. ralphd is correct, we should all monitor threads and help 'admin/moderator/keith' out.

Are thermal stores pointless?

Bowman — Wed, 11 Feb 2009 15:09:29 +0000

Google "guide to trolls"

who's that stamping over my bridge!

Are thermal stores pointless?

cookie — Wed, 11 Feb 2009 15:12:03 +0000

So you when a thermal store is used with a pumped heat exchanger you extract more energy out of the water and return it to the bottom of the tank at say 20C where as with a cylinder with a coil you can only extract water till the tank is down to say 40C so in effect your using 20C more energy from the volume of water before further heating of the water... so you get a longer shower then you would in a conventional tank of the same size.

If that makes sense?

Cookie

Are thermal stores pointless?

Bowman — Wed, 11 Feb 2009 15:25:40 +0000

Cookie, pretty much, but to complicate things there's coils and there's coils.

I suspect that an big issue with stores/bank/accumulators/buffers is that there are a vast array of types, combinations, sizes, setups etc etc. Now this will really annoy some people, there are even unvented stores, or maybe these are just very large unvented cylinders.

I think both Tom and WS are being pedantic in their opinions, neither are wrong as such, but there is huge middle ground that neither seem recognise.

You could argue that any cylinder is a thermal store - they all store thermal energy after all.

Are thermal stores pointless?

cookie — Wed, 11 Feb 2009 15:49:31 +0000

Yeah I assumed a cylinder was a type of thermal store... and assumed a thermal store was a fancy name for using the buffer of stored energy in a tank of water.

I think the key here is how you extract the energy and how you put the energy in... :o) oh and you should be commenting on my thermal store / bank design thread so I can get a nice PDF created for anyone to use and we could all agree on it!! or not :o)

or as my Mum would say... love love love love when things get heated, or perhaps we should let the water cool down before we jump back in... or its a bit tepid in here... its a bit frosty or or oh I could go on :o)

Cookie

Are thermal stores pointless?

Mike (Up North) — Wed, 11 Feb 2009 21:11:55 +0000

Hi,
I pretty much always use the term thermal store these days. They are all storing thermal energy and all giving it up in different ways for different usages.
After that I prefer buffer & accumulator but there are many many more names and many are particular to certain countries. For example, just because we call it a boiler doesn’t mean others don’t call it a kettle! (cos that’s what it does) – literal translation.

My favourite translation recently is Thermic Flywheel – which again is just what it is. Dont think that one will catch on though - or will it ???

Another point – Crusoe touched on above – using both. Its not uncommon to find a large volume thermal store (Direct – sealed or unsealed) to be feeding the heating coil within a DHW cylinder (vented or U-V) just as you would expect the boiler feed to do. So I might have a thermal store supplying heat to my UV cylinder.
Then I suppose I would have to have an argument with myself on the relative merits of each of them.

Cheers, Mike up North

Are thermal stores pointless?

crusoe — Thu, 12 Feb 2009 00:10:36 +0000

Ee bah gum, we're clipping along at a fair rate now...

To our cousins across the pond: This thread moveth somewhat quickly.

Gosh guys, in the short time available to me after work (I am self-employed) there is NO way to answer all your questions, even forgetting responding to non-personal attacks ralphd!

Ralph, with the greatest respect for your possible nous as a 'software developer', either stick to where you are in your depth, or concede error gracefully. To say that you didn't change your mind, having bemired both feet by claiming UV were 100% safe and needed no maintenance, and we should fit them over here (are you an exporter by the way?) would be to call Mr Obama caucasian, to wit:

" The statement that periodic checks of the equipment are essential for safety are wrong, at least for North America." I pointed out that North America experienced problems from time to time, and you said "I challenge you... (pistols at dawn, etc etc yaawn...)" I was hurt, and surprised, at your challenge, having given you the url to the latest in a series (not one as you tried to claim to WS) of problems (trad British understatement here Ralph, but even we phlegmatic Brits don't take kindly to the side of the house and half the roof departing rapidly, especially if it interferes with essential somnolentaic maintenance (dictionary a/c to crusoe, don't bother googling it)) with NA unvented water heaters, which, according to the authorities, should be maintained to avoid these kinda troubles.

Next, Paul from yourneckathewoods kindly donated a few urls of his own, showing these tanks still exploding. We are going to see a lot more of this. My opinion. Your subsequent comment 'now I accept' said it all to anyone watching this thread. And you wonder why I use sarcasm. Go ponder.

James Norton - Low flow saves all. Pass Go and collect 200 euros (£ in decline right now). Your comment is probably worth reflecting on as, yes, in a carefully-designed TS we CAN exceed DHW flow rates of combis and unvented (while giving rads or UFH to boot). But why should we? This may be an essential difference in approach of WS and myself? It's late. Can we debunk a few theories I don't go along with?

I mean, we spend lots of lolly on special woodburners, ASHPS, GSHPS, Solar Thermal, PIFCAS, ZORGi-COGs and other creations I may invent in the near future (the PIFCAS and ZORGi-COGs are by way of hyperbole before you google them ralph, and don't exist - yet - to my knowledge, or at least not on this planet....Douglas Adams may beg to differ here) to SAVE FUEL, CO2, energy and things we believe are hurting the planet AND (!!) costing us money.

Then, we reinforce our shower walls with Zylocrete, toughened glass panels using space-program-derived NASA boroscilicate, extract huge amounts of heat and steam with fans, strap ourselves in and get flagellated half to death by a shower with an appetite for aqua so voracious it makes a bath seem positively anorexic.

Other than the fairground experience of these devices, which I can get more cheaply on the Wall of death or Oblivion, I miss the point. I have to close my eyes and am less clean than with a more moderate shower which doesn't be-dimple me in places unmentionable. Your pumped power-shower's (or unvented's, or Thermal Store's) ability to extract your hard-earned energy from your thermal whatever and chuck it wholesale down the drain leaves me cold. I joined CAT to get me in the mindset to design appliances to avoid this kind of thing as it's analgous to getting the most fuel-efficient car going (Blue-motion Polo maybe?), getting to Asda and leaving the engine running while you lovingly search for the newest Beaujolais and Port Salud.

I must agree with Tom... (arch-obfuscator, and he of the 'I try not to muddle things up' genre)

(ok, ok, a historic moment here folks, raise your glasses) ...who, despite his protestations of innocent amusement, appears to have a deep and dark agenda. (I believe, moderator/admin, that he is in cahoots with ralph to import illegal US-spec unvented to the UK - can't otherwise understand why he won't post his (legally required) approvals of the system he clearly so lovingly cherishes and wishes to persuade others - on three threads and counting - to employ. Please, when you have time off from spanking WS, one of the few sane voices on the subject, do check this out...)

...that after my experience fitting small (300 litres or less) TSs, I wouldn't have one for combined Htg + DHW.

Ooh la la. Cat, pigeons, where are you? Gleeful rubbing of Tom-type hands...

Problem I have come to realise is that while we like our hot water hottish - at, say, 60C to allow a bit of overnight decay and still allow a hot shower next morning, the UFH will pootle along, if regularly topped up, at 35C or even less in most weather. It's like warm beer (a Brit thing, NA cousins) and hot tea....BUT, in a typical TS, we insist in storing both in one flask. As we drink less tea than beer (?) I would reasonably suggest that storing them in separate flasks might be beneficial and help preserve essential characteristics of both.

Which solves a lot of the stratification, laddomat-type issues. Cos I don't like using energy to stratify when I don't need to. Or using energy to employ a x-flow plate exchanger pump when I don't need to.

Yes, your pressure might be lower. Pressure vs flow is another debate, but give me adequate flow every time.

End of mini-rant. And no, I won't play into Tom's hands and argue the merits or de-merits of my view with WS or N E Other professional, as I understand that they hold viewpoints which, within the TS market, differ from my own. I understand that, like unvented, in certain circumstances, they have a place. But I have moved on, and am pursuing, where possible, a powerdown, natural (read low energy) approach in my system designs.

You can like it or lump it, but before commenting, whatever your poison, at least try to understand it.

Jezza22 I installed Faral Tropical sectional-aluminium Italian rads (interesting joining tool) for a customer on an IDM-energie heat-pump with 1000 litre store which also has 6m2 of ST. Rads were disappointing. Probably because they were upstairs, and the contrast with the ground floor UFH was so great. Personally I wouldn't go unvented here - maintenance issues more than safety in this case - I like genuinely low maintenance appliances - but a small TS (layered if you prefer) just for DHW near point(s) of use, fed by your solar and WB, plus back-up immersion on E7, which all divert, on DHW satisfaction, to the TS, merrily gathering waste solar all the while.

Not trying to avoid answering indivuidual questions cookie et al, but as you note, a general thread like this is just that...general. For more detailed stuff, e-mail me and I'll try to help.

Regards

Crusoe

Are thermal stores pointless?

SimonH — Thu, 12 Feb 2009 01:27:26 +0000

Posted By: BowmanStratification: Hot water rises so (in theory) the top half of a thermal store could be at 80C while the lower half is at 20C, these layers (strata) have a well defined boundary. The advantage of this is that for any given amount of stored energy, that energy can be extracted at a higher (more useful) temperature.

Eh? I Previously asked ...
From a cold start - the bottom of the tank will be at cold water supply temp - say 10 degrees. So the heatpump/solar supplying 35 degree water can provide a 25 degree uplift. Now that heat will stratify - but how fast and will the bottom of the tank remain at say 20 degrees for a fair while allowing the 35 degree source to still provide a 15 degree uplift. Over time an even with stratification effects the bottom of the tank will reach 35 degrees, and therefore not take anymore heat. At this point will the top of the tank be 35 degrees? I suspect not, but will it be 40, 50, 60, 70 or 80 degrees?

You said this...

Posted By: BowmanStill no I'm afraid, you'd eventually end up with a whole tank at 35C.

The example you give above means that the 35 degree at the bottom - WILL cool, as the heat has risen. So the heat pump/solar can contribute some more. It's for this reason why I've always assumed the solar input is at the bottom of a tank - so it can go on adding heat. Even when the top is at say 50-60 degrees - the bottom may still be only 25, so the 35 degree from the solar can add a 10 degree uplift.

Are thermal stores pointless?

SimonH — Thu, 12 Feb 2009 01:52:21 +0000

Here's a great link showing how stratification works - real stores in operation - showing how the stratification is affected as water is drawn off in 4 different models. With model B - featuring an external heat exchanger looking like a winner - if I'm reading it right.

http://www.itw.uni-stuttgart.de/ITWHomepage/TZS/Literatur/TZS_10.pdf

Are thermal stores pointless?

Bowman — Thu, 12 Feb 2009 08:32:46 +0000

SimonH, there is no law of physics which allows you to put 35C water into a pot of 10C water and end up with water at 50C? Temperature and energy are not the same thing, a bit like velocity and momentum are not the same thing, in the example I gave the input would need to be at least 80C, and if you just keep filling up with 80C you would eventually end up with a tank full of 80C.

Are thermal stores pointless?

tom.harrigan — Thu, 12 Feb 2009 08:51:38 +0000

At last some facts, and not a word of abuse hooray!

Just skimming the excellent document from the University of Stuttgart, certain points stick out:

1. The stores are ~3x the size of the ones I think are pointless. (and probably cost well over that)
2. The storage temperatures are reasonable (unlike the ones I think are pointless)
3. If you want hot water at 50C, none can match a cylinder half their size.
4. If you don't mind hot water at 43C, then you can get loads of it from a store with an external heat exchanger!

The paper clearly demonstrates that stratification is disrupted in stores with an internal heat exchanger. I would be very interested to see what happens when the stores are partly depleted, say overnight. I suspect the temperature profile of these stores will become practically uniform.

The store with an external exchanger preserved stratification well during discharge, but was only capable of providing "hot" water 17C below the fully charged temperature, at a flow rate of 10l/min! Even more impressive is the fact that it was able to deliver 900l of warm water, when it was only 750l big!

Are thermal stores pointless?

jon — Thu, 12 Feb 2009 09:37:55 +0000

Yes, that looks like a useful document. I've put it aside for a read later. Thanks Simon.

Are thermal stores pointless?

SimonH — Thu, 12 Feb 2009 09:53:56 +0000

As I recognised store D - I've just checked their pricing - about £2,500-£5500 depending on the size you go for - in the range 500-1000l. Or you can go bonkers and get the 2,200l for just short of £7000.

I think you mistook the drawn volume with tank size - it doesn't state the tank size anywhere (which is a pain). My guess is they used the typical 500l tank (as it's cheapest).

Now... I just hit the jackpot when it comes to acedemic research on solarcombi thermal storage systems...

http://www.iea-shc.org/publications/task26/index.html

With explanatory notes here...
http://www.fys.uio.no/kjerne/task26/pdf/newsletter_03_english.pdf

Basically this is research funded by the EU ALTENER programme - usually requiring 3 or more participant countries to get involved in project that increases the understanding of alternative energy in some form.

And don't miss their other reports here...

http://www.iea-shc.org/publications/publications.aspx

By the look of that lot - i'll see you in a few months

(I spend a long time reading up on any stuff that costs more than a months salary - probably at least as long as it took me to earn it in the first place! )

Are thermal stores pointless?

tom.harrigan — Thu, 12 Feb 2009 10:24:11 +0000

The effective store volume Vs is given in the table, maybe not the actual volume, but good enough for me.

Are thermal stores pointless?

Mike (Up North) — Thu, 12 Feb 2009 12:14:05 +0000

Hi,
Some non technical observations on the above report: Combi being the term used for combined / integrated DHW and space heating within a thermal store

i) Test was carried out for a consumer magazine, in the uk consumer magazines seem only to deal how big is your plasma etc
ii) From 10 Combi thermal stores 4 were chosen, suggests a far greater variety which suggests a well developed market.
iii) Report states about 1/3 of solar based heating applications are Combi store type. Which again suggests a well developed market.

This suggests that the product is freely available with plenty of choice of suppliers, with slightly different operating principals but all are valid, it’s not considered quirky or esoteric. It could be seen to be the norm.
Here in the UK we seem to be still debating even the very existence of such systems, their plausibility let alone their technical abilities. A great many installers of heating and water supplies will tell you “no such thing” which translates to “I don’t know about it so it does not exist”.

The attached link (district heating symposium Sept 2008) is for a report dealing with small T/S with attached to a PlateX for hot water from a district heating system, could equally be charged from a boiler. Normally district heating powered DHW does not have a store at the house hold it would feed straight the PlateX for indirect preparation of DHW, the space heating could be direct or indirect as appropriate.

www.dhc2008.hi.is/session/greinar/p20_Paulsen.pdf

There are a lot of interesting papers out there, and companies (eg Danfoss and many others) are sponsoring studies and post grad work on DHW preparation, space heating etc etc. I would be interested if any other links to similar sources from UK based establishments, I would hope there would be some, I suppose its a case of where to look. Hopefully, we don’t find too many post grad research programs in “leisure centre management” (i.e. Brittas . . .), but instead studying something that might help us.

Cheers, Mike up North

Are thermal stores pointless?

Johan — Thu, 12 Feb 2009 12:39:58 +0000

Mike, Excellent post!

I concure. Combi systems are the norm in Germany, Austria and Scandinavia. UK is severly lagging behind here, which is probably why the discussion is like it is...

Thermal stores are not pointless, but a a 200-300l thermal store is questinable. It's really just a bloated hot cylinder, no?
Anything >500l starts becoming more useful as a TS...

Price wise, I paid just over £1000+£300 shipping for a 750l tank. If you want one be prepared to have it shipped from Germany or one of the Scandinavian countries.

Are thermal stores pointless?

andy500 — Fri, 13 Feb 2009 09:58:35 +0000

<blockquote><cite>Posted By: crusoe</cite>

Please, when you have time off from spanking WS, one of the few sane voices on the subject, do check this out...)

...that after my experience fitting small (300 litres or less) TSs, I wouldn't have one for combined Htg + DHW.

Ooh la la. Cat, pigeons, where are you? Gleeful rubbing of Tom-type hands...

Problem I have come to realise is that while we like our hot water hottish - at, say, 60C to allow a bit of overnight decay and still allow a hot shower next morning, the UFH will pootle along, if regularly topped up, at 35C or even less in most weather. It's like warm beer (a Brit thing, NA cousins) and hot tea....BUT, in a typical TS, we insist in storing both in one flask. As we drink less tea than beer (?) I would reasonably suggest that storing them in separate flasks might be beneficial and help preserve essential characteristics of both.

Which solves a lot of the stratification, laddomat-type issues. Cos I don't like using energy to stratify when I don't need to. Or using energy to employ a x-flow plate exchanger pump when I don't need to.

Crusoe</blockquote>

I fear you may have my 'sense of humour' down to a T. My wife isn't so complimentary.

So who wants my store for scrap value then?

BUT, if you use a blending valve, as most ufh kits do anyway, you can nick a bit of 60 degree, sorry Tom, 80 degree water and blend it down to whatever floats your boat/floor. Store gets stone cold water back in, flow out and destratification is minimal due to recirculation thru the blender. I have rads, so doesn't apply here.

One store/cylinder/buffer must be better with the appropriate controls - oppose that with one store for ufh, and a separate store/cylinder/unvented bucket for hot water - heatlosses for two cylinders must be worse than one, shirley? And you would have hassle heating your house with solar doing it that way too.

Space is a big problem - to keep the store within your heated envelope, instead of heating a shed in the garden, a point I personally think is seriously important from an energy point of view - size matters.
I've got a boiler, a store, an accumulator, an expansion vessel, zone valves/pumps, plate X, solar pump station, laddomat and now 1000VA UPS as well, in the floorspace previously occupied by a bath. There's no room for even the soap now! 500 litres would mean a no-go. 1000 litres would be divorce. Or a warm shed. Trade-offs are essential in every retrofit. Clean sheet, I'd have a monster eg here - [url]http://www.navitron.org.uk/forum/index.php/topic,5875.0.html[/url] , but it'd have to be indoors.

Note personal experience so far to allow such technical input includes two combis, an open vented system with some broken shower pumps, owning and spec'ing one store and no store fitting experience at all, just some tea-sipping and mouse-clicking so I reckon I may be out of my depth in present company

Are thermal stores pointless?

andy500 — Fri, 13 Feb 2009 09:59:38 +0000

Now why hasn't the quotes worked? Argh, computers!

Are thermal stores pointless?

Water Systems — Sun, 15 Feb 2009 15:22:21 +0000

Posted By: tom.harriganAt last some facts, and not a word of abuse hooray!

Just skimming the excellent document from the University of Stuttgart, certain points stick out:

1. The stores are ~3x the size of the ones I think are pointless.

You keep on with this inane mantra. You have been asked what configuration these thermal store are you have reservations over. So far you have not. If you do I can help you with your understanding, however I doubt I will succeed But I'll try as some people might actually believe you.

The store with an external exchanger preserved stratification well during discharge, but was only capable of providing "hot" water 17C below the fully charged temperature, at a flow rate of 10l/min! Even more impressive is the fact that it was able to deliver 900l of warm water, when it was only 750l big!

The plate heat exchanger was clearly undersized. As Andy and others have displayed, you can get DHW at 5C less than the heat bank setpoint.

Are thermal stores pointless?

Water Systems — Sun, 15 Feb 2009 15:34:11 +0000

Posted By: BowmanI think both Tom and WS are being pedantic in their opinions, neither are wrong as such,

I am not into opinions. I am only interested in fact. Tom is very wrong indeed as he is basing his mantra on opinion.

Are thermal stores pointless?

Water Systems — Sun, 15 Feb 2009 15:41:45 +0000

Posted By: Mike (Up North)Hi,
I pretty much always use the term thermal store these days. They are all storing thermal energy and all giving it up in different ways for different usages.
After that I prefer buffer & accumulator

thermal store = coiled DHW take-off
heat bank = plate heat X DHW take-off
Accumulator = a cold water storage vessel to improve flow rates when cold mains have pressure but no flow.

but there are many many more names and many are particular to certain countries. For example, just because we call it a boiler doesnt mean others dont call it a kettle! (cos thats what it does) literal translation.

The official name is a water heater. That is the term used in exams. It doesn't boil.

Are thermal stores pointless?

Water Systems — Sun, 15 Feb 2009 16:17:16 +0000

Posted By: JohanThermal stores are not pointless, but a a 200-300l thermal store is questinable. It's really just a bloated hot cylinder, no?

No. A heat bank is far from pointless, please do not be influenced by people who do not know what they are on about. Do not underestimate the CH buffering, boiler anti-cycling and enhanced condensing efficiency when using a condensing boiler.

Are thermal stores pointless?

Water Systems — Sun, 15 Feb 2009 16:34:29 +0000

Separating the DHW and CH functions can be done cheaply, and as in an integrated heat bank gives the same benefits. Having a stainless steel 80 to 100 litre CH buffer cylinder is a great advantage - no boiler cycling, etc. A 200 litre heat bank cylinder for DHW. The two can be on the one unitstrut frame with the DHW over the CH. Using a Broag gas "dual temperature boiler", a 3-way diverter valve, controlled by the boiler, can be use to heat the DHW ASAP, and the temperature of the CH buffer controlled the boilers internal weather compensator. Both at separate temperatures and one does not influence the other. Both cylinders will give very low return temperatures.

The CH buffer can have a Smart pump and TRVs all around. Still has instant heat in the morning.

Well, a well designed integrated heat bank will not have one influencing the other anyhow.

But when using other heat sources as well, especially solar and the likes, another low temp cylinder can be used. When the temp inside the solar cylinder is higher than the CH or DHW it pumps its heat into either.

Are thermal stores pointless?

Willrob — Thu, 12 Mar 2009 15:25:26 +0000

So I've read 7 pages of stuff, some of it constructive.... but I am not much closer to forming a clear view on what type of system I should go for. I had a mains pressure unvented cylinder in my last place & we were pretty happy with hot water flow, pressure & recycling times. Tried to see past the 'potential bomb in the airing cupboard' issue although I could never completely put it out of my mind (thought it best not to mention this to the wife).
However the newscenario is a Victorian terraced house with an 8 year old ailing Worcester Bosch gas combi boiler located on the ground floor (in a kitchen cupboard). Want to change for a new condensing unit, but getting decent HW flow is the priority and I am not keen on another combi. At the moment, running a bath takes so long you've forgotten you wanted one by the time it finishes....
The only place to site any tank is the basement (conveniently, also where the mains water comes in at a reasonable 2+ bar). The loft has been converted so there is no space for tanks up there & I'm not really interested in tanks upstairs, with the pipe runs that entails anyway. There is no gravity fall for unvented discharge & (I understand) this causes complications, requiring stainless steel sump tank & automatic pump to drain to comply with Building Regs if I go unvented (Megaflo etc.). These extra bits are not just mega-flo but fairly mega-cash too apparently, on top of the cylinder & plumbing work.
I'm intrigued by the thermal store idea. However, a decent set-up (say 250 litre size - max. possible height is 1.7m, with 160KW plate heat ex.) is not going to be cheap either. As mentioned I'm not interested in F&E tanks upstairs, so it will need to be a sealed system, either heated directly (so that the store & rads are on the primary circuit, with appropriate diverters) - is this possible? - or as seems more common, heated indirectly by a coil.
All help appreciated.
PS If I do go for a new boiler that is not a system boiler (i.e without integral expansion vessel inside the casing) & indirect heating of the store, can the expansion tank for the sealed primary - boiler - circuit go in the basement (along with the store & its expansion tank) too, or does it have to be above the boiler?

Are thermal stores pointless?

Mike (Up North) — Fri, 13 Mar 2009 16:20:44 +0000

Hi, Will,
Firstly decide if the store is going to cover just DHW (ie just like any other cylinder) or DHW + Heating
For the system you describe to be sealed so that the boiler - store - rads are all primary it would ahve to be a steel store rather than copper and these tend to start around 500lit, although there are 300 & 400 ones on the manf specs they dont tend to be so common. At 250lit un-sealed it would be copper. having a sealed loop for heating from a unsealed store (boiler + store) is ok except that its is not a very efficient way of transfering the heat. The boiler & store can ve un-sealed if in the celler so long as the feed & exp tank is above them by about 0.5m approx, its the radiator circuit to the upper level which is a problem. You would have to use a "another" plate heat exchanger to link the sealed and unsealed systems.
The sealed expansion vessel can go elswhere although it is usually near the boiler.
Cheers,
Mike up North

Are thermal stores pointless?

Willrob — Mon, 16 Mar 2009 18:02:24 +0000

Thanks Mike

I am starting to crystallise my thoughts... I gather that having a a store / heated directly on a primary circuit (i.e boiler, store & rads all sealed & therefore slightly pressurised ) would still be caught by the unvented Regs even though the pressure is probably 1 bar, not 3 or more as you'd get on an unvented storage cylinder.

So I will be heating the store indirectly by coil or plate heat exch from the boiler circuit and running the CH off the sealed primary circuit. Not sure whether I need the faster recovery & complexity of a plate heat exch. on the store input side - most seem to think a decent coil will work? Any views?

As I will have the CH circuit running off the boiler (primary) circuit - (Y plan?) the store will only be used for HW. Seems you can only use a store for CH in a vented configuration, which is not an option for me.

Any views on whether I would be better with a system boiler & diverter valves located with that in the kitchen cupboard (boiler location) or use a 'heat-only' boiler & have the primary circuit kit i.e circulating pump / expansion vsl & CH diverter located adjacent to the heat store in the basement (probably 10 metres & down one floor from the boiler to the store location in the basement)?

All help appreciated!

Are thermal stores pointless?

Mike (Up North) — Tue, 17 Mar 2009 09:09:22 +0000

Be clear on what is on the primary side and what is on the secondary side. For DHW the secondary side is the potable i.e. hot water. This is secondary (in-direct) tap water that is heated by a coil in Domestic Hot Water (DHW) cylinder. This is the other way round in a thermal store â€“ i.e. a DHW only store. The heat comes from the boiler primary but could also be coming from a thermal store to a separate DHW cylinder â€“ lots of combinations. Or the thermal store could include the entire primary, but then the DHW would be heated in a coil in the store or a heat exchanger. The term vented or un-vented (UV) applies to the DHW potable side. If un-vented the tank of hot drinking water is at mains pressure when heated (by whatever means) which is potentially dangerous hence the control. If DHW is heated in the coil of a thermal store the DHW is within a pipe (not a vessel) so thatâ€™s different design considerations /Regs. An vented DHW cylinder has a 50 gal tank in the roof that supplies it with the necessary flow/pressure of potable water. In this case the heat is supplied in the same way as for the UV.
Try to stick to the term vented/un-vented for the DHW side and sealed/un-sealed for the boiler primary side. This is not set in stone but then we/you wont gey mixed upp as you can have pretty much any combination in between but they are different combinations.
As to using the store on a sealed primary you can use the store on a sealed situation it just means a bigger expansion vessel and would have to be steel, the limitation you describe would be for copper. Which would be OK size wise for a DHW only store.

Cheers, Mike up North

Are thermal stores pointless?

squattermadras — Wed, 01 Jul 2009 00:06:33 +0100

Totally confused by all of this, so many differing opinions.

I just want the most efficient/uncomplicated mains pressure hot water system for my 3/4 bed home with a bathroom/shower and seperate shower for 3 occupants - how difficult can it be?

Tank needs to go in the loft (insulation is important)and work with my normal open vented c/h system which has a 32Kw condensing boiler. I'm not bothered about attaching any further heat inputs apart from perhaps a solar coil, but from what I can see so far, solar heating payback is rubbish, but i guess thats another story.
Loft height is limited to 1500mm. I would rather do without a separate F & E tank if I could.

I saw a technical website saying that you need at least 2 metres height for a thermal store in order to achieve stratification and yet they seem to be available at much shorter heights. Does this mean that shorter ones don't perform well? Also what volume do I need. I am thinking of also adding a solar coil, but from what I can see so far, solar heating payback is rubbish, but i guess thats another story.

I don't understand how it is that Thermal stores are meant to be simpler - unpressurised vessel etc etc. than Unvented systems and yet cost a lot more?

There just seems to be so much disinformation, unless I'm askimng the wrong questions. Water Systems posts seem very convincing in favour of thermal stores and are easy to understand, thanks. But I don't buy the exploding unvented tank scenario - I don't doubt they do explode on occasion but aeroplanes crash and so do cars, that doesn't stop me flying or driving and I'm sure that in the UK we exceed any sensible safety requirements anyway.

Thanks in advance for any help