Green Building Forum - Are thermal stores pointless?

Are thermal stores pointless?

Water Systems — Thu, 05 Feb 2009 20:36:45 +0000

Posted By: jerseymandiffering views are still valid.

Something which is factually incorrect is not 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

You need to read this thread more.

Are thermal stores pointless?

brig001 — Thu, 05 Feb 2009 20:54:47 +0000

jerseyman, they do it by reversing the flow. Water from the store flows in to the heat exchanger where the DHW comes out. The water back to the tank will be at the end where the DHW goes in.

store 70 -----> 20 back to store
DHW 60 <----- 15 from mains

If that helps.

Are thermal stores pointless?

jerseyman — Thu, 05 Feb 2009 21:24:35 +0000

<>

It does indeed, many thanks

<>

Thanks Water-systems .....

Ok re-read it all.

Nope I cannot see that a sub 250 litre TS uses less KW than to a Non vented cylinder.

Yes more adaptable heat input sources, Yes, no annual maintenace but following previous threads a 250 lire store at 70 c has the same usable energy as a 225 ltre non vented tank at 60 C.

Where am I going wrong?

Are thermal stores pointless?

tom.harrigan — Thu, 05 Feb 2009 22:47:47 +0000

Jerseyman,

My unvented cylinder does not require annual maintenance. It depends entirely on the judgement of the installer. An unvented cylinder contains its entire volume of usable hot water. You might be interested to know that the heat exchanger in my cylinder is rated at 24kW. That is a far more powerful heat exchanger than many thermal stores! It can recharge very quickly!

A thermal store produces its hot water from a fraction of its volume, often only half of it. Furthermore, it has to heat exchange, the rate of which is proportional to temperature difference. For a 250l thermal store, it is difficult to accept the equivalent hot water storage to be more than a quarter of its volume, and I think that is generous!

I also don't accept that a small thermal store offers more adaptable input from alternative heat source. Take solar for instance. Why not use that high temperature source for hot water production in the summer, rather than diluting the temperature with a store that cannot stratify, rendering it useless. The whole point of hot water cylinder sizing is that the high temperature water is used, not wasted.

It is unquestionable that high temperature storage is less efficient than lower temperature storage; unvented cylinders win. It is unquestionable that unvented cylinders offer better flow rates. It is unquestionable that unvented cylinders offer more space-efficient and flexible hot water storage than thermal stores. If stratification does not happen, stores are a complete waste of time; It probably doesn't happen in small stores. The high temperatures specified by manufacturers are utterly ridiculous; the added complexity utterly pointless.

If I turn my CH on when my store is hot, then it comes on quickly. If I turn it on when my store is cold, it comes on slowly. If I don't have a store it comes on soon enough! Pointless complexity, when there is a simple, cheap, robust alternative!

T

Are thermal stores pointless?

SimonH — Fri, 06 Feb 2009 00:38:13 +0000

Tom - why is unvented so important? If thermal stores are naff, then why not a traditional hot water tank with stat and header tank?

To the thermal store supporters, why a store? Will very big radiators [which go cold at the bottom], plus a tradiational hot water tank for DHW?

In my mind unvented tanks will be slower to reheat than a store because they are fed by a coil. But then a store will need to be hotter to get a decent output temp.

What I actually want is a system that can give me hot water when I want it, make my central heating as efficient as possible and as yet aren't convinced either way.

What happens in winter when the cold feed is circa 5 degrees? Can a store lift the temp to 45 degrees with a top temp of 50-55 degrees?

If you have a 200l unvented system - with rads and the hot water is at 60 derees (so no demand) rads are up to temp as the heating has been on 1/2 an hour, what does the boiler do to keep the water temp at 80 degrees to the rads? What about when a few of the TRV's have shut down? Sound like you'll get some cycling - so a buffer would be a good idea.

What actually seems the best solution is a store for the CH but a tank of hot water for the taps, with a 1/2 inch pipe to the bath.

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 03:29:37 +0000

Posted By: tomharrigan
Jerseyman,

My unvented cylinder does not require annual maintenance. It depends entirely on the judgement of the installer.

It does. Please read the thread.

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

Do you want your house to blow up?

An unvented cylinder contains its entire volume of usable hot water. You might be interested to know that the heat exchanger in my cylinder is rated at 24kW. That is a far more powerful heat exchanger than many thermal stores! It can recharge very quickly!

A directly heat t store can take what the boiler gives. Want a 55kW boiler it it? Easy, connect it up - and still no cycling.

A thermal store produces its hot water from a fraction of its volume, often only half of it. Furthermore, it has to heat exchange, the rate of which is proportional to temperature difference. For a 250l thermal store, it is difficult to accept the equivalent hot water storage to be more than a quarter of its volume, and I think that is generous!

That was actually utter bunkum. Lets see a plate heat X has 10C cold main water in and 60C plus out. The hot T store water (primary) is 70C in and say 20 out. That is a temperature difference of 50C plus. An unvented cylinder has 60C out and 10C in, that is a difference of 50C. Get it?

I also don't accept that a small thermal store offers more adaptable input from alternative heat source. Take solar for instance. Why not use that high temperature source for hot water production in the summer, rather than diluting the temperature with a store that cannot stratify, rendering it useless. The whole point of hot water cylinder sizing is that the high temperature water is used, not wasted.

That is the same with any cylinder. One diagram has been given on another thread using two solar coils, one top and one bottom and the when water is hot it is diverted to the top and cooler to the bottom. Easy to do.

It is unquestionable that high temperature storage is less efficient than lower temperature storage; unvented cylinders win.

What do they win? Modern cylinder are exceptionally well insulated to the point they loose little heat. As say an ACT unvented is storing water at 65C to prevent legionella and store at 70C, there is little difference in it.

It is unquestionable that unvented cylinders offer better flow rates.

You made that up.

If stratification does not happen, stores are a complete waste of time; It probably doesn't happen in small stores.

Maybe physics eludes small stores.

The high temperatures specified by manufacturers are utterly ridiculous; the added complexity utterly pointless.

What makers are these? Obviously the Ladas of the T store world.

If I turn my CH on when my store is hot, then it comes on quickly. If I turn it on when my store is cold, it comes on slowly. If I don't have a store it comes on soon enough! Pointless complexity, when there is a simple, cheap, robust alternative!

The idea of a store is to have a "store" of heat. Why would it be cold? What is this alternative you are on about?

Best re-read the thread and get back to me on any points you have problems with. Which appears most points.

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 03:44:16 +0000

Posted By: SimonH
In my mind unvented tanks will be slower to reheat than a store because they are fed by a coil. But then a store will need to be hotter to get a decent output temp.

A heat bank can run at 70C. storage cylinders need to be at 60C plus. ACV recommend their unvented at 65C.

Heat banks/t stores re-heat quicker as they heat directly and top down. They produce useful DHW within a few minutes when using a plate heat exchanger.

What I actually want is a system that can give me hot water when I want it, make my central heating as efficient as possible and as yet aren't convinced either way.

A heat bank will do that the most efficient way.

What happens in winter when the cold feed is circa 5 degrees? Can a store lift the temp to 45 degrees with a top temp of 50-55 degrees?

It can with a plate heat exchanger. If the store is around 75C you can get just under 70C out. The kW of the plate what matters.

If you have a 200l unvented system - with rads and the hot water is at 60 derees (so no demand) rads are up to temp as the heating has been on 1/2 an hour, what does the boiler do to keep the water temp at 80 degrees to the rads? What about when a few of the TRV's have shut down? Sound like you'll get some cycling - so a buffer would be a good idea.

Yep.

What actually seems the best solution is a store for the CH but a tank of hot water for the taps, with a 1/2 inch pipe to the bath.

No. The heat bank/t store can do it all. A heat bank can act like a combi when the store is exhausted. It runs off the output of the boiler burner, so never running out of DHW.

Are thermal stores pointless?

nigel — Fri, 06 Feb 2009 10:14:57 +0000

This topic is becoming a bit like some of the ASHP discussions with commercial interests seemingly weighing in with their "impartial" and "objective" advice.
These people have a story to tell because they make a living out of it.

I don't think I will be specifying a thermal store, unless it is to simplify multiple heat sources, for a number of reasons:
1. Most plumbers do not understand them.
2. There is no evidence that they facilitate higher energy efficiency and efficient modulating gas boilers do not cycle. Even oil ones can be configured so that cycling is reduced.
3. Most of the so called advantages put forward are sales blurb that offer no significant benefit to users.

As a point of interest why do Viessmann, who are acknowledged leaders in integrated heating systems,not offer a thermal store.
After all they do specialise in solar integration as well.

Dont get me wrong I do see a useful role in thermal stores in certain situations but I dont see how they provide the so called benefits that are being promoted.

Are thermal stores pointless?

StuartB — Fri, 06 Feb 2009 10:22:38 +0000

Far too complicated for me and too many conflicting opinions on how they are supposed to work. Can't see any significant benefits in opting for a thermal store. Also agree that 99% of plumbers wouldn't have a clue how to fix them if something went wrong. Sorry, I won't be investing in this one - I'm out!!

Are thermal stores pointless?

tom.harrigan — Fri, 06 Feb 2009 10:46:05 +0000

My hot water cylinder does not require an annual service. I shall not revisit that point again.

"Under normal circumstances the “MULTIBLOC” combination control valve should not require any
maintenance. However, annual inspection and / or cleaning of the integral line strainer, pressure
reducing valve, cartridge, expansion relief valve cartridge and seating may be necessary depending
on local water conditions."

Unvented cylinders are a modern, efficient, simple, robust technical solution. Mains pressure hot water, no header tank, can be placed anywhere, lowest feasible heat loss if sized correctly.

My position is simply that you gain absolutely nothing in return for the complexity and expense of a small thermal store. To overcome their inadequacies, high temperatures or all sorts of contraptions are bolted to the side, requiring pumps valves and plenty of other things to go wrong. It has even been suggested that you might require internal and external heat exchangers, or even use the store as a combi boiler! I will go a step further and suggest they cannot perform anywhere near as well as a simple unvented cylinder for the price.

Now, instead of name calling, why doesn't someone answer this simple question:

How long will it take the top half of a store to cool from 80degrees to 60degrees, given that the bottom half of a store is at 20degrees, by thermal conduction. You may assume the conductivity of water is 0.6 W/(mK) and temperature independent.

Are thermal stores pointless?

PASB — Fri, 06 Feb 2009 11:16:05 +0000

I can understand the reluctance for unnecessary complexity with a small thermal store. But what about a much bigger one - if you've got the space for the tank, and the panels + wood-chip boiler to heat it?

PASB

Are thermal stores pointless?

Bowman — Fri, 06 Feb 2009 11:19:33 +0000

dt=dQ*x/dT/A/k

Simple

Are thermal stores pointless?

PASB — Fri, 06 Feb 2009 11:24:16 +0000

Er...that's well beyond what little I can remember of my O-Level Physics...help?!

PASB

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 12:14:30 +0000

Posted By: nigelThis topic is becoming a bit like some of the ASHP discussions with commercial interests seemingly weighing in with their "impartial" and "objective" advice.

If you mean me I have no interest in promoting t stores. They are just plain better. I have twice pasted the piece from the OSO manual and its link. Yet one poster continually keeps giving misinformation that unvented cylinders, and his Oso, do not require a service.

I don't think I will be specifying a thermal store, unless it is to simplify multiple heat sources, for a number of reasons:
1. Most plumbers do not understand them.
2. There is no evidence that they facilitate higher energy efficiency and efficient modulating gas boilers do not cycle. Even oil ones can be configured so that cycling is reduced.

Firstly modulating gas boilers "DO" cycle when connected directly to rads. read my comments on the by-pass. They wear out quicker and then more prone to breakdown. Secondly, heat banks will run the boiler at 90% plus of re-heat time, in efficient condensing.

Oil boilers do not cycle as much as gas as they have a large water content in the heat exchanger, while gas boilers have hardly any at all.

As long as you understand the t store you tell the plumber how it works. Also get one that knows how they work in the first place, not a bathroom changer.

3. Most of the so called advantages put forward are sales blurb that offer no significant benefit to users.

Have you anything back this up, beside going on old wives tales.

As a point of interest why do Viessmann, who are acknowledged leaders in integrated heating systems,not offer a thermal store.
After all they do specialise in solar integration as well.

That is for Viessmann to answer.

Dont get me wrong I do see a useful role in thermal stores in certain situations but I dont see how they provide the so called benefits that are being promoted.

Do not go on old wives tales.

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 12:36:55 +0000

Posted By: tom.harriganMy hot water cylinder does not require an annual service. I shall not revisit that point again.

It does. I posted the passage from the Oso manual for you. Stop giving misinformation. Best you contact your insurance company and ask them if they would pay up if the side of your house blew up, and you never had a service, when the manual states you should. If you want to put your family at risk then that is your choice. I would not have one in the house.

Also, they have complex, expensive safety controls. A heat bank only has a standard CH pump and flow switch which anyone can fix it is so simple.

"Under normal circumstances the “MULTIBLOC” combination control valve should not require any maintenance. However, annual inspection and / or cleaning of the integral line strainer, pressure reducing valve, cartridge, expansion relief valve cartridge and seating may be necessary depending on local water conditions."

In short, an annual service is needed. Yes, £60 a year for keeping water in a cylinder

Unvented cylinders are a modern,

Invented by an Englishman in 1860, yet the UK would not have them until 1986 because of EU pressure to standardise products.

efficient, simple,

Not efficient. They heat from a bottom mounted coil, unlike heat banks which efficiently heat directly taking all the boilers output, and and top down. They are not simple as they have complex safety controls. They deemed so dangerous only approved people are allowed to fit and service them.

robust technical solution.

sales speak.

Mains pressure hot water, no header tank, can be placed anywhere, lowest feasible heat loss if sized correctly.

Like any mains pressure cylinder.

My position is simply that you gain absolutely nothing in return for the complexity and expense of a small thermal store.

You have been repeatedly been told heat banks are simple and more efficient especial the heat bank version. And that they are not a direct comparison to an unvented cylinder, they are much more and far superior. You must sell unvented cylinders to have this misguided view.

TO ADMIN.
Mr Harrigan is clearly spreading misinformation - the bit about the Oso service clearly proves this. He has been repeatably put right on just about every point, yet he continues with the same diatribe. In short, HE IS SPAMMING. This is clearly not just a bit of fun.

I am not into opinion, well only when two systems are about equal, then opinion matters. I am interested in facts, and facts prove that heat banks are just plain superior in about every respect to any DHW only unvented cylinder. DHW only heat banks also leave an unvented cylinder standing too.

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 12:47:37 +0000

Posted By: PASBI can understand the reluctance for unnecessary complexity with a small thermal store.
PASB

Please do not suck in the misinformation being spouted by rank amateurs or spammers on this thread.

Small thermal stores and heat banks are not complicated at all. They are very simple indeed using standard off the shelf parts. If the DHW pumps fails, B&Q even sell them on a Sunday.

One poster is attempting to make a direct comparison to an unvented cylinder. That is bunkum as they are not comparable. Only a DHW only T store or heat bank is comparable, yet an unvented cylinder has complex, expensive safety controls, that only an approved man can touch (and they cost) and is a potential bomb. See how much it costs if your multifunction valve cluster needs replacing on an unvented cylinder. Ouch!!!

Once again they are not directly comparable and it is a shame many here don't see that.

Are thermal stores pointless?

Paul in Montreal — Fri, 06 Feb 2009 13:05:31 +0000

Posted By: Water Systemsyet an unvented cylinder has complex, expensive safety controls, that only an approved man can touch (and they cost) and is a potential bomb.

I probably shouldn't jump in here but it's interesting to see the fear of unvented cylinders. Over here in North America, all domestic hot water cylinders are mains-pressure unvented devices with simple pressure relief safety valves. Never hear of them exploding or requiring servicing. It's great to have mains-pressure hot water without having to use pumps! Doesn't matter if they're gas heated or electrically heated, the pressure relief systems are the same. There was a nice episode of "Mythbusters" where they tried to get a tank to explode - most weren't strong enough to go off like a bomb (or, as the myth was reported, to fly up out of the house like a rocket). BTW, in that show they had deliberately fixed the pressure relief valve so that it couldn't operate. I'll jump out of this debate now. Funny old thing UK plumbing

Paul in Montreal.

Are thermal stores pointless?

PASB — Fri, 06 Feb 2009 13:15:44 +0000

Thanks, Water Systems - I have in mind a solar-assisted (10.5sqm panels/tubes), largish heat store (difference between "Heat Bank" and "Thermal Store"?), for a 360sqm house, and a 25KW (or maybe more) wood-chip boiler. Ground floor under-floor heating; rest radiators.

I'm afraid I wasn't up to understanding "Bowman's" formula in his earlier post in response to mine just before it -

Posted By: PASBBut what about a much bigger one - if you've got the space for the tank, and the panels + wood-chip boiler to heat it? blockquote> - how would you answer that question. please? And what is the tank size-limit...on the above heat-sources, and assuming that 'bigger is indeed better', providing you've got the system to get it up to, and maintain it at, the 70+degC that seems to be sensible for Heat Bank-type installations?

PASB

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 13:31:17 +0000

Posted By: Paul in Montreal
Posted By: Water Systemsyet an unvented cylinder has complex, expensive safety controls, that only an approved man can touch (and they cost) and is a potential bomb.
I probably shouldn't jump in here but it's interesting to see the fear of unvented cylinders. Over here in North America,alldomestic hot water cylinders are mains-pressure unvented devices with simple pressure relief safety valves. Never hear of them exploding or requiring servicing.

Look at these. Water heater blasts:
http://au.youtube.com/watch?v=pu3FwgIHsQA
http://au.youtube.com/watch?v=GF_Wrm-Ns0I

In North America it is quite common to have the odd one go boom. North America only has one mechanical safety device. The UK specified extra control and only approved people to work on them for good reason. The things were reluctantly pushed onto the UK by the EU.

In France the pressure relief leaks water into the drains, they generally do not have expansion vessels. You can fit French unvented cylinders in the UK. Many go to France and buy bathroom stuff there and come back with the wrong cylinders.

A vented DHW only heat bank is simple, far less to go wrong, far more efficient, gives higher pressures, and infinitely safer than any unvented cylinder. FACT!!!

Not only that, the average direct cylinder can be easily converted to a heat bank very easily and very cheaply too.

It's great to have mains-pressure hot water without having to use pumps!

Heat banks, T stores, combis give that.

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 13:38:46 +0000

Posted By: PASBThanks, Water Systems - I have in mind a solar-assisted (10.5sqm panels/tubes), largish heat store (difference between "Heat Bank" and "Thermal Store"?), for a 360sqm house, and a 25KW (or maybe more) wood-chip boiler. Ground floor under-floor heating; rest radiators.

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. See my post at the beginning of the thread.

I'm afraid I wasn't up to understanding "Bowman's" formula in his earlier post in response to mine just before it -
Posted By: PASBBut what about a much bigger one - if you've got the space for the tank, and the panels + wood-chip boiler to heat it?
- how would you answer that question. please? And what is the tank size-limit...on the above heat-sources, and assuming that 'bigger is indeed better', providing you've got the system to get it up to, and maintain it at, the 70+degC that seems to be sensible for Heat Bank-type installations?

PASB

They have to be calculated. However they are becoming rule-of-thumb and many here will help you giving their experiences of using them and the size they have.

Are thermal stores pointless?

Bowman — Fri, 06 Feb 2009 13:39:21 +0000

PASB, Tom wanted to know how long to cool the top of a body of water, the equation is the answer. I think Tom simply showed his thermodynamics knowledge with the parameters he gave. I've been watching this thread and couldn't resist.

Are thermal stores pointless?

Paul in Montreal — Fri, 06 Feb 2009 13:40:25 +0000

Posted By: Water SystemsLook at these. Water heater blasts:
http://au.youtube.com/watch?v=pu3FwgIHsQA
http://au.youtube.com/watch?v=GF_Wrm-Ns0I

yeah, that's the Mythbusters episode I mentioned. They had a hard time making them explode (and had to rig the safety valve).

Posted By: Water SystemsIn North America it is quite common to have the odd one go boom.

Yeah, about as common as houses in the UK blowing up due to gas leaks.

Posted By: Water SystemsA vented DHW only heat bank is simple, far less to go wrong, far more efficient, gives higher pressures, and infinitely safer than any unvented cylinder. FACT!!!

How can it give higher pressure than mains pressure (without pumps)? How does pressure affect efficiency? How can there be less to go wrong when there are more components to leak/freeze (header tank)? Why does nowhere else in the world use header tanks like the UK? Writing FACT does not a fact make.

Posted By: Water SystemsThe things were reluctantly pushed onto the UK by the EU.

LOL, nasty evil EU.

Sigh.

Paul in Montreal.

Are thermal stores pointless?

tom.harrigan — Fri, 06 Feb 2009 13:50:22 +0000

Watersystems, I don't have the cylinder you think I have. I have quoted from the instruction booklet. I have asked the MD of the firm of heating engineers (who laughed). It doesn't need an annual inspection. Going on about explosions and insurance makes you come across as an alarmist.

I am directly quoting from the installation manual:

"Under normal circumstances the “MULTIBLOC” combination control valve should not require any
maintenance."

To Admin: Please don't ban me! I am just quoting manufacturer's literature. I thought unvented cylinders required an annual inspection, but have recently discovered they don't. The fact is that the regulations leave it to the discretion of the manufacturers. There is even more discretion if the heating engineers and manufacturers have a close working relationship.

Rarely do people who actually know what they are talking about need to resort to abuse. If I have missed some technical issue with small thermal stores, then I'd be grateful for the enlightenment.

It does concern me that no advocate of small (domestic tank replacement size) stores is able to answer my simple "stratification" question.

It is useful to know that when your thermal store pump breaks down you can get a replacement at B+Q on a Sunday. You can probably get all the other spare parts relatively easily too. I also suspect that water softening maintenance won't be too onerous and that you only infrequently need to descale your internal heat exchanger if you have one.

Are thermal stores pointless?

Bowman — Fri, 06 Feb 2009 14:07:45 +0000

Tom, I'm no plumber but there isn't a sensible calculation that can satisfy your "simple" question. If you remove all other losses and ignore convection the answer becomes invalid.

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 14:24:56 +0000

Posted By: Paul in Montreal
Posted By: Water SystemsA vented DHW only heat bank is simple, far less to go wrong, far more efficient, gives higher pressures, and infinitely safer than any unvented cylinder. FACT!!!

How can it give higher pressure than mains pressure (without pumps)? How does pressure affect efficiency? How can there be less to go wrong when there are more components to leak/freeze (header tank)? Why does nowhere else in the world use header tanks like the UK? WritingFACTdoes not a fact make.

A plate heat X can operate at 10 bar. Most unvented cylinders operate at 3.5 bar maximum, so higher pressures. The odd one or two operates at more but these are very expensive and I would not like a cylinder at high pressure around.

I keep repeating myself. THERE ARE LESS COMPONENTS in a t store or heat bank !!!!!! They are simple. There are no complex, expensive, safety devices bolted on.

Why would a system with an integrated header tank be more prone to freezing than one without? Many do not have header tanks and are open vented: Albion, Advanced Appliances, DPS Pandora.

When vented there is no annual service charge.

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 14:46:16 +0000

Posted By: tomharrigan
Watersystems, I don't have the cylinder you think I have. I have quoted from the instruction booklet. I have asked the MD of the firm of heating engineers (who laughed). It doesn't need an annual inspection. Going on about explosions and insurance makes you come across as an alarmist.

Only what the makers say is valid, nothing else. You haven't contacted them. You should do. Contact the insurance company too.

I am directly quoting from the installation manual:

"Under normal circumstances the “MULTIBLOC” combination control valve should not require any maintenance."

Give me a pdf link. Oso have them all on-line. All I looked at stated service. I have never come across any maker that never stipulated a service on an unvented cylinder. Do you value your family? Boooom!

Rarely do people who actually know what they are talking about need to resort to abuse. If I have missed some technical issue with small thermal stores, then I'd be grateful for the enlightenment.

Abuse is not necessary I agree. You repeatedly spout tripe after being put right, that is clear. Ask and you will be filled in. Appreciate and use other people's experience and knowledge. You do not, which leads me to believe you have an agenda.

It does concern me that no advocate of small (domestic tank replacement size) stores is able to answer my simple "stratification" question.

You were saying t stores/heat banks do not - they do. Sorted.

It is useful to know that when your thermal store pump breaks down you can get a replacement at B+Q on a Sunday. You can probably get all the other spare parts relatively easily too. I also suspect that water softening maintenance won't be too onerous and that you only infrequently need to descale your internal heat exchanger if you have one.

Anti-scale measures should be undertaken on any water system. Using an external plate plate X, it can be screwed off and de-scaled, very easily. Using a large one the steel plate flex and resists scale build up.

In an unvented cylinder the whole inside, and coil, get clogged up with scale.

Are thermal stores pointless?

tom.harrigan — Fri, 06 Feb 2009 14:51:49 +0000

Bowman, an order of magnitude calculation will do for a start!

I accept that circulation set up by convection or even Coriolis forces might persist for some time, but I'm happy to ignore them for the sake or argument.

You mention losses, so lets examine them. I suspect a store at 90c will lose 6c a day. The interface between hot and cold water is 1/6 of the area of the enclosing cube, so 1c would be lost through that interface if it was as good an insulator as urethane foam. We know that water conducts 30 times better than urethane foam. The upper (ie hot water) half of the store could go from usefully hot to uselessly tepid over night.

I suspect that in reality most small stores are kept almost uniformly hot, putting frequent demands on a boiler (Watersystems suggested using a 50kW one), which is fequently not condensing.

I have misled everyone on one point. A quality unvented cylinder is no cheaper than a store from a volume manufacturer. Add to that the costs of hiring a qualified person to install it etc, the cylinder will be more expensive.

T

Are thermal stores pointless?

Water Systems — Fri, 06 Feb 2009 15:31:39 +0000

Posted By: tom.harriganYou mention losses, so lets examine them. I suspect a store at 90c will lose 6c a day.

None run at 90C you have repeatedly been told that

The upper (ie hot water) half of the store could go from usefully hot to uselessly tepid over night.

Which doesn't happen in modern well insulated cylinder.

I suspect that in reality most small stores are kept almost uniformly hot, putting frequent demands on a boiler (Watersystems suggested using a 50kW one), which is fequently not condensing.

You suspect wrong. Using anti-cycle stats the demands are not that great and the boiler only comes in tot full re-heat on one efficient burn.

"is fequently not condensing." You made that up.

You can couple a 50kW boiler to a small heat bank and it will condense and reheat in zippo time....and it will not cycle. Put a 50Kw boiler on a rad system that requires 16KW at worst case and the boiler will be clicking on and off for ever. A store will prevent all that. Also a large 50kW boiler will acts as a combi when the store is exhausted giving around 22 litres/min, meaning you will never run out of DHW. Great for multiple or body jet showers. See if an unvented cylinder can do that.

Just by coupling up a large boiler onto a heat bank you have a two stage DHW system.

1. Fast flow using stored hot water
2. Reduced flow using the gas burner.

But the great thing about a store is that you can size the boiler for average heat loads not peak meaning a smaller cheaper boiler can be used. The best of both worlds.

I have misled everyone on one point. A quality unvented cylinder is no cheaper than a store from a volume manufacturer. Add to that the costs of hiring a qualified person to install it etc, the cylinder will be more expensive.

And again you missed out that a store can do CH too and prevent boiler cycling, etc, etc, etc. A lot of selective amnesia

Are thermal stores pointless?

Bowman — Fri, 06 Feb 2009 15:41:30 +0000

Hmmm, sorry Tom I can't made sense of your maths there, any chance you could show your working?

Are thermal stores pointless?

nigel — Fri, 06 Feb 2009 16:45:42 +0000

Posted By: Water SystemsNone run at 90C you have repeatedly been told that

Perhaps you can explain this in the context of your post on the screwfix forum.Quote"

The AlphaSmart system is poor design. A vented solar thermal store can have an integral solar panel drainback facility (DPS do them http://www.heatweb.com).

The solar thermal store can operate up to 95C storing as much free solar gained heat as possible. This unvented cylinder can only store hot water up to 60C. Cold water enters the DHW take off coil and a blending valve set to 50 to 60C into the combi inlet. The combi tops up to suit if under its DHW setpoint. Electric backup in the solar thermal store too if the combi is down.

A solar thermal store is much simpler than their setup, which is over complex and expensive to make and expensive to run. A whole cylinder of hot water at 55C can be wasted."

Unless there are two "water systems" out there one of your posts must be wrong - heaven forbid.