Green Building Forum - Are thermal stores pointless?

Are thermal stores pointless?

crusoe — Tue, 10 Feb 2009 21:32:32 +0000

andy500

Classic post! Methinks, apart from being a Storaphile, that your humour smacks of Goonism - aka Milligan, Seacombe et al, not Arsenal? Or maybe Pratchett?

PS: If the wife isn't bald, will wearing a bald wig still count? :)

brig001

If initial cost is the main issue (and you'd be surprised at how many times it is...), a combi will suffice. You can, of course, connect a combi to a TS, or an unvented come to that, if you want to. More involved though. My own preference, having fitted much 'small' Gledhill stuff, is for larger stores as they give you more options. I also like, wherever possible, separate DHW storage.

If however your crystal ball is twitching every time Putin and Nogastothewestovitch start flexing their energy/ political/financial muscle, sending gas prices soaring, then a TS can make you relatively 'future-proof' by giving you the options you won't have with a combi when the gas finally putters out.

Suggested reading before you decide:

High noon for natural gas - Julian Darley
The Long Emergency - James Howard Kunstler

Regards

Crusoe

Are thermal stores pointless?

crusoe — Tue, 10 Feb 2009 21:59:44 +0000

Paul in Montreal

You mentioned in an earlier post using a large TS as a buffer between an uncontrolled appliance such as a WB boiler and, say, an unvented. Is that an installation technique you use that side of the pond? I ask as I have something along these lines which has been requested and I am currently weighing the design options, to keep in line with both current approvals (most WB stove mfrs and UV mfrs here don't approve their product to be installed to an unvented) and safety considerations.

Regards

Crusoe

Are thermal stores pointless?

ralphd — Wed, 11 Feb 2009 01:22:31 +0000

Crusoe:

I did not have a "rapid change of mind on UV safety". I still know they are safe (just like Paul and millions of other North Americans know). I acknowledged that it CAN happen & HAS happened.
I have yet to see any evidence that they are a material safety risk. Or even that there is a greater risk than getting struck by a meteorite... and I'm still waiting for "Water Systems" to comment on meteorite-resistant roofs.

You and "Water Systems" appear to be redefining "safe" to mean something which poses absolutely no danger. And I'm not going to waste my time trying to convince people to use a dictionary.

-Ralph

Are thermal stores pointless?

brig001 — Wed, 11 Feb 2009 08:03:15 +0000

Hi Crusoe,

Thanks for that. I have already decided to keep the store to give some kind of future-proofing ie wood burning or solar etc. The question I am struggling with is which is the most efficient/green way to provide heat and DHW for the house in the mean time. I have tried to calculate the losses from the increased firing from a combi against thermal losses from a TS, but have not come to any solid conclusion. I guess that is why this topic has gone on for so long...

Are thermal stores pointless?

tom.harrigan — Wed, 11 Feb 2009 09:39:19 +0000

Meanwhile...

Small thermal stores <250l, designed as typical hot water tank replacements to fit in a typical British airing cupboard are compromised by their size. To compensate for their lack of volume, and to keep heat exchangers cheap, high temperatures are specified by manufacturers - typically >70C.

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.

So, the question is why bother with one? They certainly have the advantage of being DIYable, and it may be simpler in some situations to connect multiple heat sources to them. If this is the case, and you don't have a condensing boiler, then a store might be perfect for you. However, nothing that can in principle be done with a small store cannot be done with a cylinder, plus your condensing boiler will condense.

It seems there has been a lot of misinformation spread by manufacturers and salesmen. Some even quote old BS documents in the hope we won't notice that they have been superseded by new BS EN documents. Some even quote from university research that claims thermal stores to be 100% efficient!

So, there is another question. Why do some people become abusive when they are presented with simple checkable facts, and a reasonable opinion?

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 10:18:43 +0000

Posted By: tom.harriganMeanwhile...

Small thermal stores <250l, designed as typical hot water tank replacements to fit in a typical British airing cupboard are compromised by their size. To compensate for their lack of volume, and to keep heat exchangers cheap, high temperatures are specified by manufacturers - typically >70C.

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.

It seems there has been a lot of misinformation spread by manufacturers and salesmen.

..and a lot of trolling or spamming by Mr Harrigan and few others.

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 10:28:48 +0000

Posted By: brig001I am new to this, but have a 12 year old TS (Albion Mainsflow) 250 litres, IIRC, but can check. I have found it poor at stratifying when only running radiators and/or UFH, but not bad when running DHW. I also have no idea of the total system efficiency - it is fed from a Baxi Solo boiler.

That is not the most efficient TS setup and design available. There is better these days.

I was hoping to learn something from this topic about thermal stores, for and against, but since I haven't got very far on this, I'll ask the question directly.

It is simple. Heating them direct is better than with a boiler coil. Using a plate heat exchanger is far better than an internal coil. Using a condensing boiler is better than a non-condensing boiler.

You could fit a plate heat exchanger, pump and flow switch on the Mainsflow. The efficiency will rise when using a condensing boiler.

The CH. You can have TRVs all around and have a Smart pump on the CH. Then no nuisance central wall stat. Do not underestimate the buffer effect of the store to CH.

A combi? There are some nice models around with high flowrates. But you will have to add cylinders/stores to use alternative energies in preheating the cold inlet - the combi has to be suitable.

Are thermal stores pointless?

Bowman — Wed, 11 Feb 2009 10:30:32 +0000

Tom, I think your muddling things up:

The major reason to keep smaller stores at higher temperature is to store more heat, a 260l store at 60C will hold 6kWh while at 85C will hold 12.1kWh, nothing at all to do with heat exchangers.

Losses can be mitigated by extra insulation (my wife complains that there isn't enough heat to air the washing).
A 260l store at 85C has double the heat available compared to a 260l UVC at 60C (and that is a very large UVC)
The flow rate will be dependent on your mains flow (beside the fact that we should be trying to minimise water use not maximise it (there is a flaw in that argument but I bet you don't know what it is, three guesses...))

There is lots that can't be done with a UVC.

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 10:36:04 +0000

Posted By: Admin

Declare your interest (position) please 'Water Systems' as some are suspecting/reporting commercial interest!

I have no commercial interest at all. I have not promoted one product, just the efficiency of thermal storage. There are a few who just plainly spout misinformation that commercial interests appear to be at play. BTW, all TS makers make other products as well.

Are thermal stores pointless?

jezza22 — Wed, 11 Feb 2009 11:04:44 +0000

Crusoe / WS, thanks for input. I have an architect drawing up some plans and once that is done I was going to see if I could pay one of you to design / fit a new CH/DHW system for me as you both seem to know your stuff and have vast amounts of experience. To give you an early view on my property / plans:

- large 4 bed detached with 1 family bathroom and 2 es (1 upstairs, 1 downstairs)
- mix of new v-efficient aluminium rads and UFH for CH (I also have an air to air HP for a large open area)
- no mains gas but have 7 year old vented oil boiler (I'm keen to minimise use of / get rid of altogether)
- I'm thinking large as pos TS/heat bank heated overnight on E7 by ASHP (to about 40c to keep COP high) then 'topped-up' by WB in winter or oil boiler in summer or combination of both. Immersion backup.
- have a solar connection to the TS for future possible use

My aim is to have mains pressure showers, and minimise oil use, keep it simple and efficient.
I've been looking at DPS stores and was thinking 500 or 100 ltr square direct fed tank with heat plate X.

Crusoe - if I had a separate DHW tank how would this be fed? By the TS or other direct input? Also, would this have to be unvented to give mains presure DHW even though I have a TS? Or could this be another smaller TS?

Anyway, not the best place to discuss all this in detail but it gives you an idea of what I'm thinking if either of you are interested?

Thanks

PS Looking at design around May and install later in the summer.

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 11:09:34 +0000

Posted By: BowmanTom, I think your muddling things up:

The major reason to keep smaller stores at higher temperature is to store more heat, a 260l store at 60C will hold 6kWh while at 85C will hold 12.1kWh, nothing at all to do with heat exchangers.

A smaller store can be at a lower temperature when using a plate heat X and heating the store directly with an on-demand heat source like a gas or oil boiler. The boiler and store combine to supply the DHW. This means a smaller store cylinder can be used and still be at lower more efficient temperatures. It also means you never run out of DHW. The flow reduces when the store is exhausted of heat. This is a great advantage for DHW.

Once again. A DHW/CH thermal store/heat bank is not comparable to a DHW unvented cylinder. A TS is much. much more. People must get this firmly in their heads.

To those who are now confused. Just look at my first post on this thread. That gives the pros and cons.

Are thermal stores pointless?

brig001 — Wed, 11 Feb 2009 11:30:49 +0000

Thanks Water Systems. I know this is not a very efficient setup, but since the boiler has to go, it's worth a re-visit. When you say efficient, do you mean good at getting the heat out or keeping the heat in, if you know what I mean? I have plenty of DHW performance, but at what cost? I appreciate that a combi is unlikely to achieve the same DHW performance as even my TS, but which is the most efficient/cost effective/green?

Are thermal stores pointless?

Bowman — Wed, 11 Feb 2009 11:34:37 +0000

How does the DHW flow reduce when the store is exhausted?

Are thermal stores pointless?

James Norton — Wed, 11 Feb 2009 12:21:04 +0000

Why is high DHW flow so important in a low energy house? Surely this will just reduce the likelihood of reducing DHW demand, which is I would have though the first priority of energy design.

J

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 12:22:10 +0000

Posted By: brig001Thanks Water Systems. I know this is not a very efficient setup, but since the boiler has to go, it's worth a re-visit. When you say efficient, do you mean good at getting the heat out or keeping the heat in, if you know what I mean? I have plenty of DHW performance, but at what cost? I appreciate that a combi is unlikely to achieve the same DHW performance as even my TS, but which is the most efficient/cost effective/green?

It is efficient in promoting efficient condensing boiler operation (oil and gas). Keeping heat in depends on insulation and store setpoint, and the setpoint can be reduced using a directly heated condensing boiler and large plate heat X. Get a boiler as large as you can afford as this means a lower store setpoint and a v fast reheat and more energy when combining with the store supplying DHW. CH buffering is efficient too. No boiler cycling is efficient too. As I say, you can eliminate a central wall stat and have TVRs all around using a CH Smart pump. Then each room is individually temperature controlled.

The greenest is the TS, as the boiler always heats up very efficiently, with no stop starting. The W-B Highflow 550 gives 25 litres/min and then drops off, but in average use it is unlikely you will run out of high flow DHW and it also drops to a lower flowrate so you never run out of DHW.

The problem is the CH, as it heats the rads directly, so cycling is there. It does not come with an internal weather compensator, however I believe one can be hitched up. The Ethos 54C is another high flow combi and has weather compensation. It may take preheated solar water too. This maintains the flowrate for ever as it has no stored water. About 23 litres/min and wall mounted too.
http://www.ethosboilers.co.uk/products_54c.php

Alpha have the FlowSmart which is a combi assisted by a small thermal store to raise flowrates. They also have a solar preheat models too. This seems to be a trend to drastically reduce cylinder sizes, gain efficiency, and/or use free energy.

The FlowSmart really does give decent flowrates (OK only 18 litres/min, but fine for most). This is a two stage flowrate combi, similar to the HighFlow in operation. The small thermal store is remote rather than in the same box. This is a 35kW combi with a thermal store assist to raise the flow rate. It delivers 18 litres/min for 30 minutes. That is 540 litres in 30 minutes and 5 - 7 minutes reheat. It reverts to around 16 litres/min as the combi is a 35kW model, when the store is exhausted. An economic advantage is that the thermal store can be turned off in summer if only showers are used, and switched on if high water usage at any time. Or, kept off all the time and only switched on when high flows are needed like a few showers or a bath. It takes around 5 -7 minutes to reheat so not a problem. If a few people are in two bathrooms just hit the switch and higher flowrates in a few minutes. Very economical when using it that way.

The W-Bosch 550 is around £2k. The FlowSmart without the GasSaver is around £1300,but near £2K with the gas saver. The great thing about the FlowSmart is that the combi can be fitted and space left for the thermal store at a later date if say an extension is planned. It also has a backup electric immersion heater boss.

This is just to show that pre-heating DHW into combi does work and gives high flowrates too. At 540 litres in 30 mins it knocks cobs off any DHW only unvented cylinder in DHW delivery. It take 5 to 7 mins to recharge and off again. A 540 litre cylinder will take an hour to reheat.

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 12:26:46 +0000

Posted By: James NortonWhy is high DHW flow so important in a low energy house? Surely this will just reduce the likelihood of reducing DHW demand, which is I would have though the first priority of energy design.

J

Good point. People design water systems for near peak use (when all showers and baths are on, as in a hotel) not average use. People want a bath to fill in 3 minutes. I find a combi filling at 8 minutes is fine. I can get in when 1/3 full and it fills while I am in the bath. No inconvenience at all.

Are thermal stores pointless?

tom.harrigan — Wed, 11 Feb 2009 12:45:06 +0000

Bowman

I try not to muddle things up - but, I think you will agree if you think about it, that high temperature primary water, not only provides more energy storage, but allows the fitting of a smaller and cheaper heat exchanger.

More insulation can be fitted to any water storage if you have the space. The simple fact remains that high temperature storage results in higher losses, not to mention all the extra pipes, a header tank etc.

You make a strange claim about heat storage. If you mean that a store at 85C has more hot water potential than a cylinder at 60C you are simply wrong. An equal sized store will hold less than half the hot water potential of a cylinder at the temperatures you suggest. We already know that a 300l store is only good for 2 showers in the winter!

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 12:45:09 +0000

Posted By: BowmanHow does the DHW flow reduce when the store is exhausted?

The boiler pumps heat into the top of the cylinder. The DHW pump immediately pumps this into the DHW plate. The rest of the store can be cold. It then is operating on what the boiler's burner can give. It only occurs when using a plate heat X. The bigger the boiler the more DHW and the faster the TS reheat and still no cycling. Installing larger boiler will not compromise the system, as it would on a directly heated rad system, which can mean big trouble is greatly oversized. Larger boilers are not that much more expensive these days.

DPS made a small DHW only under worktop heat bank unit that operated like this. It think the GXV or something. The Alpha FlowSmart does the same, as does the W-B Highflow 440 and 550. They are two stage flowrate units, that fall back to what the burner can provide.

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 12:54:07 +0000

Posted By: tom.harriganBowman

I try not to muddle things up - but, I think you will agree if you think about it, that high temperature primary water, not only provides more energy storage, but allows the fitting of a smaller and cheaper heat exchanger.

But a well designed unit with a large plate heat exchanger only needs a store setpoint 5C over tap temperatures. So, not an issue in modern units.

We already know that a 300l store is only good for 2 showers in the winter!

Who are these we? What tripe! laughable. The boiler will also be pumping hot water to the top of the cylinder to assist as well.

Nice trolling try.

Are thermal stores pointless?

brig001 — Wed, 11 Feb 2009 12:59:19 +0000

Thanks again Water Systems. Lots of food for thought there. I am going to do some experiments on my TS and see if I can heat it up in one go like you say. If that is the greenest way, that's good enough for me. I never run out of hot water even with a 14kW boiler, so if I can get a good clean heat up, I will probably keep the store with the intention of buying a better one when funds allow.

Are thermal stores pointless?

Bowman — Wed, 11 Feb 2009 13:01:45 +0000

Water Systems, you're obsessed with gas and oil boilers, I don't have one and don't want one, you're making huge assumptions...

Are thermal stores pointless?

tom.harrigan — Wed, 11 Feb 2009 13:04:13 +0000

I was simply reiterating the experience of someone who has a 300l thermal store, who wrote earlier in this thread that:

"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."

I have no reason whatsoever to doubt the author.

Furthermore, his experience backs up my calculations that a store has at best half the hot water potential of a cylinder.

Are thermal stores pointless?

Bowman — Wed, 11 Feb 2009 13:06:52 +0000

Tom can you show me how you calculate two showers from 300l please my simple chartered engineer level maths can't keep up with you. (I assume you have done these calculations as you seem to be very sure of yourself.)

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 13:08:53 +0000

Posted By: BowmanWater Systems, you're obsessed with gas and oil boilers, I don't have one and don't want one, you're making huge assumptions...

Well most do have gas and oil boilers. Few do not. So I am preaching to the majority. Also, using natural gas is the best option for being green and economy. Beats a heat pump. The future of gas prices? Crystal ball gazing time.

I like coupling alternative heat sources to thermal stores.

Are thermal stores pointless?

Johnh — Wed, 11 Feb 2009 13:16:17 +0000

I've got a woodfired W23 linked to our oil CH. I want to go over to using a wood fired system and get rid of my oil boiler (or at least further reduce its use)
Could Water Systems please go into more detail about TS/Heat Banks (and their benefits)when linked to log boilers?
Many thanks.

Are thermal stores pointless?

Water Systems — Wed, 11 Feb 2009 13:16:33 +0000

Posted By: tom.harriganI was simply reiterating the experience of someone who has a 300l thermal store, who wrote earlier in this thread that:

"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."

Note the word "might", to emphasize that it does more than what is asked of it. He later said it was not an issue as it never happened. He is also storing water at less than what your beloved unvented cylinders stores water (ACV say 65C and their cyl stat has 60C as the lowest setting)

Furthermore, his experience backs up my calculations that a store has at best half the hot water potential of a cylinder.

Strange that he says the opposite to you. He also goes on about the efficient boiler usage, no cycling, instant rad heat ups, etc, etc.

Are thermal stores pointless?

PASB — Wed, 11 Feb 2009 13:17:09 +0000

...Meanwhile, back at the coalface (not literally, of course: it's so un-green..), I might be getting closer to a decision on this angst-ridden subject.

Although by no means an expert (total amateur, in fact), I've always suspected that the strongest supporting argument FOR a heat-store-based system comes from the tank itself being as large as possible - provided it isn't so big as to defy being heated up to a sensible temperature at all (65degC, it seems, is practical today with modern plate heat exchangers), and lose heat so rapidly that it's in constant need of being topped-up.

I have in mind both "passive" (solar) and "active" (wood-chip boiler) elements. At the passive end of the spectrum, first, what size tank will a given area of solar panels/tubes (10.5sqm in my case) support at my lat/long (40/03degs respectively); and second, what rate of temperature fall-off can be expected, with modern tank-insulation techniques when the sun goes in for a few days?

At 120kW from the 10sqm solar array, on a good day it would appear that up to 2,000litres COULD be heated; and at my lat/long, it seems that for about 7 months of the year this could be satisfactorily maintained against expected usage. In this, the rate of heat fall-off (according to a well-reputed tank manufacturer this can be as little as 1degC in 24 hours with a properly-insulated system) would be a big contributor to the overall performance efficiency and operating economics. However, 2,000litres is physicaly pretty large, and was only given as a maximum-possible performance: more practical will probably be 1,200 or 1,500 litres.

Sticking with the passive end for a bit longer as the baseline for the system's sizing: heat-extraction usage in this property will vary greatly with seasons; but visitors will probably be more numerous in summer months than winter, and in winter regular usage will mostly be confined to the ground floor (underfloor heating) and 1 bedroom & 1 bathroom on the 1st floor.

With solar performance in Nov-Feb being statistically only just over 20% of that in the best month (July), the slack will need to be taken up by the active heating component, a wood-chip-burning boiler of about 25kW output. The heat store's volume and heat-retention characteristics should mean that the intervals between the boiler light-offs will be fairly extended (overnight in winter, for example, the 2nd floor heating will basically be shut-down to frost-only 'stat setting, and on the 1st floor only a corridor, a bedroom and a bathroom will be in regular use; and the ground under-floor system will be turned down). All this of course assumes that the house itself will indeed have been properly-insulated.

Anyway, I hope I'm getting closer to reaching a conclusion on the basic characteristics of my intended system...at least so that an expert can take the brief and calculate required heat-exchanger performance, tank layering requirements, and so on. I am grateful to this posting for giving-up - however reluctantly - so many useful nuggets that have greatly helped me grasp some of the essentials underlying the protracted discussion of "whether heat stores are pointless". I now don't reckon they are.

Thanks to all concerned.

PASB

Are thermal stores pointless?

Tuna — Wed, 11 Feb 2009 13:47:54 +0000

Posted By: tom.harriganI was simply reiterating the experience of someone who has a 300l thermal store, who wrote earlier in this thread that:

"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."

I have no reason whatsoever to doubt the author.

Furthermore, his experience backs up my calculations that a store has at best half the hot water potential of a cylinder.

Um.. not wanting to continue this ridiculous fight, but doesn't the supply of radiators in the depths of winter draw off rather a lot of heat? How you compare one system that is providing heating to rads with another that supplies only hot showers is beyond me.

Are thermal stores pointless?

Bowman — Wed, 11 Feb 2009 13:51:46 +0000

Tuna I think some of the arguments being made are beyond those making them...

Are thermal stores pointless?

cookie — Wed, 11 Feb 2009 14:06:30 +0000

I keep looking at this thread and its annoying me, lol...

Could one of you guys explane why / how STRATIFICATION works and how this extracts more of the useful energy out of the water and perhaps this will end the argument lol. I have tried to put this in to words but I keep using the wrong terminalogy.

I did have a really good link but I've lost it sorry... Thermal banks rock.... lol p.s. please comment on my thermal bank / store design thread :op

What was the original question again :op

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