Six reasons not to install a thermal store...

[tom.harrigan]

I was going to buy a thermal store and heat it with a woodburner. I planned for it to deliver hot water and perhaps pump the central heating from it - though this could be pumped directly from the stove if it had enough connections. I don't have oil or gas and would prefer to keep it that way, but decided to plan for an oil boiler in the future if necessary.

The bad news is:

1. Thermal stores need to be maintained at a high temperature (70 -

90degrees) according to manufacturers. This is bad news for heat loss, and makes the utilisation of a future condensing boiler problematic.

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

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

1. Although they don't require an annual inspection, neither do unvented cylinders, so this isn't a benefit.

2. You can incorporate multiple heat sources with a thermal store, but you can do that with a sealed system as well - again no benefit.

1. You can't put radiators and store anywhere - you can with a sealed system.

The good news:

1. You don't have to involve building control with a thermal store.

So, I've decided on a sealed system and unvented tank powered by a multifuel stove, to which I can add an oil boiler (which will condense) when I get too old to carry logs.

T

[Andy Dingley]

Look at better makers. Those with better stratification and heat exchangers designed for this can accept lower input temperatures. Take a look at the Conus / Consolar kit and their quite detailed descriptions for an example of just what's possible for "best practice" (and a high budget!) Then find a way to do it cheaper (my own involves buying things from scrappies in the Forest of Dean, and you don't get more bodge-tastic than that.

If it's not stratified, then IMHO it doesn't count as a thermal store.

Easier with a good thermal store though.

[tom.harrigan]

I take your point. However you must agree that the behemoths from Consolar are best suited to properties with plenty of space and perhaps even high demand. If demand is low, even one of those will lose stratification through conduction! I did look at them, but decided I couldn't justify the loss of a room!

T

[Dave Liquorice]

I thought unvented cylinders *did* require an annual inspection.

I hope you've given consideration to what happens when the system is up to temperature but the stove has just been stoked up. Where does that energy go in a sealed/unvented system? A stove does not switch off instantly like a gas or oil boiler...

Also think about what happens if the power fails, the boiler circuit needs to be gravity, not pumped or the moment you would really like to have the stove on you can't and if it's blazing away when the power goes you have a lot of heat to dump possibly fairly quickly to stop the boiler boiling.

[Andy Dingley]

I have the figures somewhere (although not worked for a Consolar, a chap with a very big anorak worked it out) and in fact it _won't_. Heat loss through conduction gets it first (although this was assuming no demand to disturb things) as water's such a poor conductor and so good at convective transfer. It's crucial to get the top lid insulation adequate, as any cooling there _will_ set up convection and chill the hot layer.

[Rod]

If you search for these things, you find this:

"No sacrificial anode (eliminates need for annual inspection)"

repeated all over the place in reference to SS Megaflows. But some appear to have sacrificial anodes - / them require annual inspections?

[David Hansen]

On Tue, 2 Dec 2008 07:35:58 -0800 (PST) someone who may be [e-mail] wrote this:-

Where can't you and why?

[tom.harrigan]

I'd be amused to see that calculation. Putting the complicated maths to one side, how could a tubular heat exchanger in a consolar work if water were such a poor conductor? It's nearly 30 times better conductor than urethane foam. Water is about 0.65 thingies at temperatures one might expect to encounter in a thermal store, compared to about 0.022 thingies for urethane foam.

T

[tom.harrigan]

must be 500mm below the top of the store (or the feed and expansion tank if you have one). You could of course have a sealed radiator or U/F system heat exchanging with the relatively cool water in the lower half of a store with all the problems of exchanging heat across a small temperature difference. That's probably why none of the manufacturers I've spoken to recommend it - it doesn't work.

T

[David Hansen]

On Wed, 3 Dec 2008 03:13:21 -0800 (PST) someone who may be [e-mail] wrote this:-

Manufacturers specify many things, in order to cover themselves. One must be able to see through their claims to get the best out of anything.

Do they say why they specify this? The only reason I can think of quickly is that you are referring to a thermal store with built above F&E tank.

Newark don't seem to have anything like that, though I have not looked hard . Obviously radiators must be below the F&E tank, wherever this happens to be located. If it is located on top of the thermal store then the radiators will need to be below it. However, thermal stores work perfectly well with radiators above them, one simply has to design and install things properly.

[Alan Braggins]

I'm glad I didn't learn that before adding radiators to my thermal store system (with U/F heat), but since the manufacturer's drawings show it as an option and it's working, I'm not overly worried.

[Doctor Drivel]

Bad news? New to me.

Nonsense. No stipulates 90C operation. Even if a 80C store temp a condensing boiler will be condensing 80% of re-heat. Most heat banks (thermal stores with external plate heat exchangers) operate between 70 &

75C. The larger the plate heat exchanger the less the stored temperature.

two anti-cycle stats on the store eliminates boiler cycling. The boiler is on full flow at all times. See above, re: large plate heat exchanger.

Total tripe. You will get stratification (thermal layering) giving very cool temperatures at the bottom of the store. The DHW plate ensure water 20-30C return to the bottom of the cylinder. This ensures highly efficient condensing boiler operation.

Unvented cylinders require an annual inspection by an approved fitter - that costs. Unvented cylidners can do this, water heater blasts:

?v=GF_Wrm-Ns0IThermal stores are NOT a direct comparison to DHW only unvented cylinders as they also provide a superb CH buffer and allow for TRVs on all rads using a Smart pump. Thermal stores offer so much more. They are a different animal altogether, and so much better.

Urrrr? Come again?

There are pressurised heat banks - they do not require an annual service. These operate at 1 bar and can run a system boiler direct. They ca be anywhere. Even with a vented store the CH can be taken off a coil in the store ensuring the rads can be anywhere on pressurised loop.

Yep.

What a bad choice, you poor sod. See:

will supply stainless steel, vented or pressurised, DHW internal coil or external plate heat exchangers (heat bank). Best get two cylinder stat probes to give boiler anti-cycling. They will insert solid fuel and solar coils as well. The business indeed.

[Tim Downie]

Can you provide a link or any evidence for this? I couldn't find any on the net. Not saying it doesn't exist but that I haven't found any.

What would you say to this?

[Doctor Drivel]

The makers instructions state so. I am pretty sure schedule 1 para G3 states a recommendation or maybe words like that. If the makers also say so, then you do need the service. Just get a heat bank - better and cheaper and does the heating too.

One unvented cylinder went boom:

[tom.harrigan]

I'm just reporting what manufacturers have told me - the upper part of the store must be kept between 70-90 degrees. Actually one stated

80-90 degrees.

You miss the point. An unvented cylinder will hold its volume in hot water, a thermal store will have to be significantly bigger to compete with the tank, or heated more often. If we consider the upper half of a store, with 20degrees to play with, it would have to be about 4 times the size before it could begin to compete. With less that perfect heat exchangers (whose performance diminishes with temperature difference) its more likely you would need a store significantly larger. If you don't use it, the water will of course cool down more quickly and mix with the colder water below.

I can see you have read the marketing literature. I must say that if you believe that a heat exchanger will return water at a lower temperature than the outgoing hot water, then I'm probably wasting my time with this reply. The heat exchanger will in fact return water at a higher temperature than the water it heats, depending on how good it is.

Perhaps you could point to the legislation?

The fact is they provide a central heating lag when cooled down, and since they must be maintained at such a narrow temperature range, they don't provide much of buffer. Even worse, they prevent condensing boilers from operating properly due to the high return temperatures!

OK - You can incorporate multiple heat sources with a thermal store, but you can do that with a sealed system as well - again no benefit.

What would be the point of that? You would need a store at a high temperature and an enormous heat exchanger. A technically better solution would be to not bother with the store at all!

See:

I can think of at least six reasons not to install one of those. Did you spot in their installation instructions that the thermostat should be set at 75C ?

Its a technically inferior solution to a sealed system and unvented cylinder.

T

[Doctor Drivel]

OIl and gas boilers only reach 82C.

Assuming the whole of the cylinder is at the temperature you want it to be at the taps - stratification will stop that happening, unless a shut pump is operating most of the time.

No. A store full of water which is 70C - just heated and stat satisfied. Large DHW plate heat exchanger. DHW is drawn off in bath. The DHW temp sent to the blending valve will be around 60C, the blender mixes to say 55C. The returned water to the bottom of the store will be about 20 to 30C from the DHW plate. Stratification will ensure that 75C water will just rise up the cylinder with 20-30C water rising up as the store is depleted. You end up with a store of water 20-30C, just above the cold water mains temp. Coiled stores are not so efficient, plate make a big difference.

No. I built and tested them.

I think you are wasting your time full stop. You really don't understand something fundamentally simple.

I think we have one here.

Schedule 1 para G3. If makers say you need an annual service then you need one - all state so.

The stats are set so it doesn't cool down enough to make any difference.

What temperature range might this be?

They provide a wonderful anti-cycle buffer. Have a system boiler with an integral weather compensator heating the CH section maintains the lower section at what the compensator dictates, which is very low and lower than condensing return temperatures 90% of the time. DHW draw-off reduce the bottom section to very cold temperatures.

You have been told the return temperatures are very low. Do you understand? You could have a high return temp from the CH, but it will drop significantly at the bottom when DHW is drawn off. ACV have a pancake coil at the bottom to pre-heat cold mains water which keeps the bottom very cold indeed.

This one is three sheets to the wind.

But you are not very good at thinking.

At let less than 90C isn't it. and that is with a coil DHW take off. That can go down to 60C using a large plate heat exchanger.

You have had all of this explained to you on these thread and still you cannot understand. Also a T store is not equal to an unvented cylinder as it does CH as well. Do you understand? I doubt it.

You are a planted spammer, as no one can be that dumb.

[Doctor Drivel]

Thermal storage is big in Germany:

[Hugo Nebula]

The Building Control involvement in an unvented cylinder is an electronic notification by the installer. No money changes hands. No inspections are carried out.

[dennis]

It should be quite possible to do a heat exchanger that returns water cooler than the DHW output. You feed the hot water from the store at one end and cold mains water from the other. If you get the flow exactly right you can drop the store water down to almost the same temp as the cold mains and the mains water almost as hot as the store water. Getting the flow exactly right will be hard and will probably require some electronics and variable speed pumps.

It is an unvented cylinder with a few bits added.

[Doctor Drivel]

Some Gledhill models have variable speed pumps and no blending valve only pumping out of the store the exact amount of hot water needed. A large plate heat exchanger will drop the return temp into the store dramatically.

You really know little about thermal storage.