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| Green Building Bible, Fourth Edition |
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These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment. PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book. |
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Posted By: jerseymandiffering views are still 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
Posted By: tomharrigan
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.
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!
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.
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.
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.
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.
Posted By: tom.harriganMy 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.
Posted By: PASBI can understand the reluctance for unnecessary complexity with a small thermal store.
PASB
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
Posted By: Paul in MontrealPosted 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.
It's great to have mains-pressure hot water without having to use pumps!
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.
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
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
Posted By: Water SystemsIn North America it is quite common to have the odd one go boom.
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.
Posted By: Paul in MontrealPosted 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.
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.
I am directly quoting from the installation manual:
"Under normal circumstances the “MULTIBLOC” combination control valve should not require any maintenance."
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.
Posted By: tom.harriganYou mention losses, so lets examine them. I suspect a store at 90c will lose 6c a day.
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.
Posted By: Water SystemsNone run at 90C you have repeatedly been told that
