>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?

Because you need a heat exchanger to transfer heat from the vented primary to mains pressure secondary. If that's done with a plate heat exchanger it needs a pump, controls etc.

See also reply here..

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=4215&page=1#Item_5

Mmmm - lots of comments here, some I agree with and some I don't!

My situation is:

I have an old boiler in the airing cupboard upstairs that is about 15 years old and is pretty much knackered.
4 bedrooms, 2 en-suites and 1 bathroom. One knackered and noisy pump which was servicing 2 showers.

I would like to have the boiler replaced with a condensing unit sited in the loft (yes I know, ladder with handrail, lighting, barriers around the loft hatch, all boarded etc etc - already done.)

The real question is - what to do about the high pressure required for the showers....

Option 1: Simply buy a new pump.
+ Cheap (£120)
- Very noisy (especially at night and it wakes the kids) - not really a viable option

Option 2: Replace the showers with Trevi Boost venturi-effect showers. I like these as I've had one in the past for about 8 years and it worked brilliantly - good powerful shower without pumps.
+ Fairly cheap (about £500 for 2 showers plus about £60 to replace the high-pressure Grohe tap in the bathroom I bought by mistake....)

Option 3: Replace the tank with an unvented one.
+ High presure to all taps
+ No need for shower pumps
- Cost
- Danger (?) (I'm not sure that I believe that they are dangerous provided that they are installed competently and have the required and correctly set pressure relief valves)
- Cost of servicing anually (£60 on top of the boiler servicing charge??)

Option 4: Thermal Store
+ High Pressure to all taps
+ no need for shower pump
+ no stored water under pressure
- Cost!
- Most plumbers/heating engineers don't seem to know much about them - I was told by one last week that he gets called out to them often because they are 'very old technology' and 'very unreliable'
- heat exchangers prone to furring? (I live in Poole - water hardness is 280mg/L CaCO3)

So what to do? I've rung 5 heating engineers to see what they recommend and had 5 completely different suggestions, including;

1. You want a big combi !
2. A big combi simply won't supply 2 showers adequately, in winter
3. You want a pressurised system
4. Ooooo wouldn't touch a pressurised system, mate......
5 What's a thermal store...?

So even the trade seem pretty clueless when it comes to tradeoffs between systems. Manufacturers also seem to have different thoughts - DPS seem to be quite sanguine about thermal stores in hard water areas. Gledhill seem quite anxious about it.....

I guess that I'm looking for some guidance here!

...and finally, some questions....

1. If I have a high pressure system fitted, what exactly does the annual service include? Do they test the overpressure valves and if so, how? How does the annual service guarantee that it's all still safe or is it all a bit of a con?

2. If I go the unvented route, do I have to replace any taps or valves in loo cisterns to cope with the increased water pressure?

3. If the boiler is put in the loft (as my neighbours has been) then what stops the boiler air-locking? (possibly a dumb question)

Thanks!

Nick

Just had 2 chaps from Brit Gas round.... Oh dear......

They recommended anything other than a combi.
Didn't have a clue what a thermal store was....
Had never installed solar and didn't think that it was possible with an unvented system...
And told me that the boiler coldn't go in the loft as the F&E tank had to be at least 6m above the boiler (!)

Oh well, another chap arrives on Wednesday.

I get the strong impression that many (not all) plumbers, probably like most people go down the route of least resistance.

The number of them who had recommended 'combi' boilers to me for my old 4 bed, 2 bath house when it was obvious that they could never adequately cope with demand....but they were easy to install and hence provided a greater profit margin for the installer. Any reaearch would reveal that at the time you would never install a combi in anything bigger than a 2 bed flat! I must add that this was a few years back and I have no idea whether modern combis are any different.

I just metion this to illustrate that you really have to do your own due diligence and not take anything at face value without digging deeper. Why do most plumbers install unvenyed systems? they say because thermal stores are 'sludge buckets' very helpful! I think its because they genrally just stick to what they know.

So I am still undecided as to whether I should use UV or TS for DHW/CH.

Oh - another thing - I found out what I get from my £60 annual maintenance of a pressurised tank - they patially drain the system, remove and clean the filter, re-pressurise the system and check that the pressure relief valves operate (by turning them dowb and then back up to the correct pressure)

Seems sort-of reasonable given that it takes about an hour.

Hi Nicky

- the susceptibility to hard-water furring of the plate exchanger
ANY hot water system is susceptible to furring. Combis, traditional hot water tanks and thermal stores. I'd suggest that anyone with very hard water consider a water softener. They're brillant. Clearly they are costly to buy but the lack of limescale in showers, baths, basins and on any tap is a real bonus. As is the lower need for cleaning products (personal or household). We save more this way than we spend on salt (it uses 2 bags / year)
- the lack of decent manufacturers guarantees
Yes I agree.
- the loss of energy when storing that much water at 75C
This is a complete red herring as you don't need to store the water at that temperature. EVEN IF some manufacturers say so. I'm sure we went through this earlier on this thread. There are some people who've posted in the past on this issue who have with strong views and no practical experience of thermal stores. Have a look back through the comments and if you're still not sure why this is, please say so and I'll have a go at explaining.
- the massive amount of inhibitor needed whenever the system needs draining
Yes this is an issue, I wouldn't say "massive" though
- the fact that solar heating is less effective due to the water in the TS being at 75C as opposed to 65C in a
normal tank
I'm not sure you've thought through how solar works in practice. In practice for me there are three situations:
1. Winter = minimal solar input anyway, store heated to 60-65C (effectively the temp you want your radiators at) by boiler to provide DHW and heat radiators. Store means that boiler is running at full effiency.
2. Transition = some solar input topped up by boiler. Also applies on very cloudy summer days and sunny winter days. You can minimise the number of days in this instance by correctly sizing panel and store.
3. Summer = only solar needed.
In the transition months, the thermal store really shines, as you may end up with a full tank of not quite warm enough water (say 35 degrees). So you run the boiler for say 5 minutes in the morning which loads 60C water into the top of the tank but does not need to heat the whole tank. Bingo, two or three showers, plus all the hot water one needs for washing. We've been working this way for the past month as our solar panel is undersized (design cockup) and our gas usage including cooking is about 50p / week.

If I haven't made sense please say so and I'll have another go. Might be better to start a new thread though.

That is poor. Try again and if no luck then name them!

One more comment - I installed a QT-45 shower pump last week to replace the old shower pump and it is suprisingly quiet - the old pump screamed - this one has a deep hum that is a lot more acceptable.

Unfortunately the new shower then decided to give 2 minutes of warm water, then go cold.... A new thermostat (thanks, Bristan!) and all is working again....

This thread has got a bit long - but perhaps I have few comments pertinent to the last few posts:

You should know I am working as a plumber and I am "qualified" to install unvented systems and solar thermal systems.

Having said that, the issue of unvented system vs thermal store is still not clear to me.

First: Combi's
A decent, basic shower pump will deliver about 20l/s from a vented system, assuming everything else is workingfine. If it is plumbed right, someone else turning on a tap won't affect this. A 30kw (large) combi froma well known manufacturer produces max 12.8l/s (35degtemp increase) - so this large boiler only just about makes it (assuming you have 8l/s cold) - turn a hot tap anywhere else in the house and you are stuffed. You can bolt on solar thermal - but that means you need your cylinder back, and you get a system that is not as well optimised as a twin coil unvented or thermal store (yes I have heard of them). 'nough said. Except perhaps: most plumbers fit combi's (as a preference) - most plumbers HAVE unvented cylinders and vented or unvented boilers in their own homes........

Unvented vs Thermal store?

Only one straight technical comment here - is unvented dangerous? It depends. Potentially of course, but practically not - i.e. not many documented failures. Having said that most of the time I go to new customers, the unvented system has not been checked since it was installed (at least it is not documented). The checks are basic (as previously mentioned), but there is little you can do to check a relief valve other than open it, ditto with the pressure vessel (check the pressure). Interestingly there is no requirement for a pressure guage on unvented DHW - although you have to on unvented boilers!

Most of the other technical points have been well dealt with in this thread, but it is hard to find a bottom line. But the reason is: it depends - depends on who is going to use it and what else is going on in the property. There is nothing fundamentally more complicated about a combi/ unvented/ or thermal store set up. It is just a question as to where the controls are - the combi's are complicated inside, unvented systems have a pile of safety devices and thermal stores are not necessarily more complicated - but they can be made that way.

e.g. DPS add a pump for plate heat exchanger, and an extra pump for central heating - Gledhill just use a high capacity coil (no pump) and plumb the central heating off a single pump. But DPS can get 45l/s DHW off theirs, Gledhill 25 (for comparable cylinders). DPS's c/h set up does optimise boiler operation and reduce cycling - great if you have to run an oil boiler which won't modulate, but so what if you have a gas boiler. The different way that DHW is heated also affects the way heat is extracted from the store and thus the likely variation in temperature within the store will be quite different - but does this matter - I am not sure. One is twice the price of the other! My own experience of the manufacturers was similar to above - it took 3 phone calls to get anything out of one manufacturer, the other got back to me within 1 hour.

A c/h pump will probably empty/ circulate the volume of your cylinder in under 10 mins, so don't overestimate the size of the solar contribution to your central heating.

Back to the bottom line - I need to make my own mind up for my own domestic refurb project. I think my decision (and perhaps most peoples) could be based on the 3 points below; but understanding that thermal stores do (or at least can) work (i.e are not pointless) and either option can represent an "equally" green solution:

a) If I want decent £ payback on my solar thermal, paying £2k for a cylinder is not an option.
b) If want to futureproof against changes in boiler operation or other alternative technologies (e.g. pellet burner) then a thermal store is answer. There will higher pressure for this, for those properties without mains gas.
c) If I reckon I am UNlikely to have any other heat source than boiler/ solar thermal - then swapping my vented cylinder for an unvented will be cheapest and most straightforward.

I hope this helps!

There were constant assertions on this thread that unvented cylinders do not require servicing. This is totally incorrect.

BS-approved UDHW systems training manual, under the heading of maintenance, p40:

"the installer is responsible for the safe INSTALLATION of the unit. He/she must also make his customer aware that periodic checks of the equipment are ESSENTIAL FOR SAFETY... some manufacturers recommend a MAXIMUM of 12 months. Experience of local water conditions may indicate that MORE FREQUENT inspection is desirable."

BS6700: 1997 states that with unvented cylinders, the “maintenance and periodic easing of temperature relief valves is particularly important”.

That clears that one up.