French Pressurised Hot Water Cylinders - fitting in the UK

Hi, I have another query with regard to my own plumbing this time that I would like some advice on please. We are building an extension at the moment and need to run water from the current house into the extension. Without going into all the reasons why, I am considering upgrading the gravity fed hot water cylinder to a pressurised one.
The cost of these cylinders in the UK seems over the top, but a friend recently told me they were much cheaper in France & he had a recent Brico Depot catalogue to hand. A 150 ltr cylinder costs 99 Euros fitted with a 1800w immersion heater, a 200 ltr is even cheaper at 90 Euros! I understand that these do not have a coil inside them so I could not hook it up to my existing C/H system. This does not bother me, it does not seem to cost much more whether I heat off gas or electric from past experience. According to him, you just plumb them in, connect the water pipes up and away you go.
Has anyone any experience of fitting these things & can offer any advice?
My questions are:
Will these tanks fit up to my existing 22mm plumbing connections OK ?
Can anyone forsee any problems modifying my C/H system as the pipes won't go through the HW cylinder any longer ?
Would I be breaking any regulations and/or should the building inspector be involved ?
Do I have to fit a vent pipe (think I do)?
Can anyone forsee any other problems ?
Thanks,
Peter
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I would be very surprised if it is legal. The building regulations are very strict about the safety controls of these devices. In the rest of the world, where they've been used for decades, the regulations are much more lax, although I suppose there could be a European trade directive saying that if they're legal in France, they have to be accepted here.

That would make it an unpressurised cylinder and, therefore, would almost certainly make it legal, but it would require a header tank and not provide mains hot water.
Christian.
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I suspect that this may well be true, certainly that is the ultimate aim. It has been found (see threads here a while ago) that a house wired to UK standards and with UK sockets etc. is OK in Spain.
--
Chris Green

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Christian McArdle wrote:

Er. not so. There should be a a vent pipe coupled to a pressure valve.
They 'blow off' at about 2 bar I thnk.

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Well, I would call that a pressure relief pipe, not a vent, but it's only a matter of nomenclature.
After all, they are called "unvented" cylinders, which sort of implies there is no vent.
Christian.
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What! Gas is 3 to 4 times cheaper than electricity per kW. You haven't looked at your bills closely enough.

If not adaptors are available from continental sizes to ours. The French use BSP sizes for threads

A BBA approved installer has to fit an unvented cylidner. You can't do it yourself.

No. a 1" over flow pipe with tundish on it.

Britiush unvented cylidner have more safety protectiopn on them than continental versions.
An unvented system consists of:
- Expansion vessel. An external pressure vessel or internal pressure air pocket. - A high-pressure relief valve. If pressure exceeds the set limit the valve opens releasing pressure to a safe level. - A high-temperature relief valve. If high temperatures occur, typically 90-95C, the valve opens to amtosphere.. - Non-return valve. To prevent cylinder implosions and back-pressure into the cold mains supply pipe. - Pressure reducing valve. To reduce the inlet pressure to a working pressure of typically 2 bar for copper and 3.5 bar for steel cylinders.
If it doesn't have the above it will not conform to UK specs and be illegal.
Have you tried looking at a thermal store, as you can DIY these. There are some cheapish ones about.
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Or his boiler is running at 25% efficiency ! :-)
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When I lived in France the house had these mains pressure hot water tanks. The heating was electric only (no coil) and there was no internal expansion diaphragm so there was a pressure relief valve at the *bottom* that normally dribbled when the water was expanding. It's at the bottom so the overflow was from the coldest point. This overflow was collected and drained via a waste pipe (which periodically got blocked with fluff, etc. causing overflow - that's why the French are keen on all tiled rooms :-). Since these tanks are at mains water pressure, a simple open vent pipe won't work, you need the adjustable pressure relief valve.
If I remember right, the valve had an in-built lever that could be manually raised against the adjustment spring to drain the tank and/or test the operation. The fitting incorporated a cup to catch the drips and a connection for the overflow pipe, which was normal copper size (never measured it though...). The whole cup thing was at air pressure so I suppose if there was a boil-up due to a faulty thermostat on the electric heater, it could vent the hot water/steam into the room if the overflow pipe couldn't take the volume (we had this happen once and it was a right mess...)
--

John

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says...

You had drips because your internal air bubble disappeared. The lever is to reinstate it one a year. There is a technique to do this. Unvented cyidner are a PITA.
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berlin.de>, says...> You had drips because your internal air bubble disappeared. The lever is to

Well, the French plumber didn't know about this either. Even replaced one of the cylinders, which still dripped. Since the house was rented from new, professionals were used (say no more - the workmanship was terrible, I really wanted to fix things properly myself - and don't get me on to the electrics...)
--
John

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Not all cylinders need rebubbling. Some use an expansion vessel, which needs different sorts of maintenance.
Christian.
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needs
Those with levers are to reinstate an air bubble.
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Thanks for all the comments/replies so far - much appreciated. Sorry if some of my terminology is wrong, as they say 'a little knowledge is a dangerous thing'!
In response to the cost of gas v elec. I have Econ 7 so when I use the immersion its usually running off this, but I have the immersion set at about 50 degrees. When the GCH heats it, it depends how high the boiler is set but it always seems to make the water hotter. There is no cylindar stat so that is why I probably see little cost difference.
Anyway, onto the technicality's again. When I mean't vent pipe, I should have said pressure relief valve. My friend described the same one as somebody else mentioned on here - an egg cup device to catch the odd drip. However his in France does not have a vent pipe/pressure relief pipe going from it, which I'm not very comfortable with. So I think I might just tap a hole in the wall and give the water somewhere to go if it did go wrong, especially as this tank would be housed on the first floor!
When somebody mentioned a BBA installer must do the work, I am not familiar with the BBA. I would get a CORGI registered installer to help me connect it up, would this be OK ?
What does an overflow pipe with 'tundish' mean ?
What exactly is a 'thermal store' ? Will this still provide me with mains pressure hot water & where is a cheap'ish place to get them ?
Does anyone know if the French cylinders generally come with all the fittings, such as the pressure relief valve (with expansion lever), the egg cup bit, etc ?
If the cylinder has a lever to provide an internal air bubble, would that be legal in the UK (instead of having a separate expansion vessel) ?
Can anyone forsee any problems adapting the C/H system if I go with this idea ?
Thanks again to everyone, this is really useful.
Peter
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Personally, I think you'd find it very difficult to get such a system approved in the UK. By the sounds of it, they're deathtraps in comparison to UK certified designs. The French never seemed to have quite a same safety culture that they do here. It's what makes their water parks more fun. (4 down the flumes simultaneously and the "rapids" where you get crushed by successions of dive bombing kids and always end up with scratches and grazes).
Christian.
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No. He has to be BBA approved. Many Corgi are, many are not.

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says...

When our thermostat on the electric heater stuck on, the excess pressure was relieved via the same "egg cup valve". The spring holding the valve shut was unable to hold the valve closed against the pressure of greater than the preset point (it was adjustable to accommodate differing water pressures). The hot water was ejected, slowly at first, until the overflow pipe couldn't carry the flow. We then had a hot water wash of the upstairs carpets... (concrete floors upstairs) I suppose this cycle would have continued with the balance between the cold entering the tank, being heated and ejected via the pressure relief valve until we decided we'd had enough of the carpet wash... Needless to say, the tank didn't blow up, but the pressure at the hot taps was somewhat elevated.
I liked the mains pressure hot water but I agree there does seem to be a potential safety problem... Perhaps that's why they are cheaper than the approved UK equivalent.
John
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On UK systems:
An unvented system incorporate:
- A high-pressure relief valve. If pressure exceeds the set limit the valve opens releasing pressure to a safe level. - A high-temperature relief valve. If high temperatures occur, typically 90-95C, the valve opens to amtosphere..
Whichever comes first, the high pressure or temperature, the valve concerned will open. There are two high limit safety devices.
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Indeed. The UK spec version would have had
(1) pressure relief overflow to outside. (2) temperature relief overflow to outside (3) a second manual reset overheat thermostat on the immersion heater.
Christian.
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Peter Cherry wrote in message
My 2 euro cents worth, a few random ramblings about these cylinders.... ..

The French ones are set at 60 deg C, you can't adjust them AFAIK. So this will cost you more in electricity but the French ones seem to be much better insulated than the Irish (which I suspect are the same as English) ones, the water stays hot for days. Also they look nicer :-)
If perchance the 60 deg issue makes you think you might buy a French one with a coil and plumb it into your existing system - don't! They are very expensive and they need a separate pump since the coil is horizontal and water won't flow through it by gravity. This is not mentioned on the packaging or in the sales literature. We found this out to our cost when we fitted ours and it didn't work off our carefully newly plumbed system to a wood burner. We have never got around to going back and redisigning the system and adding the pump.

We have two of these cylinders. One, which we fitted ourselves, appears to have no pressure relief system at all, I suspect it has some inbuilt but not easily visible system, no idea where the overflow would go, will check with husband and post again if he knows more. The other one (Thermor brand) has a valve at the bottom, with a lever as someone else described. This is plumbed into 32mm plastic waste pipe and goes into the rest of the drainage system. Not sure what would happen to 32mm pipe when confronted with boiling hot water under pressure!

It would seem it depends which one you buy as regards the pressure relief bits, you would need to check. BricoDepot should have them on display.
You asked about fittings. Don't think you will have a problem with these because the threads seem to be the same, but it might be worth just buying the two connectors to get you from the thread on the cylinder to your 22mm copper, just in case.
You will need to think about the electricity supply too. The French ones come with two cables, each 3 core 2.5mm, both of which can be hard wired or put onto a plug and socket. One is to run the immersion and the other to run an anti-corrosion monitoring system (French tanks steel not copper) The A-C system one has to be connected all the time, the immersion I expect you would put on a timer. AFAIR the English regs will require heavier cabling than this for the immersion so you would have to check whether you can get into the electrical system bit under the cylinder to change the cabling right back to the heater. Hope that bit makes sense and that the rest of it is of some use.
Dim question for everyone else.....this pressure relief system is obviously a serious issue since you are all so concerned about it, can someone explain why please? I would have thought that since the water in the cylinder is open to the mains, any expansion of the water caused by increase in pressure would sort of push its way back up the feed pipe. Mains pressure not being all that high, and variable, why wouldn't this absorb the pressure sufficiently? I'm sure there is an obvious answer to this but I can't work it out.
Holly
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There is a non-return valve on the mains pipe. With only one point of safety, the pressure relief valve, if this fails you are in trouble. The British have temperature and pressure releif and a cut out on the immersion if it goes above a certain temperature. If you can, fit two pressure relief valves on the French cylinder. If one failes the other works. Otherwise if it explodes it can kill.
If the immersion is set to 60C then install a blending valve on the DHW draw-off. Then the user can select whatever temperature.
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