Can all copper hot-water cylinders withstand mains pressure?

Many conventional copper cylinders are rated at up to something like 4 bar - so in theory may withstand mains pressure depending on the local mains pressure. However what they would lack are all the ancillary controls that make unvented cylinders safe to use.

If you attempted to just feed mains into a conventional cylinder and did not have things like the expansion vessel, they it would likely rupture the first time you tried heating it.

For an example of what is required, see:

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Reply to
John Rumm
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If there is a header tank in addition the cylinder, then its not unvented.

(It may be an indirect cylinder with a sealed system primary - but that is a different beast to an unvented cylinder).

Should be fine...

Reply to
John Rumm

If going to the hassle of moving it, would it not be easier to just install a real unvented one in the new location and be done with it?

Reply to
John Rumm

A case where DIY is the way to go if you want exact control over what is going on!

Reply to
John Rumm

And be careful who you get to specify the size of the boiler expansion tank. Our oil-fired combi boiler has an expansion vessel (as opposed to header tank in loft) for the central heating system. On our old boiler, the vessel was inside the boiler. When we had problems once with the boiler we called out our supplier of oil who are also (so they advertise) qualified heating system maintenance engineers. They diagnosed a perforated bladder in the vessel and fitted a new one. When that boiler failed a year or so later (corrosion of the heat exchanger *) we called out a different independent engineer who was definitely qualified, member of some professional organisation (I forget which).

This second guy made three comments: firstly the boiler was not economically reparable (he checked prices with the manufacturer) and we'd need a new one; secondly the expansion vessel was far too small for the volume of water in the pipes and radiators, and thirdly the new oil tank that the first company had fitted for us some time previously when the original one was badly rusted, was too close to neighbouring properties. He said he was obliged to mention the tank placement to us each time he serviced our system, but he wasn't obliged to report it to anyone (so it was an intermediate level of non-compliance). As regards the expansion vessel, he said that someone had wrongly specified the size that was needed (and maybe our oil supplier had just replaced like-for-like). Sadly the correct size vessel is too large to fit inside the new boiler (**) so he had to install it high up in the downstairs loo, tee'd off the main CH output from the boiler (or maybe it was main CH return to the boiler) in logically the same place as one in the boiler, except further along the pipe.

So don't always assume when the expansion vessel fails that it should be replaced with the same size, if the original one (installed when a previous person owned the house) was the wrong size.

(*) The second guy wasn't prepared to say that the small vessel *had* accelerated the failure of the heat exchanger through too much cycling of water pressure, but he did say that it was plausible that it may have been...

(**) The original one had been 5 litres and the correct one was 25 litres - not a trivial increase :-)

Reply to
NY

And consider using a thermal store which appears to have the benefit of mains pressure operation without the attendant safety issues of a pressurised cylinder.

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Reply to
Tim Lamb

All fair enough. But to put the counter-argument, we've just replaced our boiler and if the well-qualified person who replaced it had taken too literal a view of the regs we would have had a major problem with resiting it, and consequential plumbing. And regards the expansion tank if there is at least about .5 bar cold and it doesn't go above say

2 bar at maximum temperature then it is big enough, whatever theory says. Perhaps a little bit of Hydrogen in some of the rads helps here!
Reply to
Roger Hayter

There was a case a few years ago, where a faulty immersion caused hot water to go up into an ordinary plastic cold water tank. Eventually, that split, and of the couple in bed underneath one died and one was somewhat less seriously injured.

Reply to
GB

Very valid - practice often wins over slavish observance of theory. I

*think* in our case the second guy found that the CH pressure actually *did* go over 2 bar (or maybe we told him that this is what we'd observed at times before the boiler failed) which turns it from a theoretical to a practical problem. It was a while ago but I've a vague memory that he may have tried the original vessel with the new boiler in case it worked OK, despite the theory.

I do remember him saying that gas in the radiators reduces the pressure variation but is not something that can be relied on in case one day someone bleeds all the radiators and removes this buffer, causing hot pressure now to exceed the acceptable maximum.

Reply to
NY

Indeed. Sorry John, my typo! It's a standard vented system. I'd like to replace with unvented eventually but otehr things have got very much in the way, so first I want to move it to make space in the bathroom so I can refurbish that, re-route a bunch of pipework etc, then subsequently go to unvented. Unvented will have no effect in the bathroom (apart from maybe the odd leak if I don't deal with the pipework first), but will allow me to get DHW to the top floor, above the level of the current F&E tank.

Reply to
GMM

NY wrote: snip

I was being a bit facetious, and certainly if your (rather large?) system needed a 25l reservoir you will have run into problems with 5l. Where we agree is you have to have good knowledge *and* judgment to know what rules to break.

Reply to
Roger Hayter

no. buy a proper tank, anmd also install water softener if in hard water area

Reply to
The Natural Philosopher

For a cylinder heated from a boiler, there is not really any attendant safety issue - the boiler is not going to be able to super heat the cylinder.

For an electrically heated one there is more of an issue, but even then to have a serious failure would require defeating something along the lines of five separate protective devices i.e. stat in the immersion, secondary safety stat in the immersion, main stat on the cylinder, then the over pressure release valve, and the over temp / combined pressure release valve.

Reply to
John Rumm

Yup I saw your later posts.

Having done nearly what you describe for you future plans (i.e. installing an unvented cylinder on the ground floor and decommissioning the cold cistern and CH F&E tank on the first floor. I would say there is not much difference in the amount of work to do - although that will depend a bit on where your cold mains is in relating to the cylinder and how much extra piping you need to do.

In my case I could loop back the mains that used to feed the cistern to what used to be the cold feed from the cistern to the hot water cylinder, and use that to feed the unvented cylinder. The only extra bits were installing the drain pipe and tundish etc, and hanging the expansion vessel.

Reply to
John Rumm

A while since I read the rules.. firstly ISTR a requirement for the installation to be carried out by a *trained* person.

Second an annual inspection recommendation morphs into a requirement if the property is to be let.

I'll report back if the heat transfer from my thermal store proves inadequate:-)

Reply to
Tim Lamb

Yes the person installing needs to know what they are doing. There was a time where part G3 of the building regs only applied to unvented cylinders, and normal gravity fed ones were outside the regs. However all hot water cylinders are now included in G3. So in terms of officialdom the treatment of both is the same. (although the technical competencies for unwanted are different).

Having said that, I would not be surprised if there have actually been more injuries from conventional cylinders than unvented ones in the uk.

(Note that the actual annual servicing required is fairly trivial - on mine one needs to manually operate the safety valves to check they work, check the pressure in the expansion vessel, and check and clean (if required) the strainer on the input multifunction valve).

More than likely. Something to add to the annual gas safety check, electrical inspection, smoke and CO alarm inspection etc.

Reply to
John Rumm

If you've got a header tank, it's unlikely to be an unvented cylinder!

Reply to
Roger Mills

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