If a house has copper-cylinder hot water that is currently fed by a header tank in the loft, is it likely that the system can be converted to feeding the cylinder from the rising main instead (so hot and cold taps are at same pressure) or are cylinders for tank-fed made to withstand a lower pressure than those for mains-fed?
no! not using the copper cylinder.
NT
Ah so the cylinders fitted in modern houses with mains-fed hot water (like my previous house, built in 2000) are a thicker grade of copper to withstand the extra pressure?
NO. Emphatically NOT.
Yes.
British Standard BS1566 Grade 3 copper for domestic use is rated to 1. bar or 10 metres maximum working head. Grade 1 copper for commercial is rated to 2.5 bar 25 metres max head.
Grade 4 is even thinner
Unvented systems are typically pressurised at 3 bar working but must be able to withstand higher pressures up to the setting of the relief valve which is typically about 6 bar.
and unvented systems over 15 litres must only be installed by someone with an unvented hot water qualification, and inspected annually by same.
Owain
Yes, although many unvented cylinders are made of steel.
Owain
Ah, I didn't know that. It's difficult to tell now that cylinders are usually covered all over in foam insulation so you can't see the metal.
I presume "unvented" is synonymous with mains-fed and "vented" with header-tank-fed, because only the latter allow steam to push water back into the header tank if the heated water gets very hot and to compensate for the expansion of water as it gets hot.
An unvented cylinder is a totally different animal and, apart from being much sturdier, has lots of built-in safety features such as over-pressure/temperature valves which discharge into a tundish. The mains feed needs to be via a pressure regulating/non-return valve, and there needs to be an expansion chamber to allow the water to expand without causing excessive pressure when it gets hot.
Precisely.
Plus they have built in expansion vessels and pressure relief valves (that need to be connected to a drain).
Without wishing to hijack the thread, I have been thinking of moving my unvented cylinder down one storey, leaving the header tank in place (which is just above the cylinder at present). In principle, this should leave the DHW pressure through the house unchanged, but increases the head to the cylinder by (say) 3m.
Anyone care to quote the odds on this ending in tears?
We're having one such fitted as part of the modernisation of my late mother's property.
What I don't understand is why they need the expansion vessel and pressure relief valve. Why doesn't any overpressure just expand back up the water main? Or put another way, why doesn't the mains water pressure define the maximum pressure experienced in the tank? Or are they there as a safety precaution to cope with the situation when someone closes the main stop-c*ck, deliberately or otherwise?
The PRV is essential, given how dangerous very hot water is. And yes the system has to be robust enough not to explode if the mains tap is turned off, or a stop c*ck elsewhere in the supply is turned off, and in any case you are not supposed to feed water back into the mains.
The expansion vessel is necessary for when the water heats up (and expands). Otherwise, water could be forced back into the mains.
UNvented cylinders do not usually have header tanks.
If it's a vented system, unless you have a very tall house, a grade 4 cylinder, or something's worn out (most likely) then moving the cylinder should be possible.
Assuming you have a non-solid-fuel boiler with a pumped primary. If you have solid fuel or gravity primary moving the cylinder may end in disappointment, if not actual tears.
Owain
Not quite, many installations these days are sealed on the heating circuit but not the supply side. These use a conventional vented copper water store but no expansion tank. Instead of an expansion tank the heating circuit and hot water supply circuit to the tank coil (but NOT the tank) are sealed and pressurised. Changes in water volume as it heats and cools are taken care of by an expansion vessel (often built into the boiler) rather than a small feed and expansion tank in the roof. The heating circuit pressure is usually about 1 bar. However, all the stored water in the hot water cylinder is fed from the main tank in the roof and is NOT at mains pressure. See the left hand diagram under sealed system at
A fully un vented pressurised system using mains water pressure to feed the storage tank and needing no cold water feed tank at all is a different beast and requires pressure vessels, installation by a qualified fitter and annual checks. It is shown in the right hand diagram under sealed system at the link above.
At a 5 bar pressure, the water could be at around 150C.
Oops - sorry, that must hvae been a freudian slip! UN-vented is my aspiration, but vented is what I have and this move is an intermediate move to put the cylinder where the new one will be so I can sort the bathroom first.
It's a tall house but there is currently only DHW on two floors, indirect, pumped from a regular gas boiler. Moving the cylinder down will give it maybe 4 - 5 m of head in total, which *should* be safe, but we all know what that means....
I would expect it to burst.
Possibly fatal too if someone gets a tank full of hot water on them.
A mains pressure un-vented system will have:
A pressure reducing valve to drop the pressure to about 3.5 bar. A relief valve on the tank that vents if the pressure exceeds about 5 bar *or* the temperature exceeds about 90C. The vent will discharge outside the house somewhere like the one for a sealed boiler system. It will also have a none return valve to stop the water going back to the cold taps as it expands (legionella and such stuff) and will require a fairly large expansion vessel to cope with the expansion.
HTH.
Get an un-vented cylinder, they work vented and you can add the rest later if you don't want to do it now. However I don't see much saving in work doing it that way.
"dennis@home" wrote in news:596cd724$0$64285$b1db1813$ snipped-for-privacy@news.astraweb.com:
A case where you hope the tradesmen have read the instructions and regulations.
How old is it?
Brian
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