Since it has been mentioned a few times recently, I have updated and expanded John Stumbles' original wiki entry on unvented cylinders to include some more detail etc:
Fast re-heat is listed as an advantage. Is that only because unvented cylinders /tend/ to be manufactured with more effective coils than vented cylinders?
[...]
Are you sure this bit is true:
'The discharge pipe must terminate outside of the building in a safe place such as a gully where it can't cause injury'
Our discharge pipe terminates inside the house in an internal soil stack, running via an Hep2O valve after the tundish. The whole estate, built in 2007, is like this so I am assuming it must be fully compliant with the regs?
Mathew
running via an Hep2O valve after the tundish. The whole estate, built in 2007, is like this so I am assuming it must be fully compliant with the regs?
Clause 3.60 of G3 applies, i.e., it is permissible if the system conforms to the G3 specification.
Yup... (and slightly better insulation as well perhaps)
Its a slight simplification - you can also take it out a few other ways such as at low level onto a surface, or onto a roof etc
Secti3.60 The discharge pipe should not be connected to a soil discharge stack unless it can be demonstrated that the soil discharge stack is capable of safely withstanding temperatures of the water discharged, in which case, it should: a. contain a mechanical seal, not incorporating a water trap, which allows water into the branch pipe without allowing foul air from the drain to be ventilated through the tundish; b. be a separate branch pipe with no sanitary appliances connected to it; c. if plastic pipes are used as branch pipes carrying discharge from a safety device, they should be either polybutalene (PB) or cross-linked polyethylene (PE-X) complying with national standards such as Class S of BS 7291-2:2006 or Class S of BS 7291 -3:2006 respectively; and d. be continuously marked with a warning that no sanitary appliances should be connected to the pipe.
such as a gully where it can't cause injury'
running via an Hep2O valve after the tundish. The whole estate, built in 2007, is like this so I am assuming it must be fully compliant with the regs?
I was more or less told to stick it out the wall by the BCO. Into a gully I think
Id be interested to know if a soil stack is an allowed discharge route. I guess in a block of flats the tendency is to have all the drains in a 'core'
I found an NHBC newsletter which says it is, subject to a few provisos:
In your section concerning expansion vessels you say " This is also normally connected to the multipurpose valve described above."
We've recently discovered that our plumber had fitted it to the HW outlet. This resulted in premature failure of the pressure vessel and is NOT recommended by Albion. It also creates a HW "dead leg" which I believe is bad from the bacteriological point of view.
He's coming back to sort it but it might be worth highlighting the two problems associated with expansion vessels fitted to the outlet.
Tim
Some modern boilers that support split temperature operation can also run anti legionella cycles that take the whole cylinder up to 75 once per week to kill any lurking bugs.
I suppose there are a number of places you can connect it and it will work - however some are going to be better than others. Perhaps the best a general article can say is follow the directions of the manufacturer.
But that's not going to heat up a downstream expansion vessel to the same degree.
Indeed! My plumber claimed to have seen lots of installations with the expansion vessel fitted to the outlet but I'm sure it's just plain "wrong". Our first expansion vessel failed after just two years and when I phoned Albion regarding expansion vessel siting, without any prompting they said that premature failure would occur if fitted to the outlet.
Tim
Well if its on the outlet, then the first time you draw off water it will probably get refilled with very hot water... Still probably better on the cold side though.
(although its worth noting that if you have a secondary return connection to the multiway valve rather than an extra tapping in the cylinder, then the cold side may not be cold either!)
Does your one have provision on the inlet valve to attach the vessel?
Yes it does but where the valve's fitted, it's inaccessible. He's going to tee off the tank feed immediately after the PRV now.
Tim
It can be quite an achievement working out how to pipe all the connections - they seem to design them so as you have a pipe emerge from every conceivable direction!
If he sticks the vessel higher than the cylinder (if not already) it also means its easier to service, since you can shout off the water into the cylinder and open a tap, then the vessel should empty all its water back into the tank leaving it "dry".
Firstly:
"4. Connection for a secondary return (often provided by an extra tapping on the cylinder)" appears in the diagram legend but, unless I've missed something, isn't mentioned in the main text. Since 'secondary return' is usually down the drain I'm wondering what this connection might be used for. I can only think of pumped secondary circulation loops - often used in hotels etc., but rarely found in a house?
Secondly:
"Small unvented storage cylinders of less than 15 litres don't have these, [...]"
Sometimes they do: pressure reduction valve & expansion vessel kits are always available as an optional extra. Also that volume should read "of
15 litres or less.""[...] and the small amount of expansion is accommodated by backing the water back into the mains. The only requirement is to check that all stop valves upstream are not of the 'loose jumper' type which could act as a non-return valve and block the expansion)."
Yup, pumped circulation loops for "instant" hot water at point of use.
Yup good point. Perhaps "are not required to" would be better phraseology.
The small cylinders still have the same protective valves IIUC...
Well they're exempt from building regs, but to install them safely you need to follow the manufacturer's instructions. These generally will require a pressure reducing & non-return valve and expansion vessel (supplied as an optional kit) if the mains pressure exceeds a stated amount (typically 4.5 bar) or if the expansion volume can't be accommodated before a CW draw-off point. So "may not be required to" would be better, IMHO.
As above. If the expansion kit isn't required you only need the pressure relief valve, AFAIK. RTFM :-)
I have rephrased the bit a little, see if you prefer that.
The only really significant mention in doc G now seems to be:
"3.19 Water heaters with a capacity of 15 litres or less that have appropriate safety devices for temperature and pressure will generally satisfy the requirement set out in G3(3)."
Its actually quite tricky finding a suitable FM to read for the dinky ones. This looks like a reasonable example:
(must admit I am tempted to exclude the teeny cylinders from this article anyway, since they are not really the focus of it, and add extra "noise" to the narrative... what do you think?)
Yes, it's fine now.
Probably a good idea, but as the title appears to cover all unvented DHW you might want to add a sentence in the introduction to say that they're excluded.
Here's another manual:
OK, added that.
Also added a section on selection of cylinder size (although that might really belong in a separate article since its applicable to any type of cylinder)
Thanks for that.
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