For Low pressure with Unvented Cylinders

[CopperCylinderMan]

Chris is spot on with his description of a potable water expansion vessel. At max expansion, the balloon wont fill the vessel, but it will be close, although you generally oversize the expansion vessel (so you'll never get max expansion) to prevent the pressure from exceeding the safety valve setting on an unvented system - too much expansion volume is always a good thing.

As for patents - whats to stop someone fitting such a vessel 'to combat water hammer'? Seems to me OSO are just good at marketing (I am biased btw so take any anti-OSO comments with a pinch of salt )

When does a shock arrestor become an accumulator? 0.5Litre like on a thermal store? 12 Litre like on a 150 Litre unvented cylinder? 100 Litre on the incoming mains?

CCM

 

[Paul Barker]

I use oso bubble tops in prefernce to megaflows, more useable water less cost.

Folliwing this discussion, is any of this worthwhile? If someone is running a bath, the most likely time you'll have a problem, won't the capacity of this vessel soon be used up and you've fixed nothing. If you can't get sufficient flow up the mains you have to store your own water and pump it, simple as.

 

[CopperCylinderMan]

paul - I dont think we are talking about 12 litre vessels here - some of the OSO units are 100-450 Litres.

assuming a standard bath - say 120 Litres mixed water, they should work.

 

[ChrisR]

If they were all made with a rubber balloon, then there would not need to be "potable" water ones and "smelly old CH water" ones, but there are. I'm sure I've seen that some at least of the potable w ones are stainless. They're usually white or blue instead of red of course.

For some reason I have always imagined that some of them have a diaphragm. Possibly because that word is used? That in my mind is a flat thing across the vessel where the join is, crimped in with the metal. So the water would be on the entry/exit side, and gas in the other. Could be utter bullocks of course. Anyone ever cut a combi PV up?

Reflect, those who haven't worked it out yet, on the precharge pressure.
In a combi it's about 0.7 - 1 bar.
In separate PV for an unvented cylinder it might be 3 bar.
In the top of a Megaflo style cylinder it's 0 bar.
In an accumulator it might be 0 bar. *
In a surge tank - dunno - answers on a postcard. If its function is to smooth out supply surges +ve and -ve then it might be about the same as the normal supply pressure.

* For an accumulator you'd want the precharge pressure to be significantly lower than the static pressure of the incoming mains. It would charge up to the static pressure when there's no demand, and discharge until its pressure matches the incoming mains pressure , when there is a demand. I haven't decided if there would be any advantage having a positive precharge pressure.

As for Stars, Chris H, Mars isn't one! The one I thought you meant is a red bipolar in an old Ford! Is your email address in your old Potterton?

 

[ChrisR]

(Chris is spot on with his) description of a potable water expansion vessel. At max expansion, the balloon wont fill the vessel, but it will be close, although you generally oversize the expansion vessel (so you'll never get max expansion) to prevent the pressure from exceeding the safety valve setting on an unvented system

Not really true. It depends on the precharge pressure. If it's 0 bar, that means relative to atmosphere, the absolute pressure would be 1 bar.

SO if you're using the vessel as a mains pressure store (say with mains at 3 bar), then the vessel would only be 2/3rds full of water. That's cos the precharge air would be compressed, until its pressure were 3 bar - which would occur when it had been squished to 1/3rd of its original volume. See?

This is absolutely nothing to do with the expansion you need to accommodate in a combi, or an unvented cylinder without a bubble inside. That's the expansion as the water gets hot. 4% of the HW volume is all you have to have though agreed, bigger is better.

 

[Paul Barker]

I had no idea, so what you are saying is that we are no longer in the combi era where you couldn't tolerate the loss of space from the airing cupboard, we've gone to the post combi period where you simply must have mains pressure hot water, a larger cylinder than your gravity one, plus a whole load of extra pipework and controls plus expansion vessel if it isnb't a bubble top, so a huge airing cupboard required, but as if that's not enough you now want me to find space for another huge vessel bigger even than the cylinder (which until recently was completely out of fashion).

I'm glad I have my 4 story house head of water with 3/4 inch pipes delivering plenty of hot water, and a good old simple copper cylinder which will outlast any gurantee on any unvented cylinder, much less danger, zero maintenance.

I'm not a luddite I have the unvented cert. and have fitted two since I got trained (what happened before we won't talk about) and am fitting one next week. BUT it all seems a little two faced all this shifting directions of the industry.

I don't think they need mains pressure vessels at all, but I'll fit 'em for the lumocks.

 

[CopperCylinderMan]

I do think that the era of a combi boiler being the 'answer to every solution' is over.

The HA's and councils went through a stage of fitting nothing but combis, as the install time and cost was significantly cheaper. They now realise that to fulfill their statutory requiements as landlords, if a combi boiler breaks down, they have to get it fixed in 4 hours (I think this is the figure) as the tennant has no hot water - this is expensive to achieve and the cost of ownersip of the combi boiler now outweighs its benefits - local councils up here (Scotland) are now fitting tradtional systems.

The other folk that were fitting combis - the private householder now realises that if they want fancy showers with body jets, or jaccuzzi baths or more than one shower at a time, a combi boiler wont do - and the best solution is an unvented cylinder.

in some cases, the mains pressure might be good enough to run such a scylinder, but the flow rate isnt there - either the mains pipe is to small or the flow from the street isnt high enough. The accumulator is only of use when the pressure is good, but the flow rate is poor.

Not every Unvented Cylinder needs an accumulator - in fact, they are pretty rare as most properties are suitable. This thread moved away from IN12354's question - whether it would work to pump from a break tank into an unvented cylinder, into a discussion of what might be a better solution in his case.

an unvented cylinder allows a property to have a 'dry loft' assuming electric heating or a sealed system boiler, and takes up not much more room than a traditional cylinder.

Sorry to sound like a rant, not meant as such, but you just cant beat an unvented cylinder, properly sized and heated, for a superb hot water system.

CCM

OOh, and try tying in renewable heat sources witha combi boiler - cos thats the next thing the govt is going to legislate for, but thats another story

 

[chrishutt]

For anyone who isn't sick of this thread, here is some detail of the OSO accumulator. Internally it looks a little more complicated than we thought, but not much.

By the way, ChrisR, I now see that our red giant does ride around in an old Ford, but with a sting in the tail.

 

[ChrisR]

Not every Unvented Cylinder needs an accumulator

INdeed so, but I only quote for about a third of the unventeds I'm asked to consider because of hopeless mains supplies. So often, the perceived success of the Megaflo installation depends heavily on the reservoir of squashed air. When I've been called to look at a misbehaving one, it's the lack of hot water pressure which is noticed, not the dribbling discharge, when the bubble has been absorbed.

Next time I think I'll try to take a punt and sell an external, large vessel, as a combined accumulator and expansion. More HW at pressure PLUS a longer period between call-outs. If a "Surge tank" turns out to be inappropriate in some way, no huge loss in terms of installation time or cost.

I'm kicking myself for going the "other way" - I've just put a 4 bar twin pump in my own house, partly because I know the supply pipe is 20mm mdpe. When I get fed up with the noise I'll sell the dang pump on ebay and fit an unvented!
Could have been worse, I might have had a mental holiday and fitted a combi.

To

you just cant beat an unvented cylinder, properly sized and heated, for a superb hot water system.

you have to add "properly supplied with water". All too often the mains in London is not much over 1 bar.

 

[ln1234]

The OSO Dualstream system sounds ideal for our requirements. Have any of you installed this system, or seen it working? I'd be interested to know how well it performs.

 

[chrishutt]

As I think ChrisR mentioned above, the Dualstream system seems disproportionately expensive, considering that much the same performance can be achieved using a generic "accumulator" at a fraction of the cost. But if money is no object......

Again as ChrisR suggested above, there is no clear distinction between expansion vessel and accumulator. The Dualstream accumulator is also the expansion vessel (depending on connections), and any expansion vessel, however small, will function as an accumulator. Of course the duration of that function will depend on its volume.

 

[ln1234]

So the dualstream system is no different than using a normal expansion vessel? Will the water still be potable using a generic one?

 

[chrishutt]

dualstream system is no different than using a normal expansion vessel?

I don't think anyone actually said that exactly - just that the principle of operation is the same. OSO obviously would beg to differ, anyway. The water would be potable (or wholesome - isn't this the approved word these days?) provided the vessel is for potable water. Usually coloured white rather than the red of CH expansion vessels.

 

[silverback]

Try looking at GAH heating www.gah.co.uk

 

[chrishutt]

Thanks for that Silverback. The GAH Dualstream is the OSO Dualstream. I noticed the following statement

Dualstream is protected by Patent No. 9905894.3 this protection applies to the use of any accumulator with any unvented cylinder and the application of its use subject to permission being obtained form GAH (HEATING PRODUCTS) LIMITED WRAS Certificate No. 0109003.

Since all expansion vessels function to some extent as accumulators, how can the above be valid? When is an expansion vessel an accumulator? When is a surge tank an accumulator? Can only be tested in court of law, presumably.