Where to source a small accumulator for mains water

[Buzzark]

I may want to add a smallish accumulator to my mains water supply just to bolster the dynamic pressure for showering with whatever mains pressure DHW solution we end up with.

I was thinking 60 or 80L of water should be more than enough as this should recharge between showers (we're getting 20l/m now @ 2.2-2.6 bar) and it's about 5-10 mins between in the morning.
Actually, they're probably oversized? Maybe even smaller than that will do.

Plumbcenter do this one:
* JET 60 LTR POTABLE EXP VESSEL 10BAR
MAXIMUM WORKING PRESSURE 10 BAR 1%22 BSP MALE
REPLACEABLE DIAPHRAGM
Price (Excluding VAT) £180.79
Which I guess is the sort of thing I want as it's the potable water version.

Can't find much info on them otherwise, anybody know of any other places to source them?

I guess simond might have some answers here?

 

[chrishutt]

You could use any potable water expansion vessel or surge tank as an accumulator, but you'd need a lot more volume than you might think. Also much cheaper via a supplier like Antares. We had a famously long and acrimonious thread on it some time back - here.

 

[Buzzark]

Thanks Chris

I have read that one before, although probably not to the end. To me the pricipal seems sound in the application I want to use it for but let me run this past you regarding sizing.

Lets assume I just want a cold shower for simplicity as I know that different methods of DHW supply will absorb differing amounts of available pressure etc.

My figures actually were a little mis-leading. My incoming main will supply 20l/m @ a dynamic pressure of 1.5 bar - when static is 2.2
The accumulator holds water at static pressure and enables the dynamic pressure to remain nearer the static figure - or so I've concluded from the principles..

So my target figure might be 25l/m, so shouldn't that just use 5l/m from the accumulator as the main would replenish at the rate of 20l/m?
If the shower lasts 10 mins, we'd use 200l from the main and 50l from the accumulator?
The accumulator would then refill in roughly 2.5mins @ 20l/m?

I'm assuming that water volume is probably a bit over half the overall volume of the store?

I'm presuming you're pro-accumulator rather than being on the "it'll never work" side?

Erm.. lots of little questions there!

PS Is that the right website - just get page-not-found.
PPS I think the website is just down at the moment maybe.

 

[Agile]

Firstly, congratulations that you have now understood the difference between static and dynamic pressures! You are learning something from us!

However, you clearly now have a water supply which most people would die for.

A reasonable shower flow rate is 10 li/min. If you want a better flow then 15 li/min should be the target.

25 li/min is just totally wasteful and more than most film stars would want.

Consider the planet and all the unnecessary CO² emissions you are causing.

If you want to waste money then install a solar system.

Tony

 

[Buzzark]

Thanks Tony

I did understand all along the differences between the two, it just wasn't really relevant to the question I'd posed...

The 25l/m figures are "supposed" - just for calculations and giving some buffer on the sizing, I'm only really after equal to or a little more than our current pumped shower at 16l/m. Not that we open it up fully all the time anyway, it's just nice to have if I've been overdoing the sports or something and need a bit of theraputic force.

Though normal usage we're probably nearer the 12l/m flow.

 

[chrishutt]

So my target figure might be 25l/m, so shouldn't that just use 5l/m from the accumulator as the main would replenish at the rate of 20l/m? If the shower lasts 10 mins, we'd use 200l from the main and 50l from the accumulator? ....I'm assuming that water volume is probably a bit over half the overall volume of the store?

It's not that simple. The accumulator will initially supply much more than 5 l/m because it will start off with a charge pressure of 2.2 bar when the pressure of the main will be diminishing as the main flow increases. Towards the end of the discharge of the accumulator it will be supplying much less than 5 l/m since its charge pressure will have fallen close to the dynamic mains pressure of 1.5 bar @ 20 l/m.

Also you won't necessarily get 25 l/m because the mains flow will be reduced if the dynamic pressure is higher than 1.5 l/m. What you will get is a higher pressure and/or flow for longer, depending on the flow rate from the shower. For example at a target flow of 20 l/m your pressure will start at 2.2 bar and decline gradually towards 1.5 bar at which point it continues as if the accumulator wasn't there.

If you want to try for a more stable enhanced pressure you could use a Pressure Reducing Valve set at say 2.0 bar (fitted downstream of the accumulator connection), but of course that pressure would drop towards mains dynamic once the accumulator had discharged down to 2.0 bar. You will appreciate that we aren't talking about linear relationships here but curves. I'm sure I explained all that somewhere on that linked thread.

 

[simond]

Sizing the unit by calculation by notionally adding to the existing water flow rate will not work, this is a mistake.

The accumulator (in your case the Plumbcenter 30 litre actual , 60 litre volume) will discharge most of its volume in just over a minute at your desired flow rate, then you'll be back to the mains.

Which is a waste of your time and money.

You would need a >150 litre accumulator to deliver a worthwhile shower performance through a combi or unvented system.

 

[Buzzark]

It's not that simple.

You will appreciate that we aren't talking about linear relationships here but curves. I'm sure I explained all that somewhere on that linked thread.

I'm still working through that thread Chris!

Hmm ok, I see how that works - I guess I'd need to pump air into the other side of the accumulator for a linear performance. (Just for theory!)
I had kind of expected it wouldn't be constant and would fall as it emptied but I did expect that mains water would flow in as soon as the water started to flow.
You're saying that because the accumulator to begin with is charged to the static pressure then the incoming main doesn't really supply any of the water until the dynamic pressure drops enough to allow it in... the more it drops, the more it allows water back in from the mains. Yes, that does change the curve in that it would really need to be roughly double the size to maintain flow and pressure because you don't actually have the flow from the mains doing the feeding.

In my case though, I may just want to keep the pressure a little higher and the flow a little more e.g. 3l/m so while I'd need something larger than I was initially thinking I probably only want to deliver about 170l over a 10 minute period.
It's more like the mains supplementing the accumulator as it's charge falls than the other way around which is how I first assumed it worked.

The requirement is really to be able to deliver our normal shower flow which I guess is about 12l/m but to have something in reserve for occasionally blasting away dirt or mucsular pain etc. It's not often I turn the shower up to it's current pumped max of 16l/m as it is.


I wonder if it would be possible to give priority to the mains and to top up with an accumulator by plumbing it slightly differently?
I think it's normally done:
MAINS---NRVALVE--***---MAINS to OUTLETS
.............................***
.........................ACC on a TEE

Plubing to "top up" the mains supply

MAINS---NRVALVE---***------------*--------MAINS to OUTLETS
..............................***...............*
.........................NRVALVE......FLOW Restrictor (or 15mm pipe)
..............................***...............*
.............................ACC------------*

PS - excuse the crap ASCII art

It's just an idea loosely based on a venturi setup - although obviously the venturi principle isn't entering into it here, the accumulator has it's own pressure pushing water out.
I "think" this would allow the mains to flow more than the standard setup - but I reserve the right to be completely wrong!

 

[Buzzark]

You would need a >150 litre accumulator to deliver a worthwhile shower performance through a combi or unvented system.

Ah, cold hard figures - thanks! Is that vessel size or water content?

 

[chrishutt]

I guess I'd need to pump air into the other side of the accumulator for a linear performance. (Just for theory!)

Yes, exactly, to maintain the 2.2 bar of pressure forcing the water out of the accumulator. But then we're talking about pumped water supply which is another ball game.

I wonder if it would be possible to give priority to the mains and to top up with an accumulator by plumbing it slightly differently?

You could do that by putting a PRV (pressure reducing valve) on the outlet of the ACC so it would only release water pressure when the downstream dynamic pressure dropped below a certain level. Or you might fit a flow restricting valve (say set at 5 l/m) on the outlet. Either way you'd need a by-pass of the PRV (fitted with a NRV) to allow the ACC to recharge.

As for the size of the ACC, I don't think in your case there is one "right" size. The bigger, the better, but also the more expensive. Just decide what you feel is affordable and you have space for and go for that, although anything below 100 litres won't do much.

 

[Dan Robinson]

Jesus that was a long and very bitter post - which ultimately led to no real helpful advise - although it was interesting.


The upshot was (if I understand things correctly). A small accumulator is not worth squat; but a biggun will save you having to dig up your front garden. and turn your dynamic pressure into your static pressure for a proportionally diminishing period dependant on size and charge of said vessel. Flow rate will be dependant in pipe size/length & appliances in the house.


Yes? No? - and I don't want to start the argument again!!!!!! FFS

 

[chrishutt]

Jesus that was a long and very bitter post - which ultimately led to no real helpful advise - although it was interesting.

I assume you're referring back to the thread that was linked to earlier (not my post above!). Yeah, it did get bitter, but back then Simond was a newby on this forum (his first post was on that thread) and us old(er) hands were a bit resentful of this upstart, so there was too much of that stuff going on. I think we've all learned to get along more smoothly since then.

 

[Dan Robinson]

Yes, I am serioulsy considering an ACC in a clients' house in Hampstead, but just haven't has the time to run the calcs.

I did think that most of that thread was totally unneeded and a case of smoke and mirrors - and I read ALL of it, and I DO understand the principles, pros and cons of ACC's as well as break tanks.

The key benefit for me is the potable water side of things. But currently it would have to be sized to boost the flows of 3 combis, a 300 (possibly soon to be a 500) litre Unvented cylinder and one of our Multi's (safe to assume that it would be the equivalent of a 250 litre Unvented cylinder with the use it gets.


Space is not an issue. Although currently there is a major water hammer problem and not 3enough time in the day...

 

[Buzzark]

Thanks Chris, all useful stuff to ponder and potential solutions if I feel the supply comes up a bit short. Probably worth waiting until after a new boiler/DHW solution is all fitted to guage how far off, if at all we are.

Although, I'm tempted to just go out and buy one for playing with - I could get a small one, fit it and run some tests on that.


Dan, that really does make my install sound insignificant.

 

[Dan Robinson]

This polace makes most domestic installs look a bit weedy... At one point it was going to have 30 - 40 kW of ground source heating installed - we couldn't decide what would be hit first - the water table (a good thing) or the Northern Line (probably not a good thing).

Instead with the help of a certain pair of contributers here () they'll be getting 33kW of air-water heat pumps and 6 solar panels.

Possibly a wind turbine (planning permission permitting) three new condensing combi's (to replace the current TurboMax and two Microgenus')
and a system boiler to replace the ageing Mexico120 RS and a large twin coiled unvented cylinder to replace the 6" tall Megaflo - so there might be some spare parts floating around soon .

phew!