Poor pressure on a megaflo??

[chrishutt]

Simon, I've modified my previous post to get around the block, in the interests of fairness. However I have to say you brought the block upon yourself by self promotion. We're all in business but don't generally use the forum to tout for trade.

 

[simond]

Apologies for that Chris,

The original respondent way back in this thread was in our service area and I wanted him to know who we were.

I will not be so brazen in future!

Simon

 

[ChrisR]

Last time I visited the hwch site the button didn't do anything, now it does.

I looked at esp@cenet for the patent quoted on the GAH site. I used the search using the number and the name and it didn't come up. The silly balanced flue one did though...

 

[doitall]

Firstly, I'm not sure what your first sentence means. My example was based on a mains pressure of 2.0 bar (for the sake of simplifying the subsequent arithmetic). I understand that the stored energy cannot be greater that that provided by the static mains pressure (excepting the extra margin due to expansion). But are you perhaps not taking account of the low dynamic pressure that would result from an inadequate supply pipe from the main, which would be the reason for considering an accumulator in the first place?
Secondly, expansion vessels are fitted after the combination valve (PRV, etc.) and an accumulator is just an oversized expansion vessel, so I don't see why it should be in breach of regs.

The OP has a poor water supply delivering 17 litres/m, the 1.5 pressure was an assumption as we do not know the actual.

SimonD said he could fit an accumulator with a standing pressure of 1.5bar to feed the un-vented cylinder and solve the problem.

Megaflo cylinders minimum recommended is 20 litres at 1.5bar, I say the accumulator will make no difference.

To store usable energy it has to have a pressure differential in the 4-1 bracket, 3-4bar works well with an operating pressure of 1.5bar, a storage tank represents stored energy, the higher the tank the greater the pressure, lower it to the same height as the cylinder and nothing comes out the taps. The original batteries were accumulators that stored energy (electric) for low power radio's etc.

An accumulator works by storing energy at a higher pressure to suppliment the poor flow and to provide a constant, therefore the lower the drawoff the greater the benefit, Combi/shower as I said uses little water when compared to an un-vented cylinder and works well, provided you have a pressure differential as above.

As I have said before, we have installed several accumulators, and in the past used them as top up vessels for heating systems. (stored at 4bar system 1bar) where no mains has been available.

You may have noticed when you open a drawoff connected to a Megaflo you somtimes get a high pressure squirt of water before it settles down to a constant flow, this is because the pressure in the cylinder rises through expansion, the accumulator will do exactly the same, a small burst and the flow will diminish rapidly, also as I said, pressurise an 85litre accumulator to 1.5bar and isolate the supply, and measure the available water at the outlets before it reduces to a trickle, (20litres if your lucky)

 

[simond]

Mr D

The pressure differential seems to be fundamental to your particular argument but is an irrelevance.

A DualStream unvented/accumulator system is limited to 3.5 bar at the combi valve. By your somewhat dodgy reasoning this would mean that you need 6bar at the incomer. The combi valve would reduce this to 3.5bar anyway - the accumulator is after the combi valve. For reasons you are having major problems understanding, it would not work so well if it had 6 bar! (Due to the available volume of water).

Did you not read Chris' explanation of Boyles Law and how you can compress a gas?

My home installation here on the Kent/Surrey borders has a 3bar incomer. I can fill two baths and three showers simultaneously. If I fill one bath I have to turn the taps down or it comes up the sides!

We fitted an accumulator on a Vaillant AquaPlus hybrid combi (output 18 litres per min) 3 years ago, it is incredible. The Vaillant product line came over from Rochester to see it. The owner complained that it she dropped the shower hose it snaked around the base of the bath and she couldn't pick it up.

 

[ChrisR]

Aquaplus has a pressure vessel on UV hot water. Still waiting to hear why that doesn't infringe the patent.
We installed several of those on very good mains supplies. They were OK. Nothing anyone would describe as incredible about them at all.

3bar incomer.... If I fill one bath I have to turn the taps down or it comes up the sides!

That's odd, I don't have that problem with pumped 4 bar supplies. Perhaps you have small outlet diameter taps.

 

[doitall]

Pressure differential is how and why an accumulator works, by storing energy at a higher pressure so how can it be irrelevant.

A Dualstream may be limited to 3.5 bar at the combi valve, how are you going to convert 1.5bar available pressure to 3.5bar.

At 3bar you could do that without an accumulator if the volume matched the pressure, an 32mm mains at that pressure would eat it. how many baths can you fill if you isolate the mains. I suggest none.

18litres a minute at that pressure she must have a very small handset, fit an 150mm drench rose and the combi wouldn't cope with the hot water demand as the flow rate would be nearer the 45lt/m mark.

When fitting the accumulator to the Megaflo as an expansion vessel how does this effect the existing air gap, do you install an additional PRV.

And the ten dollar question, assuming you connect the accumulator to the cold feed to the Megaflo, water will only expand X times per litre capacity x temperature rise and will be taken up internally by the air gap, how can the accumulator use something that isn't available.

 

[simond]

To answer the question about isolating the water supply; it makes no difference, Mr Doitall. my baths and showers will run exactly as before until the water runs out - in my case, over 200 litres later. I have a 450 litre nominal accumulator. The flow rate and pressure does decrease gradually as the accumulator charge exhausts itself.

There are restrictions to fitting an accumulator to a Megaflo, these are commercial, not technical.

The PRV is not affected, if you understand unvented you will not need to ask. As I explained earlier in the thread, the expansion of the unvented cylinder can be taken up by the accumulator, therefore a dedicated exp vessel is no longer needed. We are not using the heat related water expansion as a motive force, we are just removing the need for unnecessary plant.

Chris R, our first combi accumulator was on this AquaPlus 3yrs back and Vaillant thought it was well worth travelling 50 miles to see. Without the accumulator the customer (with 9litres per minute) could not enjoy a decent combi. I think this is progress, if you disagree, so be it.

Like most of the people reading this post no doubt, I have now tired of defending my position. We will continue what we are doing, and some of you can keep your heads in the sand.

The final straw for me is having this DoItAll individual telling me how my own home installation works. I'll resist the temptation to tell him how his combi works - but the gauntlet is still down, he can come and see unvented accumulators working providing he reports back here.

TTFN

 

[doitall]

Shame you can't answer the question, how does 1.5bar become 3bar without a pressurization device such as a pump.

To get 200 litres without any make up you must have a huge accumulator, 1000ltrs minimum.

I suggest they are technical as in it cannot be done. the expansion is taken up by the internal expansion (air pocket) and cannot be taken out because you want to fit an accumulator.

If your not using heat related water expansion then the maximum you can charge the vessel equals the available mains pressure.

How very trying running off without an explaination.

Haven't got a combi, and it will be a pleasure next time I'm in town.

 

[chrishutt]

... pressurise an 85litre accumulator to 1.5bar and isolate the supply, and measure the available water at the outlets before it reduces to a trickle, (20litres if your lucky)

That doesn't tally with my specimen example posted earlier. According to that logic, an 85 litre accumulator with a precharge pressure of say 0.25 bar which is then pressurised to 1.5 bar will have its air compressed to 42.5 litres (50%) since the absolute pressure will have doubled (1.25 bar to 2.5 bar). It will therefore store 42.5 litres of water, not 20 as you say.

With low mains pressures the precharge pressure of the air in the accumulator does seem critical. I've assumed a precharge pressure of only 0.25 bar above just to keep the arithmetic simple, but the precharge pressure would have to be 0.9 bar (gauge) to give only 20 litres of stored water.

Thinking about it a bit further, it seems optimal to set the precharge pressure to match the lowest dynamic pressure, as happens to be the case in my specimen example. The storage capacity will then be proportional to the pressure differential between static and lowest dynamic. Does that make sense?

 

[doitall]

The precharge pressure should be 1.5bar below the static pressure, or a minimum of 0.5bar.

Its not a question of how much stored water, but how much stored water you can force out under pressure. remember the more water you take out the lower the pressure will be.

The vessel may hold 42.5 litres at 2.5bar, but if as in our example the mains is only 1.5bar then only a combined pressure of 1.5bar will be stored for use, you cannot add the precharge and call it 2bar.

The dynamic energy available is 1.5bar, and will diminish rapidly when an outlet is opened, leaving the majority of water at zero pressure still in the accumulator.

 

[ChrisR]

Chris R, our first combi accumulator was on this AquaPlus 3yrs back and Vaillant thought it was well worth travelling 50 miles to see. Without the accumulator the customer (with 9litres per minute) could not enjoy a decent combi. I think this is progress, if you disagree, so be it.

What a ridiculous thing to write.
I was challenging your assertion that 18l/min from a combi is "incredible". No, it's pretty ordinary these days.
Accumulators, and break tanks and pumps (which take up less space and are probably cheaper), have been in use for far more than 3 years of course.

 

[chrishutt]

The vessel may hold 42.5 litres at 2.5bar...

I still can't see the logic of your figures. Taking the 85 litre accumulator with a precharge pressure of 0.5 bar and mains static pressure of 1.5 bar I calculate that the vessel will then store 34 litres of water, not 20 as you suggest. To store 42.5 litres (50% of vessel capacity) would require a static pressure of 2.0 bar, not 2.5 as you say.

At least those are the figures I get applying Boyle's Law, as explained in my specimen example, which by the way wasn't supposed to relate directly to the OP's situation. I was just trying to establish the basic principles that apply. Since we come up with different figures at least one of us is getting it wrong! If it's me I'd like to know why.

One reason for discrepancies might be the difference between nominal and actual capacity. the 85 litre (nominal) vessel might I suppose have an actual capacity of say 80 litres which would make some difference.

 

[ChrisR]

I agree with your figs ChrisH in the last point.

But

The storage capacity will then be proportional to the pressure differential between static and lowest dynamic. Does that make sense?

isn't quite right I think.
Using Precharge = lowest dynamic pressure,
Vessel volume V and
Static Mains pressure M, (all pressures as measured on gauge)
Stored volume is given by V(1 - (P+1)/(M+1))

 

[Agile]

Perhaps its too early in the morning but it does not look right to me.

If there are 85 litres of precharge air ar 0.5 Bar and then water is introduced at 1.5 Bar then the water will squash the air into 1/3 of its previous volume ( adiabatically of course ). There will then be about 56 litres of water in the accumulator.

How can this be wrong?

Tony