So confused Vented/Unvented mains pressure or combi

Hi, Have moved into a house, and should have really turned on the hot tap before we purchased, but we didnt and are now left with a dribble of a shower. The property is a 1920s semi with a baxi WM 532 RS boiler, 8 radiators one bathroom usual appliances and an airing cupboard with a hot water cylinder that has very poor insulation. In the loft we have the usual header tanks (Not very high). Although the boiler works perfectly never even hear it, we were going to look at replacing with a more efficient one and was thinking of a combi, luckily we didnt make that decision.
We are now looking at replacing the hotwater cylinder with either a pressurised unvented cylinder, however my only concern is where the pressure relief would be run, also the mains supply is only 15mm pipe with no possibility of upgrading (as its a shared supply with next door), also I have found out that my boiler would not be suitable for this type of system so would require upgrading.
The other possibility is the mains pressure vented cylinder but know little about these, would they offer the same pressure as the unvented type? also have been looking at boilers and most say 22mm gas inlet the supply to the boiler is only 15 mm would this be a problem?
So in summary my questions are: 1) What would be best a vented or unvented mains pressure HWC? 2) Would increasing the header tank by a meter benefit the shower much? (at present theres only bout 1/2m drop from the tank to the shower. 3) Will the 15mm gas supply be a problem? 4) Will the 15mm mains water in let be a problem? 5) what rough cost im in Essex.
Thanks for any help Olly
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In an earlier contribution to this discussion,

Do you really want either - see below?!

No.
It probably would if you wanted a combi-boiler capable of producing 'real-time' hot water at a decent rate.

Very probably. Measure the flow rate at your main inlet point - probably the kitchen tap. If it's less than 20 litres/minute forget a mains-pressure hot water solution, be-it combi, non-vented hot cylinder, or thermal store.

No idea.
It seems to me that you are overlooking the most obvious - and almost certainly, cheapest - solution, which is to install a shower pump. It needs to take its hot feed from the HW cylinder (preferably via an Essex or Surrey flange, but that may not be necessary) and its cold feed from the cold header tank. That will give you a decent shower for very little effort.
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Cheers,
Roger
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snipped-for-privacy@hotmail.co.uk wrote:

Insulate the cylinder and pipes, and fit a shower pump?
Owain
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On Sep 30, 9:13 pm, snipped-for-privacy@hotmail.co.uk wrote:

Also look at heat banks.
A
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Yeah probably should have said we did look into the pump situation, fitting a single pump for the HW would be no problem, however because of the setup fitting a twin pump for cold water as well wouldnt be easy and wasnt too keen on the noise
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Well just tested flow rate and its 20 litres a min. Would this be sufficient for a mains presured hot water cylinder such as megaflo or would I require the pressure of the water as well. Does this mean that in theory a shower would produce about 40 l/min if both hot and cold are fully open?
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In an earlier contribution to this discussion,

Flow is more important than pressure - unless you live at the top of a sky-scraper.
> Does this mean that

Only if you can find a way of defying the laws of Physics! <g>
Of the cold water coming into the house, some will go to the cold tap and some will go into the hot cylinder, expelling hot water to the hot tap. But you've only got 20 litres/min in *total*. Where do you think the other 20 is going to come from?
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That isn't how it works. Pressure is not the same as flow.
20lpm would be just about enough for one shower with single, simple shower head - i.e. not a drencher or ody jets etc.

No it doesn't. 20lpm is the *total* coming into the house. Some will go to the shower as cold water, while some will go via the cylinder, but it can never exceed 20lpm.
The only way that you can make a difference to that is via some means of storage. This is why roof tanks can deliver large volumes of water.
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On Mon, 01 Oct 2007 23:19:49 +0100, Andy Hall wrote:

I declare a plague on both your houses.
It depends on the distribution of the flow resistance a significant amount is almost certainly the last 0.5m to and through the kitchen sink tap, especially if flexibles have been used. You might not get a lot more out of the sink even if the supply to it was in 32, 28, 25 or 22.
So you will get more total flow, certainly not double, and more than none extra. Anyway 20l/min is quite good for a shower for most people.
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Ed Sirett - Property maintainer and registered gas fitter.
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Thanks ed, I was a bit confused that if one tap was turned on the flow would drop significantly obviously it does but I would imagine a combined flow of 25-30 L/min not unreasonable. The cold inlet is only 2-3m from the bathroom at max, and the same from the airing cupboard, all fed by 15mm pipe.
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