Unvented Cylinder - Pressure, Flow Rate & Pipe Diammeter

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Hi Folks,

Been doing a fair amount of research over the past couple of days, as a refurb of our 2nd floor has given us the opportunity to do away with the HW and CH system-feed tanks in the loft.

We've had a plumber around and he has measured static pressure (hit 8 bar on the garden tap before it blew off his pressure gauge) and dynamic pressure (6 bar in the garden with me holding it firmly on this time) and flow rate (24 l/min from the kitchen tap) of the mains.

The problem starts where this is all supplied throughout the property via 15mm piping from the mains stop-cock onwards. It's clearly blue MDPE (and I'd hazard a guess at 25mm) into the property but the stop-cock downsizes the supply to 15mm immediately, and this is then piped around the bottom floor taps, and up to the tank in the loft (and, again I'd guess, is tee-d off at some points to feed the 1st and 2nd floors). All CW taps are connected to the mains - with the stop-cock turned off they all stop working.

The plumber has recommended a Megaflo but their minimum specs require 1.5 bar, 20l/min, through 22mm pipe at the point of entry into the cylinder - which will be in an airing cupboard on the 1st floor. He did say replumbing 22mm from the stop-cock up to the cylinder could be an option, but he doesn't think we'll need it at our readings - that 15mm should be OK up to there. He did mention there are equations and calcs that can be used to work it out but he intimated it was an academic side of the job that is rarely used once you get into practice and have a number of years experience behind you. He also mentioned OSO tanks and encouraged I do a bit of Google'ing over the weekend to decide.

Having Google'd the past day away, I'm now more confused! All the manufacturers I've looked at advise 22mm in-feed, and I've ended up concentrating on this more than choosing an actual make of cylinder. I've also seen many posts on this forum asking about 15mm versus 22mm in-feeds to unvented cylinders. I just tried Megaflo technical support but they don't work weekends. Valliant picked up, said the readings sounded fine, but when I mentioned it was all on 15mm, pretty much said that it would have to be changed to 22mm.

Main query then is: Should I ask our plumber to re-pipe from our mains stop-cock onwards up to the new cylinder in 22mm against his initial advice? And will this mean replumbing the whole of the house's CW system, or can the exisitng 15mm pipework on each floor simply tee into the new 22mm 'backbone'?

Love the forum by the way - have found lot's of useful advice in browsing through, and had a chuckle at some of the characters and 'debates' that have sprung up over the years. Thansk in advance for any help anyone can give.
 
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I would guess flow would be ok at the pressure you have but would go with 22mm as that is what the manufacturers stipulate. Ok to leave the existing 15mm to the other floors.
 
If feasible upgrade the stopcock to 22mm and tee off to the cylinder in 22mm. How many bathrooms are you trying to run, if it's just the one I can't see a major issue with the figures you've got and you can't run 22mm to the cylinder. Just my personal opinion.
 
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Have just fitted a santon unvented cylinder,mi's state 15mm supply is allowed provided recommended pressure and flowrate can be achieved,which i think with your readings it would be.
 
Have just fitted a santon unvented cylinder,mi's state 15mm supply is allowed provided recommended pressure and flowrate can be achieved,which i think with your readings it would be.

You think?

I have 7 bar running through a 100kW plate off 22mm installation pipe work fed from 25mm MDPE.

2 showers at once are what I would call adequate.
 
Thanks for the prompt responses.

Looks like 22mm is the way to go. I forgot to mention that part of the refurb will change a previous ensuite into a second bathroom - it used to have a pretty pathetic electric shower in and the idea was to install a better solution, along with getting rid of the tanks.

When the kids get a bit older, I can see that we might have two showers running at once, and I like the logic behind a 22mm in-feed providing high enough flow rate to power two 15mm end points.

I also browsed a bit more today and found this website: http://www.johnhearfield.com/Water/Water_in_pipes.htm which helped explain a little more about pressure, flow rate, and pipe diammeters. Still not sure exactly how to do the calcs as I got a bit lost in the theory, but I could work out that to obtain Megaflo's max spec of 1.5 l/s through 15mm pipe would have water travelling at about 10m/s ~ 22mph! (but not sure how pressure fits into the picture).
 

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