Best solution to improving DHW

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Ideally you want to maximise the hot and cold flow everywhere you can. You would want the mains increased to at least 25mm from the outside mains to your internal stop tap. Then at least 22mm up to the unvented and then 22mm out from the unvented to as close to the outlets as possible.

To give you an idea, I have an ultrasteel 180L unvented, 3 bar dynamic @ > 30L/Min and I can run 3 Mira Atom thermostatic rain showers at the same time with no complaints.

Some of the Armaflex insulation is great, Kooltherm is another one from Kingspan, there are numerous makes that would do the job. As suggested, it's always best to box in too.
 
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So. Some more numbers. as a reminder :

static measures:
my garden tap gave me 33L/m (single outlet) and my bathtub cold gave me 22L/m (as single outlet)

For dynamic measures:
Utility room (single outlet) gave me 13.5 L/m

with the utility tap on max, the bath tub gave me 17.6 L/m and causes the utility to drop to 7.5 L/m

So the question is, if I get a unvented megaflo installed, will i be likely to get two showers running without noticeable drop in performance ?

Thanks again for all the help here. It’s been amazing actually.
 
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From those numbers unless you upgrade the internal pipework to 22mm you will still get a change in flow at the shower when something else is turned on.
33 ltr/min is pretty impressive from your 15mm supply, there's clearly capacity to run 2 x 10ltr/min showers.
With good thermostatic mixers you might get away with your current setup, but imho you'd be much better off upgrading the internals to 22mm as part of the megaflo install (thats 22mm from house stoptap to cylinder). Separate 15mm runs to each shower from megaflo will work (so i'm told) and will reduce the volume of cold pushed through before hot arrives.
Have an investigate of upgrading supply to house but that could be done later as a separate scheme if you need to.
A pressure reducing valve at the shorter of the 2 shower runs may also help balance things, as will some restrictors on utility rm and kitchen taps
 
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Thanks that’s great advice.

PS
I’m assuming this is a 15mm supply but I could be wrong ! It’s clearly seen better days too.
 

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Hard to tell from that pic, could be in 3/4" steel, doesn't look like lead. Certainly fairly aged but delivering that flow/pressure you may well find you get away with it. If replacing it with 25mm mdpe was relatively easy (no concrete to cut through) then it'd be worth doing mainly cos eventually that steel will rust out...if its a 2 grand job then prob leave it be UNLESS you still have unacceptable variations when running 2 showers (with upgraded internal pipework)
 

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