I got this done recently, when I first enquired about pumping the shower I was told it was possible but with the size of my hot water tank pumping it at 3bar would mean there wouldn't be enough hot water left for a second shower so the plumber I got suggested if I was to do this properly I should replace my hot water tank aswell. So here's what I did and its cost..
1. Buy new hot water tank ( I went for a 300L Stainless steel with Solar for €1,650)
2. Buy second cold water tank for attic with 3.5 bar submersible pump in it. This was got from RVR for €525
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3. Remove electric shower from ensuite and replace with thermasatic valve shower from shires €540 I think for this and the rose
4. Labour costs of 2 days by Plumber to install €1,200
5. Sold old copper tank -€60 on buy and sell
Total: €3,865
Expensive yes but we agreed this was the best solution for us and I have to say we are very happy with it and I would recommend this solution to anyone our shower pressure is so strong we don't use it on full power and over christmas we had people staying and 5 showers where had and everyone had hot water still with the tank. Plus we can now get solar panels down the road if need and we can connect them to our new tank.
We got the whole house pumped so yes when you flush a toilet the pump kicks in in the attic but this system is much quieter than a pump in your hotpress.
Hope this helps!