Certainly the best starting point, good luck hope it works

When I lived in France I had a 220lt immersion which was turned up to max (70c as I recall but don't quote me) and left on on a timer which provided power between the hours of 4 and 6am during which it consumed only what was needed to bring it back up to 70c. Being as for the majority of the time there were just the two of us 220lt was way in excess of our nominal daily needs so the amount of electricity used was very modest and it was costing me around just €10wk.

The reason I fitted a 220lt unit was to cater for the times when we had guests which at times meant 8 people in the house all wanting showers at least once or twice a day but we never ran out of hot water!

Higher temperature will accelerate calcium build up but that said my boiler in France was some 10 years old when I sold the house and was showing no signs of distress or failure and I reckon that's a pretty good life span for such a device.

Might I be correct in thinking that your user name (Algorfa) is an indication of where you live?

I ask because I know an excellent and reliable English plumber not far from there who for sure would be able to sort out what's wrong.

Not any more, we sold our property there in 2012 and I have not got around to changing my username 😁

Just been in the shower and this is what happened after setting the boiler to 65c. I asked my hubby to keep watch on the boiler when I went into the shower.

We turned the valve by the way inside the shower bar about halfway between 38c and 45c so presumably 41 or 42c.

When I got in I turned the water flow on the left hand side to half way to reduce the water pressure and turned the temperature about 1/2" forward as the water was taking a while to heat up, I also took note of the time the temperature turned hot, it started getting very hot and so I put a thermometer under the shower head to see what the temperature was, it got to 41c which was a little bit too hot for me 😁 so I increased the flow rate slightly as the temperature lever had not much to go to the cold setting to see if it would cool down which it did slightly but was still too hot and so as I didn't want to turn the water flow full on, I turned the temperature control almost to the top (the cold setting), it then started to cool down and even though I turned the temperature up to compensate it carried on cooling down until it was stone cold. It took just under 5 minutes from when I got under the shower.

My hubby who I shouted to when I was getting into the shower and who was watching the boiler, said from 65c it shot down to 43c in 7 mins but of course I was getting dressed by that time ! He said it did fluctuate briefly (presumably when I was fiddling with the flow rate in the shower 🙄

So what the answer is I don't know ! To me I think the water flow rate is too fast and is emptying the boiler too quickly. My son in law has a different shower head (the older type) which I think we might try as I think it allows less water through, to see if that makes any difference.

What doesn't make sense is why the thermostatic control on your shower bar isn't cooling the water to correct temperature? By increasing the flow rate of the shower to bring temperature down just means your hot water in the boiler is being used quicker and then running out.

I must admit our shower head is a very basic one bought from local Chinese shop for around 4 euros. It provides us with an excellent shower

I am correct in presuming that your thermostatic shower control bar has both an hot and a cold water inlet coming into it and it isn't simply being fed from only an hot water inlet ?

As I said earlier, it's a faulty thermostatic mixer tap- Change it. My shower has a basic mixer tap which adjusts the flow rate and temperature with just one lever. Pull it
out to change the flow rate and rotate it to vary the temperature. As we have a long run from the heater to the shower, I usually run the sink tap on full hot for about 15 seconds to make sure the water coming to the shower is up to temperature. 22 years with that system and no problems. You are overcomplicating it with a fancy thermo tap.

I think you are right, I have just been back down and took down the smaller hand held shower head and turned on the water flow rate full until it came through hot and tried turning down the temperature and nothing changed, hot or cold it seemed the same !🙄 Looks like getting the plumber back in to take it out and then I think I might just go for a straight forward mixer tap. Problem is I will have to return it for a replacement, they won't give me my money back unfortunately 🙄

Yes it has both hot and cold feeds. If it was only a hot water feed there would be no way cold water would come through it🤔

yes that's as it should be i was just confirming the obvious. Im afraid from my limited knowledge of plumbing i cant offer any more.

Do you have a standard mixer tap in your kitchen? If so does that work ok i.e. can you get that easily to correct temperature and flow rate. If so if you run that how long do you get hot water from that tap. Trying that and if it gives a reasonable length of time with hot water would confirm the issue is with the thermostatic tap. Also just out of interest what is the water pressure like from the other standard taps in your house? Is it very high ? If so you could try reducing that by adjusting your stop tap at entry point to the house. Don't have it fully open just open it sufficient to give you a desirable but not too powerful flow rate. That's something we had to do when we removed the pressure reduction valve on advise of the town plumber. I adjusted the stop cock and my wife shouted down to me when the flow from kitchen tap was correct rather than way too high. That worked for us. Although it does sound like an issue with thermostatic shower bar but these are other simple things you could try if your water pressure is very high.

In common with virtually all similar mixer taps the Grohletherm 800 is preset to a temperature of 38c at it's midway/click position but on installation that can be raised to 43c. Regardless of what the boiler is set to however the temperature in the shower should NEVER exceed what the mixer is set to, that is after all the entire purpose of having a thermostatic mixer!

If that's not what's happening then it's either faulty or possibly your plumber fiddled with it and set it higher to an even higher temperature, which in many cases be done, but even then the shower temperature should still remain constant until the hot water supply becomes depleted, it will just happen sooner. My own I've set to 46c because A) we both like piping hot showers and B) with only adults ever in the house potentially scalding children is not an issue.

A more meaningful test for yours would be to set the boiler to say 45c (making sure to leave it on long enough to ensure that the water is that hot) then with the mixer in the midway/click position and the flow at maximum monitor the temperature to see if remains constant and for how long. You might also use a bowl or bucket with known capacity to see what the actual flow in lt/min is.

Forget about pressure or the shower head being the problem, they are both red herrings.

to me it could be to much pressure and the mandatory pressure release valve is just pissing water out - meaning less hot water.
The thermostat shower bar could also be faulty...
As it is new, calcium buildup is not the problem but will become one in 2-3 years.. reducing the efficiency and requiring replacement..