Your shower bar must be similar to this in photo?
The knob on left of shower bar reduces or increases pressure by rotating . The Knob on the right reduces or increases temperature of water from shower. The thermostat built in maintains that tenprature you choose so long as there is hot and cold suoply

I'm confused...

You say that your plumber etc said to increase the temp of the heater to 55c.
We have a fixed setting one and it gives water at 65c.

Does your shower not have a mixer tap???
Nobody in their right mind would only run a hot feed to a showerhead.
I like coldish showers and the wife likes to steam up her bathroom.
All we do in our respective bathrooms is move the mixer to the desired spot.
Depending how far from the heater you are, there will be a waiting time for the full heat to come through (my bathroom is the furthest from the heater)

We can do the washing up a couple of times a day, each of us shower and there is still lots of hot water left from one heat cycle.

yes that is identical to us I think OP has the mixer bar set up which will have an output pressure control andan heat control.That is point i am trying to make 80 litres of water will not last long if that is all you are using for your shower. You must have the hot water (hotter than 45 deg ?) and mix it via mixer bar to desired temperature and shower pressure. At the end of the day the shower bar is just a fancy tap combining hot and cold water.

Yes that is how the shower bar is designed, water pressure on the left and temperature control on the right. What photo?

We have a temperature knob on the electric boiler which goes to 75c but I have read that the more you turn the boiler to maximum, the quicker it will deteriorate so the plumber told us to keep it at 45c but to shower I turned it up to 55c because it is not hot enough. The shower bar itself is a Grohe Grohtherm 800 thermostatically controlled device, it obviously mixes the cold and hot but controls it thermostatically but ours is not controlling it because the water is coming from the boiler which is emptying fast and the temperature at the boiler drops fast with the cold water coming in so our showers soon run cold as the shower head is delivering 14Ltrs of water a minute, so as suggested I am going to try turning down the water pressure on the shower bar and see if that helps. I have heard from other plumbers that these type of shower bars do not work effecively in Spain off an electric boiler but when I questioned Grohe on this they said they did !!! Go figure !

Yes you are probably right, we probably should have just bought a straight forward mixer tap for 20 euros instead of paying 160 euros for a shower bar which is not able to do its job effectively off an electric boiler, but as usual our plumber never advised us what to do and we have always had a combi boiler with a consistent temperature output and controlled the temperative and water flow in the shower and it remained constant !😞

I think your thermostatic shower bar will work fine,. I would just try turning the temperature of the boiler up high and adjust the heat and shower pressure from the shower bar. I guess all will be fine if you do that as you will be putting hot enough water through the bar so it can efficiently give you the correct output. You can't use it as you are used to like you did in the UK with the combi boiler. You just need to familiarise yourself to your new set up I'm afraid. Good luck and let us know how you get on .

I would say that the shower bar is not doing its job because the hot water entering it is not allowing the thermo valve to open to let cold water in to balance the system.
Either the bar is faulty or you don't have it set right.

Knock on wood our water heater is 20 years old and has been in almost daily use for six years now (previously it was only used for a month or so a year).
So setting it higher has not affected it yet.

Something is very wrong here.

You say the boiler is 2kW, which is actually on the high side for Spain where 1.5kW is more common, but then say that it heats 80lt of water in 40 minutes which is a physically impossibility.

Assuming an efficiency of 95% and a start temperature of say 25c to heat 80lt of water to 55c would require 2.8kWh of electricity which, with a 2kw element, equates to about 1.5 hours NOT 40 minutes!

You therefore need to need to put the heater on for at least 1.5 hours to get a tankful of 55c water.

The flow rate is a factor of the available water pressure not the boiler.

BTW 45c is too low, 55c is the recommended temperature to prevent Legionella.

https://www.greenbuildingadvisor.com...naires-disease

Touch wood, let's hope ours in the same !🤞

Yes exactly the boiler heat needs to be way hotter and that will allow thermostat to work mixing hot and cold to the temperature set on the shower bar. Like you our boiler was in the house when we bought it. So is over 18 years old. It's set to near max temperature on a timer. We used it as holiday home for 10 years and last 8 years it's been switched on daily. Never had an issue with it and it's only a basic heater not a fancy or well known make but it does the job just fine. It also retains the heat well when the heating element is off so must be well insulated.

That's a very good point, our heater when timer turns it on overnight is on for 50 mins to an hour and it's heating water that isn't cold.

I would turn that temperature knob on the boiler up to 65c. That is below maximum and should be fine. Our boiler is etc at near maximum just a little down. As I have said boiler is over 18 years old touch wood never needed repairing or element changing. The element if it does need changing isn't either a difficult or expensive job anyway so I wouldn't worry about keeping setting down they are designed to run up to figure shown in your case 75c so will be capable of operating at that. Remember even though your boiler is switched on permanently the heating element is only heating so working when it needs to be.

Sorry I was out and about and tried to attach a photo which obviously didn't work. So we are talking same thing re shower bar. Have boiler water hot and adjust temperature and flow from the shower bar. What you are trying to do is run the shower purely from the temperature in your water heater letting it get to desired temperature by running off the excessively hot water. When the water from these boilers starts to cool they run out of hot water quickly as you have used the majority of hot water in the boiler tank and it's back filling itself with cold water which then needs to be heated by the element.

I didn't mean it takes 40 minutes from cold to get to 55c, it is already always at 48c and I don't watch it that closely, I was guestimating.😁

I am setting it to 65c and we will see how the thermostat works on the shower bar, we have reduced it down from 45c to about halfway as when we set it on 45c my husband said it was way too hot for him when it came through but with the boiler set at 65c we will see if the thermostat works to reduce the temperature enough for when it comes through and I will reduce the water pressure on the shower bar to see if it lasts longer.