Well today I removed the electric water heater (75 liter) to give it a decoke as this thread reminded me to get my backside into gear.
Sulfamate (aqua furte) was the only readily available product that would touch it as it is extremely hard & vinegar would have had not the slightest impact.
The gas water heater is next in line when I am satisfied that the electric unit is up and running without issues.

Great care! of course must be exercised when using Sulfamate.

Damp newspaper and wood ash works a treat.

playamonte,
yes great care. I guess they don't call it agua fuerte for nothing.

I worry that the trick with ash is using it as a mild abrasive. The chemical cleaners are cheap, and you don't use much. If you do scratch the glass, it will get dirty and then be far harder to clean.

I don't use it every day, but what I do do is wipe off any soot marks with a dry, clean paper towel. I also do that before I use the chem cleaners, as the soot comes off easily, and then the cleaner works better.

As I wrote that about the ash I thought I bet bil (the alchamist) will tell me off
You are right about the abrasiveness but I wonder how much abrasive content is there in the chemical stuff or is it purely a solvent.

Purely a solvent.

Thanks again for the science lesson.

No problem.

Try using citric acid. You should be able to get it in any chemist shop in the UK and probably in Spain also. I use it all the time in my kettle in my home home in Estepa which has very hard water, All you have to do is wash it out afterwards. Citric acid is found in lemons and it is also used for home wine making and I get it from the homebrew shops. Hope this helps.
By the way I have a degree in chemistry, and I teach chemistry in a high school in Scotland.

So why is citric acid better than vinegar?

Depends how you define better. To me augua fuerte is cheap, fast and effective.

Thud!
Is that the sound of a chemical gauntlet.

I agree, but remember that you can't easily buy big bottles of concentrated acid in the UK - the yobs would probably start throwing it at each other!

Jesus in the UK I can only but two packets of aspirin. That means I an not allowed 4 packs of daily aspirin, even tho they have half the aspirin of two normal aspirin packs.

Idiots.

Yes it is daft. You are allowed to buy Asprin in Sainsbury for 40P. Would suit me as I hardly take anything. If you go to the Doctors they will prescribe 100 para-codol at a time....free if a pensioner or on benefits!

Wouldn't you just bloody love it if common sense were used instead of all this mindless, retarded, jobsworth crap we get at every turn?