Help!!! All I want is to take a decent shower! The cold water tap on its own is too cold but I cannot mix in just a little hot water to warm it up a bit. As soon as I turn the hot on it makes it scalding hot. I've tried doing the opposite and having it hot first and adding cold water to cool it down but this doesn't work either. Is there some way to change the temperature that the hot water is set at? Or any other solution to this?

I have a Siemens box on the wall in the shower and not being savvy about these things I have been hesitant to try and open it and see what is inside until I get more info.

OK, make an model of the Siemens box, let's see exactly what you've got (include a picture if you can). Some heaters do have a half-power setting that you can get at inside, but unless you know exactly what you're doing leave well alone.

In the meantime, check your shower head for flow-restrictors and remove them, clean the shower head at the same time.

Is there a flow control on the inlet to the heater?

If you run the hot wide open does the water cool to a usable temperature that you can add just a little cold to?

I think it's a DHT 1200 from the little sticker on the side. Looks ancient.

No flow restrictors.

No flow control on inlet.

No - it stays boiling hot.

The DHT 1200 is (was) a 12kW unit which explains why your water is scalding.

It was apparently made in a 3-phase variant, do you have a 3-phase supply? If you do have the 3-phase version you could disconnect one phase to reduce the output by about 30%.

Ok so I decided that it was safer to get a handyman in to fix this. Water, electricity and me holding back a curious 2 yo toddler just didn't seem to mix. Handyman came and fixed it in 2 min. A perfect shower, finally! Thanks for your help.

Any indication what he actually did?

Glad you're sorted.

I agree, children MUST be excluded from any work area for their (and your) safety.

Ha! No idea. By the time I went in to see what he was doing he was done. And my Thai isn't good enough and his English non existent to have a chat about it. Wish I did know so I could enlighten others.