What is the 3-way switch labelled "Tag", "0" & "Nacht" for and how/when should I use this?

I have a gas boiler that both heats the house and the water however on the electricity panel there is a switch as described above.

Im quite sure it has something to do with the hot water, but I'm not sure when/how I should use it.

Any ideas?

Can't say for sure, but it could be for the immersion heater for the water boiler (hot water tank). These use an element which consumes a fair amount of lecky (high wattage), so the settings could be 0=off, Tag= heating up during the day (high tariff), and Nacht= heating up during the night (low tariff, if applicable).
Normally used when you've used up all the hot water in the tank, and you require more.

Yes, it's for the immersion heater. The local electricity company supplies two signals, one to enable the immersion heater during the night, when the electricity is a bit cheaper and the other one enables the heater during the day, at certain times, when the electricity is more expensive.

So, the switch allows you to switch between these two signals. Normally, you leave it on the Nacht position, unless you have a hot water emergency situation and then you can switch it to Tag - but don't forget to switch it back to Nacht afterwards :-)

The middle position, 'O' is off.

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Chris

Thanks for answers.

Taking into account that the gas boiler also heats the water, do I leave the switch in the "O" position in winter and in summer when the gas boiler is off (in theory) I put the switch into the "Tag" or "Nacht" positions?

Also when does Nacht start/end? Is it something like 22:00 - 06:00?

The immersion heater is only normally required in emergencies - boiler failure, unexpected high demand for hot water, so you can leave it off.

For me low tariff is something like 2100 to 0700, but it should operate automatically if you ever have to set it to night tariff.

Since the gas section of the rig described by the OP also heats the house, I assume that the electric immersion heater is used not only in emergencies but also during periods when no heating of the house is required.

Low tariff times can be widely differ between two neighboring villages, and many grids do not provide low night tariff at all. In our vacation home in the Lower Engadine we have a complicated automated switch that had to be installed for quite a bit of money a few years ago. It was meant to switch on certain appliances such as a big electric storage heater only at night, but, funny enough, just a few months after mandatory installation of that gimmickry, the night tariff on that grid was officially dropped.

Even the Swiss get switzerlanded once in a while.

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I forgot to mention that the boiler has the option to provide hot water only. So based on all the valuable responses so far, I guess I now have to determine which is cheaper, gas or electric.

Another consideration is not running/operating the boiler for several months. Based on experience and feedback on various heating forums, the viessmann units like to be operated regularly.

Might only be comparable on the cheaper night tariff for the immersion heater - it's like using your kettle for your hot water needs (works the same, but on a larger scale).

However, you need to determine if you have dual tariff lecky - as this is not a given for all households.