Bathroom fan, pure exhaust with humidity sensor wanted

...Here, as compensation, among other things, window rebate ventilators and these 3 exhaust devices were prescribed. Nothing more.

Exactly that (window rebate ventilators) is the remainder of the raumlufttechnischen Anlage.

What I have been unsuccessfully looking for a while is a dehumidifier with a connection to the sewage system.

In principle, this should be technically relatively simple and the heat would not be pumped out of the house. I don't know whether the electricity cost would be higher than the savings.

Maybe that is the reason why such a device does not exist?

There are lots of construction dryers with hose connections, so you will definitely find a cellar dryer that has that. They also look a bit nicer. Honestly, it's relatively pointless. You pass by the bathroom every day and could empty the bucket. Thanks to windows, you can also get rid of the moisture without a construction dryer, and if you are going to invest money, it’s better to invest in a proper ventilator. It also has lower electricity costs than a dryer that runs at 0.5kW.

Maybe that's the reason why such a device doesn't exist?

Oh yes, they do exist. I have an older De'Longhi device at home; I don't know the model name offhand. I just quickly looked for other examples and FOR EXAMPLE, look at the De'Longhi DDSX220WF.

There are certainly more in different price ranges. But they are probably rather the more expensive ones and not the 100 EUR devices.

Honestly, it is just relatively pointless.

The point would be not to pump the warm air out of the house ...

Then you take a decentralized fan with heat recovery. Such a construction dryer requires considerable effort to remove the moisture, which costs energy. For, say, 0.5kW of the dryer, you can get 1.5-2kW of heating capacity from the heat pump. Overall, much more efficient.