Old gas heater defective -> need a new one, but which one?!?

If you don’t have underfloor heating, a heat pump is probably not effective, since radiators require a higher flow temperature. I think the easiest solution would be a new gas heating system for you.

Only in the bathroom there is one, the rest is with radiators. I’m starting to suspect that too..

Now you just need a gas detector. The CO detector makes sense but does not measure combustible gases.
I’m actually not a fan of gas boilers but tearing down a whole system quickly seems financially risky to me. Replace the gas boiler with a simple new unit.

Okay, I have to take a look. Many advertise CO detectors and gas detectors, but can’t detect gas..
Yes, financially it would be a disaster..

Okay, the values correspond more to the expected consumption

Still too high

Roughly, you have a heating load of 17.3 kw for the house.

The values are not too high but correspond to what is stated in the [Energieausweis].
You can calculate it yourself.

KW times usable area.

201 * 180 = 36,180 kw / 10 because of gas = 3618 cm3 gas per year

Roughly, the house has a heating load of 17.3 kW

Ok, thanks. Good to know!
Next Tuesday another heating company is coming...

These are all values researched on the internet by a layperson, so do not rely on them. I am still unclear about the statement from the chimney sweep. Does it smell like gas or exhaust? Why did he want to shut it down and why didn't he do it then? He is liable for consequential damages if he allows a defective heater that is dangerous to remain in operation.

Why did he want to shut it down and why didn't he do it then?

Because the window is open after all and there are children in the house. cool logic. Unbelievable stuff.

But at least the consumption is not as dramatic as previously suggested.

Roughly, you have a heating load of 17.3 kW for the house.

The values are not too high but correspond to what is stated in the energy certificate.
You can calculate it yourself.

kW times usable area.

201 * 180 = 36,180 kW / 10 because of gas = 3618 cm3 gas per year

The reason the values are accurate is simply that this is not a calculated energy certificate but one based on consumption data. Unfortunately, these are only a rough guide. If six people live in the house, have everything heated up to 24°, and shower daily, you get a very different result than if grandma still lived there heating only the living room.