The chimney sweep wanted to shut it down two weeks ago because there was a strong gas smell in the basement, and since we keep the windows open downstairs & have two small children, he refrained from doing so.
I calculated that we currently use about ~110m³ of gas per day.
You need to tackle this immediately, your system is just dumping the gas straight into the chimney. That means it’s already life-threatening.
You have a gas smell in the house and an extreme gas "consumption." You still enter the house voluntarily?
Are you sure you didn't move a decimal place on the meter?
True... Sorry, I read it after 9 days back then and it was at 100m³ .. 09.01 meter reading = 18496.800m³ 05.02 meter reading = 18802.183 m³ Consumption: 305.383 m³ in 23 days -> 13.28m³ / per day
You have gas smell in the house and an extreme gas "consumption". You still enter the house voluntarily? Holy crap. Get the kids out of there.
In the basement, luckily not in the house. I also installed CO detectors, there was none before either ...
If you don't have underfloor heating, a heat pump is probably not effective, since radiators require a higher supply temperature. I think the simplest solution would be a new gas heating system for you.
Now you only need a gas detector. The CO detector is useful but does not measure combustible gases. I'm actually not a fan of gas boilers, but rebuilding an entire system quickly seems financially risky to me. Replace the gas boiler with a simple new device.