Thanks:) I'll try different caps and see the difference.

I tried out my set-up and logged the boiler's water temps (supply and return) and basement temperature once a minute for 3 hours.
You can see when the boiler turned on and then off an hour later, while the circulation pump stayed on for another hour.
The temps seem low, the max of only 37°C is much lower than the 55°C my IR thermometer shows. The sensors are just tucked under the foam insulation. I was also surprised that after the boiler turned off the supply temperature stayed 2-3°C over the return temp for over an hours.
Need to do some tweeking.

Due to recent spamming I've closed this thread. If anyone wants to post, please send a PM to Daox or myself to unlock it.