Thank you Christian
It’s just that I saw this.. see picture..
When I removed my one there was no ring, washer or gasket..that’s why I asked..
Looks good, but I never saw this before. The thermostat itself from eg. Bosch does not include a seal or copper ring, and I have not seen it for the house before now.
However, I guess it cannot harm to add one, and leaks from the house are often seen, if the seats have a small damage or imperfection.