M6 standard would be 1.0mm which is large compared to most modern stuff. But that is all there is avaiable so here we go. There is not a great deal of engineering (maybe not needed) that goes into heating a house with modern stuff
If you have one bathroom and 2 people and and 3 bedroom. Then a combi is perfect. In my opinion (I am married and know that my opinion means very little) anything more requires a traditional system which there a variety of configurations or maybe a storage combi