As I dye all my wool with natural dyes, I need to bring it to the boil twice, once for mordanting and once for dyeing, so there is even more chance of felting.
So, I have adjusted my technique and can relax now why I am dyeing wool.
I use the french "au bain marie" way. My dyebath is filled up with the mordant or dye solution and the wool goes in there. I put this pot into a bigger one, filled with water. There needs to be a space in the bottom between the two pots, so that the dye pot has water around it at all sides. I bring the water to the boil and the dyebath will heat up, but not boil completely.
The wool comes out perfectly unharmed.
Energywise, it would be a lot better if the dyepot was lower than the heigth of the water pot, because than I could put a lid it. I am still looking around for the right sizes that suit me.