The way in which the water is heated is irrelevant to the symptoms you are having with your hair.
It is the water. ALL water is not created equal. I have had well water supplied to my home for decades. I had to temporarily move into a relatives home to help them in their old age. They had town water that was treated with chemicals. My hair color began to change due to their water and it felt "straw-like". So, I got better conditioners and used styling hair products.
When I moved back home, my hair felt like silk again and now it is shinier and healthier looking and back to my natural color.
So, I am going to assume the same thing or something similar with the water, not the heater, is causing a change in your hair. Perhaps you should use a product that has shampoo and conditioner in one because perhaps conditioning your hair after shampooing is the cause of your hair looking greasy. I would experiment with different shampoos until you find the one that works. If you want to remove all of the product residue in your hair, put 2T of baking soda in your palm with your shampoo and it will leave your hair super clean.