What's wrong with your boiler? I think it's better not to climb there on your own, because there was a negative experience. You can wind it up in such a way that it will be scary to climb under the shower later. Now in our city there is a good service that can quickly solve problems of this kind. Maybe you should write to them? See https://chosenrooter.com/water-heater-service/ has a lot of experience not only with boilers, but with much more complex plumbing problems.