Hi, the pump runs literally all the time and it has a bit of a rattling sound to it. Amd it is hot to the touch. And, the first shower of the day never gets truly hot. Is the pump failing? If I recall, the impeller was replaced once a long time ago. I would like to do it myself if that is what it needs. I understand the cartridge is easy to obtain and replace. If this is the correct path, can you guide me on stopping all water flow to the pump while I swap the cartridge and the O-ring? Thank you for any guidance you can provide.