First, determine whether you need to replace the water heater or whether maintenance is sufficient. If you are not getting hot water, it could be due to the heating element or another maintenance problem.
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I think you should replace the piping under the kitchen sink. Thank you for enclosing photos to help you understand what the problem is. The structure is too big and weighs not a little, which is not safe. If you do something wrong, you could flood the kitchen or break the drain pipes. I would not advise changing the configuration of the kitchen plumbing drain yourself but rather using the services of an experienced plumber. I know a good plumber who has changed the [link|https://theplumbingprofessornsw.com|plumbing and heating near me|new_window=true], so I recommend him.
First, determine whether you need to replace the water heater or whether maintenance is sufficient. If you are not getting hot water, it could be due to the heating element or another maintenance problem.