I am not getting hot water out of my shower, however I do get hot water out of the sink in the same bathroom. Shower was not used for about a year or more.
There is no way to do it without plumbing work. Essentially, a strong acid needs to be applied internally.
It's not the pipes that are probably the issue, but the shower head itself. Remove it and see if water comes freely out the pipe. You can often open those things up or soak them in a solution of vinegar (or CLR) to try to clean them out.or by a new one. You can find ones that are designed to be self cleaning they have rubbery nozzles that you can periodically run your finger over to bust up the scale. You may wish to consider a water softener if this is an ongoing problem.
