If you want the ideal solution replum the house using 3/4 inch copper pipe. There are other less effective solutions others may suggest.
If the house is a ranch, with unfinished cellar or crawl space, you should have easy access to all plumming. Then, I would replum from scratch.
If the house is a 2 story and you don't have access to all plumming, you have to decide what you want to do.
Maybe one of Brian's tankless hot water heaters and resovire tank for the bathroom would do the trick.
Just tell your husband not to shave when you are in the shower...
They say "the honeymoons over when you in the shower and your husband comes in a takes a shave...
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