What is the fix for when you flush the toilet while in the shower the water gets hot?

New pressure balance faucet or you can install a pressure balance valve in the

water lines behind tub or if you have a crawl space you can install it on the water lines there. But install only too the lines to tub faucet. And they are not

easy no matter where you install. I would just tell everyone not to use any water till you are done with shower.