Heat makes things expand.
In a compression faucet (the most common kind), the water flows through a narrow gap between a neoprene rubber washer and a metal “seat.” In the hot-water faucet, the initial flow of hot water makes the washer expand, which closes down the gap between washer and seat, restricting the flow of water. With less hot water flowing than you originally selected, the mixture is now colder.
There are several things you can do:
1. Replace the neoprene washer in the hot faucet with a “sandwich” type: fiber composite on the outside and rubber on the inside. The fiber doesn’t expand and contract as much as rubber does.
2. Don’t be so stingy with the hot water. If you open the faucet wider, the slight constriction due to expansion won’t even be noticeable. Of course, to get the temperature you want, you’ll have to open the cold water faucet wider also.
3. Preheat the hot water faucet parts by running the water for several seconds after it flows hot. Then when you adjust the temperature, the diabolic expansion will already have taken place.
4. Take cold showers.
Or, for a change of pace, ask your live-in to flush the toilet while you’re in the shower. You’ll get all the heat you want. Fast.