This is a 2 story house. Our downstairs is just a shower with a single handle mixer. Hot water not a problem in this shower. On the 2nd floor directly above the downstairs bathroom is the upstairs bathroom. This bathroom is still the original and the shower/tub has a separate hot and cold valve. Problem is: shower stays hot however if running tub water for bath only hot for about 3 min then have to turn off the cold water to get luke warm water to tub for bath water. Our water tank is for both our using water as well as baseboard water heat. Any solutions?