There are two types of valves that will solve your problem. One is a pressure balancing valve that compensates for changes of pressure in the supply lines to the valve so you don’t get cold water shock. Another option is a thermostatic temperature control valve. This type of valve has additional features than the pressure balancing valve because a thermostatic valve compensates for changes in supply line pressure and temperature. What this means to you is that if you are are taking a long shower and you are using alot of hot water, with a pressure balancing valve you will need to turn the temperature handle to more hot water as you shower. With a thermostatic valve you select the exact temperature you want, there is a range from 70 degrees to 110 degrees on the handle. As you select the temperature you want, the valve maintains that temperature even as the temperature of the hot water coming into the valve starts to cool. The temperature of the hot water must be at least as hot as the temperature you have selected on the handle of course. Neither a pressure balance valve or a thermostatic valve can heat water. A thermostatic valve costs more but it has many benefits. A pressure balance valve will also provide a safe shower. Grohe has a good explanation on their web site at groheamerica.com. It’s pretty helpful.