Whether you have an old fashioned two-handle valve or a more modern single-handle one, it is just mixing hot and cold water in various ratios, and the outlet temperature of your mix is entirely dependent on both the volume of both the hot and cold supplies, and their current temperature. Take this example: supply hot = 120-degrees, supply cold 70-degrees (say it's in the summer). If you mixed equal amounts of 120 and 70 degree water, you'd just average them to get 95-degree outlet temperature. To get a hotter output, you'd have to reduce how much cold you used, and cooler, vice-versa. Now, take that same scenario in the middle of winter when the incoming cold water is now 40-degrees. Mix those two together, and the average is 80-degrees. To get it warmer like it was in the summer, you'd need to reduce the cold a lot more than you did in the summer. On a two-handle valve, that means just adjusting the cold a little bit. On a single handle valve, that mean rotating the valve more towards the hot side. Note, if you properly setup that single handle valve, and say you did it in the summer, you'd have set the limit for how hot it could get. If it's now in the winter, that might not be hot enough. Done in the winter, the maximum hot might be way too hot when summer arrives and the cold water temperatures are much higher. So, from a safety standpoint, especially with young children or older people whose skin is thinner, that can pose a scald/burn hazard. Neither of these two choices can solve that problem. Then, take the situation where you're approaching the end of your tank's hot supply...the outlet hot temperature will slowly cool off, and to maintain your desired outlet temperature, you will have to regularly adjust the valve to give you more hot water into the mix. Depending on where the limit stop is set, while it might be fine when the tank is full, you may never be able to get it as hot as you'd like as the tank is nearly the end since you've limited the maximum amount of hot water into the mix. Now, take a modern thermostatically controlled valve. When installing them, you still should calibrate it so when it says say 105-degrees, the outlet water actually is that temperature. But, most modern ones can automatically adjust the mix of hot and cold to KEEP the temperature the same, regardless of the inlet temperatures of either the hot OR cold! Even as the tank is being depleted and the tank's outlet is cooling off, the thermostatically controlled valve will just keep adding more hot and turning down the cold in the mix, all by itself. Now, they're not magic, and can't make hot water, but even if the outlet of the WH is reaching your desired temperature, you may not notice because the valve has automatically adjusted the mix to remain at a constant outlet temperature you have set. On the one I have, I only adjust it summer/winter to accommodate the air temperature in the room...making the mix cooler in the summer than the winter so essentially, what hits me ends up near the same temperature. So, consider this tech when specifying your new shower valve.