A water heater mixing valve helps regulate temperatures with your water. It is especially useful when you are taking a shower or doing dishes. The mixing valve mixes water from the hot water supply with cold water to maintain a comfortable temperature. Installing these aren't terribly complicated, but there are a few common problems.
1. Incorrect Soldering
Most mixing valves need to be soldered when installed. For those who don't have experience with this, the valve can become damaged and rendered useless. Unless a leak is detected immediately the problem won't be noticed until the temperatures aren't even when in use.
2. Manual Valves
Most valves are automatic and include an anti-scalding device. However, you can also purchase valves that require manual setting. The biggest problems with these are that many people don't understand how they work. While they seem fairly cut and dry, it's not hard to set them too high or too low. If you choose to use a manual valve, make sure you understand how to adjust the temperature, and always test the water before you take a bath or shower, or bathe a child or pet.
3. Actual Temperature versus Set Temperature
Even if you have the water set at a certain temperature, the water that comes from the taps can be as much as 10 degrees different. The length of the pipe that carries the water can cause this change. When you set a new mixing valve it's important to remember this, and test before you use it.