Toilets, Sinks, Showers, Tubs and Disposals - Mixing valve in shower keeps breaking...

We lived in our previous home for 12 years and I don't remember ever replacing the mixing valve on either shower. We've live in our current home for five years and have put three new mixing valves in the Moentrol shower in our master bath. It is now having the same problems that it's had before (water stays WAY too hot until the control is turned almost all the way to cold, then it is freezing cold--one extreme or the other) so I'm thinking we will be replacing it again. My husband's friend is a professional plumber and, I hate to keep bugging him to fix it, he's fixed all of them for free--thank goodness. But I'd like to figure how to keep it from happening again, so I guess my question is why it keeps going through mixing valves. We have two other showers that are not having this problem. The only difference is that this shower has been known to occassionally freeze up in extreme cold snaps because the pipe runs close to the outside of the house. Could the ice that builds up be jamming sediment or lime in the valve? The plumber has commented before that the valves he's pulled out were "gunked up". Is it the shower fixture itself? Any advice would be appreciated. Sorry I don't have the model number of the shower control--I couldn't find it anywhere. Thanks.:confused:

If you have galvanized piping, this can be the worst. If it freezes, the partially clogged and rusty insides break loose due to pipe expansion, then get into the small chambers, and where a metering rod has to slide for temp balancing.

You may have to work out some plan to try to stop those pipes from freezing, or forever be plagued by your problem with one of these more complex mixing valves.

if nothing else ask your plumber friend if he thinks a sediment filter would be somewhat easy to install on the pipes before the valve