The above post is my 'solution. A new mixing valve is around 100.00, and the Honeywells, Taco's, Caleffi's, Watts all work well. Probably you have the original watts' small guy' with the knob to adjust, locked in place with corrosion and lack of ever needing to be adjusted, took its toll. Ball valves need to be added if and when you change it out...it will have unions on the 3 ports......cold, hot, mixed and I always use 3/4 in, but you can keep a 1/2" one if you wish. Before this all takes place, you can carefully try to remove the packing nut assembly, however usually they are piped in VERY tight and a wrench or channel locks may not fit. The domestic coil will love the vinegar treatment also. I noticed the 'hard water comment'....you are the victim of build-up for sure, and if you can rig up a loop to pump vinegar through the coil for a half hour, you may be good to go. The mixing valve still needs to come apart, or be replaced.
I don't know how you like the new tech electrical stuff, but Field Controls has a device called Clear Wave that wraps around you water line, is plugged in to a 115 outlet and attacks hard water particles with electromagnetic waves. It is not hokus-pocus, and they claim it is a salt free water conditioner. My wholesaler said after the 'old timers' stop laughing, that they are buying them to install at hundreds of location in NH, Vt Maine and Up State, NY. I have 13 of them installed right now, and the results speak for themselves. Go to http://www.fieldcontrols.com
. One thing we know: you are having issues, and it is affecting your home systems, and I think they are about 125 dollars ' Clearwave' It is better suited to a single home, and not apartments, however it has been around for a few years, so time will tell.