Mixing valve for upstairs shower

I own a duplex that has an upper unit and a lower unit. In the upper unit there is often a problem with the temperature of the water in the tub/shower. The water will be too hot and moving the hot/cold handle on the tub/shower makes little difference(if you push the handle to the coldest position the water will then become cold-but the correct adjustment in temperature just doesn't happen). In other words when this is going on you will have very hot water or you can adjust to very cold water but nothing in between.
After about 5 to 7 minutes of this, you are able to control the water temperature normally.
I would say this happens about 20% of the time you use the tub/shower.
I understand there are "mixing" valves to control this problem.
Where in the plumbing system would I install such a valve to solve the upstairs tub/shower problem?
Is this a do it yourself project or better left to a plumber?
Roughly how much do these valves cost?
Thank you,
Bob
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surprised if your problem is a malfunctioning "mixing valve". Try calling the manufacturer and tell them the problem. They may send you the needed parts, as many of these have "lifetime" warrantees.
Bob
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Most single handle shower controls on the shelf today are pressure balanced and should solve your problem (older single handle were not necessarily pressure bal.). The valves cost between $40 and $150 and the price depends heavily on the trim plate and knob finish and brand. Its not too difficult if you take your time and understand how before you start. It will probably require some tile and wall repair as well to gain access. If you think you probably can do it you probably can.
For a little more money you can get a thermostatic pressure balanced valve which has a seperate handle for temp and volume (flow)
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