I live in a condominium unit and have a thermostatic control valve in my shower. I have just been advised that it is faulty (asymptomatic to me because I don't have any problem with water temperature which has always been set at the optimal 38 degree C ). The alleged faulty valve is causing temperature fluctuations in showers on floors above and below me. How can I verify that my valve is faulty? Can I install a check valve ( what type ?) in my hot or cold water supply lines to the shower to prevent future potential crossover problems ( will happen over time arising from a faulty in the valve due to wear and tear ) affecting showers in other unit(s) on the same supply line due to a faulty valve in my shower?
This answer is only relevant if your building has a central hot water system.
The faulty valve (anti-scald, mixing, or tempering valve) may let cold water backflow into the central hot water system giving lukewarm showers to your neighbors - except when you are showering. If so, a non-return valve (tap, faucet) on the hot water inlet would prevent this. But your system might not be like this.
I have seen this happen.
Multi unit housing sometimes have shutoffs built into the shower valves so a cartridge replacement can be done without shutting water to the entire building. You can remove the plate and see if there's a way to shut of the supply to the valve. If so shutting those supply valves for a day or two should help determine if it's your mixer or not.