Manual mixer shower valves rely on the user to adjust the shower temperature when fluctuations in water pressure cause temperature changes.

Basically if someone turns on a kitchen tap or flushes the toilet your shower will send a burst of cold or hot water through your shower.

So not the best for small children or the elderley but on the plus side they normally work on low water pressure and are cheaper to buy than thermostatic showers.

Manual NON Thermostatic valves