The shower in my en suite is only getting luke warm though the hotwater taps on the sink are fine. When i switch on the clod water tap in the sink the shower then gets hot . The shower two has

the given symptoms sound like hot and cold preasures are uneaqual if this is the case there are two options either a preasure reducing valve fitted to the cold supply or a reducer fitted in the cold supply to reduce the flow(probably less affective) If the hot is tank fed with cold being mains fed idealy you should run a new cold feed from the tank to balance the preasures.

Mixer showers are generally set by the manufacturer quite low so they are antiscald. There should be an adjustment inside that allows you to increase the amount of hot water that flows through the valve.

I also agree with Kevin. If it was a thermostatic mixer then running the cold water tap in the sink shouldnt make the shower temperature change. The fact that it does shows the cold and sink are fed from mains pressure cold and hot is fed by loft tank pressure.

I agree with Kevin, there does seem a simple pressure imbalance going on there. You need to find out how both hot and cold are fed and then rectify, bottom line they must be relatively equal

It almost certainly is a case of the hot supply to the mixer being tank/cylinder fed and the cold being mains fed. Ideally, removing the cold feed and replacing it with a feed from the same cold tank as where the hot cylinder is fed from (ensuring equal hot and cold pressures and flow rates being delivered to the shower) with a plus point of being able to install a twin impeller pump in line with the mixer to drastically increase the pressure to the mixer shower. If you do go down the twin impeller pump route though, make sure you install either a Surrey or Essex flange to the hot water cylinder to ensure an air-free supply to the inlet of the pump - failing to do this will invalidate any pump warranty in most cases. Best of luck Dave - DrainMedic

You've probably a faulty thermostatic blender within your shower bar?? Some makes are able to strip down clean repair and replace - replay small parts within the unit..others (cheaper type.) Come as sealed unit and are only repaceable when/if temperature fault occur. Your best alternative is to get a MANUAL type mixer where YOU the user sets shower temperature to your OWN desire. Or again you have an unbalanced hot and cold system in which one either the HOT or the COLD are being fed or coming from two different types of source. Hope this helps pd

Check the thermostat, maybe a faulty cartridge or unbalanced pressure.