I have recently fitted a standard shower mixer valve to a gas combi system. The shower works fine, but it doesnt run hot enough if I keep it at around the recommended temperature of 38 degrees. I can get it to run hotter but I have to overide the manual scald feature and crank it up to max. The hot water coming into the valve is hot enough as when it its at maximium temperature the metal valve body itself becomes very hot to touch, yet the shower is only now at a temperature which I would say is normal to have a shower in. Do I just adjust the thermostat as is allowed, or should I be looking at doing something else. I am presuming that as the cold water is mains fed that the flow rate may be too high versus the hot water flow rate. Should I fit a flow rate reducer to the cold water?
Occasionally the temperature setting has to be adjusted in the way you describe by the installer. This is quite acceptable. Maybe you have a flow imbalance between the hot and cold which can be caused by the boiler or the extra pipework the hot has to negotiate which is causing the manufacturer's setting to be incorrect.
Some shower manufacturers stipulate flow restrictors should be fitted in the cold, or both hot and cold depending on the type of hot water system. Check the instructions
If you can achieve an acceptable shower by simple adjustment, all is well.
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