Changing the cartridge or thermostatic shower in my bathroom

Hi Lenalee, Luke warm water is usually a sign of a faulty thermostat especially if the other hot supply's in the bathroom are working normally. If you have a well known shower valve ie Triton, Mira, Aqualisa etc then getting a replacement thermostat should not be a problem. As with everything it may be wise to get a plumber to check the entire shower system first to rule out any other possible causes. I hope this helps

the thermostatic valve is not working properly letting by more cold water than needed. isolate the shower and take off the handle exposing the shower valve. Take it out and take it to a local plumbers merchant to get a replacement. job done good luck. cheers exceeding expectations