Water not reaching 55 degrees in unvented water cylinder

There a few things that it could be here in order of possibility, Faulty immersion heater. An immersion heater that does not heat the water inside the tank, may be a fault relating to either the immersion heater or the thermostat controlling it. This is best checked with a voltmeter and an ohmmeter. To replace the immersion heater the water cylinder would need to be drained down. Checking the unvented control set up. Always ensure that the unvented controls on the unvented hot water cylinder are all in sequence. The example below is for a stainless unvented cylinder: Pressure Reducing Valve: Set at 3 Bar Expansion Relief Valve: Set at 6 Bar Pressure & Temperature Relief Valve: Set at 7 Bar Finally, Fault with thermostat and high level cut out. This particular fault allows water to become heated to 95°C. The immersion heater thermostat should have removed the electrical supply to the heating element at 65°C. When this failed the high level cut out should have operated at 90°C when this fails the last line of defence is the TPR (Temperature and Pressure Relief) valve. This opens at 95°C and prevents the water from turning into steam. It should be noted that in this instance the water inside the cylinder is still being heated as the source of heat has not been removed. The water in the cylinder is cooled by cold water from the main supply replacing the hot water that is being discharged. (Unlikely cause) Hope this Helps,