I have a thermostat on my hot water cylinder set at 50 degrees. In the summer when the heating isn't on much, the water temp is fine. In the winter with the heating on the hot water is scalding hot. Why???????????
Do you turn up the boiler thermostat when you turn on the heating for the winter? We do because water at 50deg is not sufficient to heat the radiators sufficiently in very cold weather (like now). If you do AND the cylinder thermostat is broken the water in the cylinder will be at boiler temperature rather than the temperature you've set the cylinder thermostat to. You can often (but not always) hear thermostats click as they switch on and off; if you twiddle the cylinder thermostat can you hear it clicking?
I have no idea but I can't get more than a lukewarm bath out of my. not very well, boiler...please, please, please may I come round with my towel and sponge bag? So jealous!!
In answer to questions above - I never change the water temp on the boiler itself. The hot water cylinder is only heated by gas i.e. there's no immersion heater element fitted.