I have had one similar issue. Had been heated by solar, coil scaled up inside. I disconnected mixing valve, added an elbow and a length of copper to the top section of the coil and kept pouring in a strong lime scale remover. It was so bad it took me several hours. I did plug the bottom for a while to give it chance to eat away the scale. The only other option new cylinder.
One question though did you try just adjusting the valve to only have cold through? if you have full flow cold through, there is your answer.
good luck.