Why does the hot water get hotter if you turn on the cold tap:confused::confused:

Let's see if I understand your question: You are using shower and somebody else turns ON cold water in sink. Water in shower gets hotter. Correct ?

If that's the case, it may be several problems. The most obvious is that you have under-sized pipes in your house. Therefore, higher cold water demand will cause cold water delivery to drop at each running faucet while the hot water flow remains the same.