I,ve just had a condensing boiler replaced (like for like) the system has been working fine until a few days ago. when i turn on a hot water tap, i get freezing cold water. the water will slowly come through a warm then hot. This is after about five minutes. after a couple of minutes the water will run cold and then a couple of minutes later warm again but never hot. Its totally baffled me. I cannot account for the hot cold cycle especially as the tank 300 litres just holds water. Its been working fine for the last 10 years.