Had a call from a neighbour saying that they only had cold water coming out of their hot taps when they had hot water in the cylinders and had not had any problems before. On investigation they had 2 linked indirect cylinders . Cold feed into bottom of one, hot out the top and into the bottom of the other and out of the top to supply taps. Cylinders were full of hot water but all the hot taps only produced cold.Pipework out of the top of each was hot for about 2 feet and then went cold. Still wracking my brins to work out what the problem could be. Any ideas?