We are having an addition done, and they just ran the hot and cold water lines to the addition. The lines are not used yet, so the water in the hot water line is cold because hot water never moves through it.
Whenever we run hot water anywhere else in the house it is hot at first and then only lukewarm.
Here's a picture showing where they connected the new PEX line to our existing copper hot water line. When we don't run the water for a while that copper T connection is warm. As soon as we turn on a hot water tap the copper becomes very cold there - so it must be mixing in the cold water from the new PEX line.
What could be going on? Is it just that there is cold water in the unused line that is mixing with the hot water and making it cool? Even after running hot water for 20 minutes that connection is really cold. Or could there be a hot/cold crossover in the addition?
Update: I found a hot cold crossover at the shower mixing valve in the addition.