rymojo
What is happening here, if you are getting a “slug” of hot water out of the cold water side of one or more fixtures in the house is probably this: When the heater is filled with cold or mostly cold water and begins a heating cycle, the water, as it expands, backs up into the cold water inlet piping, probably towards the meter. This places a “slug” of heated water in the cold water piping system and it will be carried along by the flow of cold water to a cold outlet by the first usage of cold water following a heating cycle. Test this hypothesis so: After a large usage of hot water( a couple of showers or loads of clothes), warn everyone in the house not to use any water for an hour. Feel the cold water piping going to the heater. As the heater recovers (heats up) you should be able to feel the cold inlet piping begin to heat up further and further away from the heater. If it gets hot upstream of the tee on the PVC inlet main, I am right. (Remember that PVC loses its strength rapidly at elevated temperatures and that if the inlet main temps are exceeding 110F an alternative method of handling this expansion should be addressed.
