I am super of a 13 story residential building in New York city. I have several apartments all on different risers al on lower floors. While people are showering the water jumps from normal to extremely hot to extremely cold. The only major job that has been done is a new house pump for the gravity tank. Any help is appreciated. If you need more info let me know. Thanks.

How old is the building and what material are the pipes made of? Did everything work OK before you installed the new jocky pump? Was this a new installation or was the pump simply replaced? Back to you. Tom

The building went up in 1930 the cold is galvanized and hot is copper the pump was replaced we had 3 hp and went to a 5 hp pump I had no problems before the new pump. Thanks

I keep coming back to the galvanized cold water pipe. I know how mineral build up in the pipe walls can affect both pressure and volume. This could explain the water getting hot through a mixing valve with multiple showers being used at the same time, (like flushing when you're in the shower). What I can't explain is the water losing heat and turning cold. The only thing I can think of is a cross connection made at the time the pump was changed out. You do know that if even one of the shower valves in the units,or any mixing valve, has a cross connection it affects the entire system don't you? Regards, Tom

I know that but I have not found any bypasses anywhere in the building.