hot water piping

Hi all - My water heater has a T off from the hot line that runs though a pressure relief valve, and eventually exiting outside on my patio. I'm not sure why it's there but maybe the original builder (about 22 years ago) did that because there's no drain in case the heater's TPR valve triggers...

We have very little HOT water coming out of most of our sink and bathtub faucets. We believe that it is sediment from the hot water heater. We didn't know you were supposed to drain the water heater to get rid of sediment. Is there a way to flush this through the pipes or do we need to have...

I've had two hot water rolled copper pipes develop leaks under slab within 6 months of each other. So I've routed new piping overhead in the basement for the three hot runs and abandoned the hot water underslab piping. Should I be concerned about the cold lines developing leaks and go ahead and...