我一老房子的50多年的銅水管，怎麽辦？

My tenants somehow got a plumber to flush an electric water heater for like $20-30. He said water heater 20 years old so does not make sense to flush it anymore. 

He then came back with a quote to change water line to PEX piping for $10k (new hose bibbs, shut off valves, supplies and laundry hoses. Sheetrock repairs NOT included.) and One Story 2 Bath Whole House Re-Pipe: Drain Only ( I assume with PVC) for over $10k

so I posted this on reddit. 

https://www.reddit.com/r/Plumbing/comments/1tisv0q/50_year_old_copper_water_pipes_for_whole_house/? 

(for whatever reason I could not post photos today)

He then quoted me to Install a high-efficiency tankless water heater. Save space, enjoy continuous hot water, and start saving on energy from day one. Navien NPE240-A2 with recirculation pump. Includes Isolation valves. (it is closet now with no safety pipe outside of house. Neither drainage pan and pipe to outside of house. New tank water heater would require that. 

$7k