California Plumbing repaired the plumbing to the recirculating valve copper pipe that was leaking.
The Re-Circulating pump is what keeps hot water in the system throughout the building. Have you ever had to wait for hot water to get to your faucet or have a shower that goes hot and cold, Well the recirculation pump fixes that.
Unfortunately, when you hire plumbers that don’t understand the system, they turn off the water and forget to bleed the system so that the recirculation valve does its job. Air in the line causes hot cold, hot cold showers for everyone in the building. Then the HOA plumber has to come back out to fix the issue which cost the HOA more money and increases your monthly dues. Always hire a plumber who is experienced with these systems.
For even more scientific information on how this works see this video.