I have a 120 gallon indirect water heater. I notice that when I take a shower with a 3 gpm shower head, the water starts to cool after about 20 minutes, which would be 60 gallons or only 50% of the tank. I assume this is because any hot water removed from the tank is being immediately replaced with cold water which then starts to cool the tank down. Is this correct?
If so, is there anything I can do to increase the duration of hot water, such as somehow only having it start to refill the tank when it is 20% full or something like that?
The system is heated by hot water coming from a boiler. The pressurization is by city water supply at 80 PSI. The temperature set point is 130F.