# Hot water heater running out at 50%

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.

• There does seem to be a problem, but just to get the facts, what is the temperature set-point for the water in the tank? What is the indirect heat source for the tank? Commented Nov 30, 2018 at 13:01
• You should describe the hot water system in more detail to get the best advice. Is the tank pressurized or does it deliver by gravity? city water or a well?, etc. Usually cold water is fed into the tank at the bottom and hot withdrawn at the top and stratification keeps the output hot as water is used. But if you have a 120 gal tank maybe stratification is harder to maintain as water is drawn out. Commented Nov 30, 2018 at 13:10
• The 3 GPM is assuming a specific water PSI so you maybe be getting more or less depending on your actual PSI. Answering the questions the other people asked will help figure out the issue. And no, you can't use X % of water from the tank and then start to refill it because of the reason you gave. Even though cold water starts to refill the tank, it won't mess up the hot water significantly unless there is something wrong on the inside. Commented Nov 30, 2018 at 13:31
• @JimStewart I have updated the question with answers to your questions. Commented Nov 30, 2018 at 15:29
• When you have not used hot water for a while (say first thing in the morning) does the boiler come on immediately when a high flow rate demand is made on the hot water? This is a "combination" boiler, right? That is it also heats water for space heating of the house. Since it does double duty maybe the "recovery rate" for water heating is compromised. Commented Nov 30, 2018 at 22:09