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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.

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    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? – Jim Stewart Nov 30 '18 at 13:01
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    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 Usually cold water is fed into the tank at the bottom and hot withdrawn at the top and stratification keeps the output hot as water is used.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. – Jim Stewart Nov 30 '18 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. – HazardousGlitch Nov 30 '18 at 13:31
  • @JimStewart I have updated the question with answers to your questions. – Tyler Durden Nov 30 '18 at 15:29
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    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. – Jim Stewart Nov 30 '18 at 22:09
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You absolutely cannot delay delivering cold water into the tank to replace the hot water output. You have a pressurized tank which must remain full of water to deliver water. If you had an unpressurized water tank which delivered hot water by gravity, you could do this, but you don't have that.

I assume that the cold feed is very cold in Cambridge MA. You may not be starting with a full tank of 130 F water. It could be that the controls on the boiler are set to reduce the number of heating cycles per unit time. Where is the temperature sensor located vertically in the tank?

Some recommendations on the temperature set point of tank water heaters are that it be set at 140 F (60 C) to prevent the growth of legionella bacteria. Other recommendations give 130 F. Does your shower valve have an anti-scald temperature control?

  • The temperature sensor for the tank is about mid-way up the tank. There are no controls on the shower heads. – Tyler Durden Dec 1 '18 at 0:56
  • AFIK there are no shower heads with controls on the shower head itself (except for electric resistance heads used in less developed countries), but does the shower valve have a thermostatic anti-scald feature? If you were going to raise the set-point the temperature of the water in the tank to 140 F, you'd probably need that. – Jim Stewart Dec 1 '18 at 1:29
  • There is no anti-scald feature that I know of. – Tyler Durden Dec 1 '18 at 1:36

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