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In a 40 gallon electric water heater with incoming water at 42 degrees,
how long should your hot water last for a shower? Is there a table available to calculate that figure?

Thanks
Dennis
Southern IL plumber
 

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There are too many unknowns in your question. What temperature shower are we talking, what is the HW GMP and how cold does it need to be to be considered depleted.

Look at the first hour recovery from the manufacturer than test to see what your flow is. Or you can do it the hard way and figure it takes 2.47 watts to heat one gallon of water one degree.

Mark
 

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About 32 minutes, most flow requirement on fixtures is 2.5GPM, figure you will mix the water 50/50, so 40 divided by 1.5=32. Electric heaters don't have a quick recovery rate, so I base is upon the stored water, even at 32 minutes is pretty conservative, gas heaters do much better recovery rate is higher.
 

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The roll of thumb is 80% of the size of the tank. So a 40 gallon is only going to give you 30. The time it takes to run out depends on who is using thne shower and how hot they run the water. I take pretty hot showers after work. My daughter can drain a 50 if I let her.
 

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height x width x depth / 231 will give you gallons. A standard tub will get about 6-7 inches of water with a 40 gallon tank before it drops below 100 degrees. Should get 120 for 3 inches or so.

If you are getting less and the initial temp is 120, you, most likely, have a cross connection, tank filled with sediment, bad tstat, or a dip tube problem.

Run the above calculation to determine if your water heater is working normally. If it is operating correctly, run the shower with a stop watch :)
 

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The roll of thumb is 80% of the size of the tank. So a 40 gallon is only going to give you 30. The time it takes to run out depends on who is using thne shower and how hot they run the water. I take pretty hot showers after work. My daughter can drain a 50 if I let her.

Rheem in their technical bulletins says that the first hour delivery of any tank is 70% of gallon, plus recovery. That's the first HOUR, so for purposes of calculating length of a shower, leave out recovery. A 40 gallon tank will provide 28 gallons ( at a temp approx. 20º less than set point.) Naturally, very cold inlet water probably fudges this number down a little.

At 2.5 GPM, 28 gallons gives you about an 11 miute shower. Only solutions are :
> ultra low flow head
> bigger tank
> set the temp higher, but them MUST use a tempering valve for safety.
 
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