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scouter t1_jcuqy71 wrote

Reply to comment by JohnProof in Hot Water Tank's Hot Water Runs Out Quickly by Pomegranate4444

Right so far, but consider a thought experiment. After the first 20 gal of hot water has been drawn, the “bottom half” will be full of cold that the water heater is heating. But we know that there will be mixing, thus the hot water coming out after 10 minutes will be colder than the first gallon. Notably colder, because the tank is pulling in 50-60 degree water (typical in North America). And this will continue, so the “40 gal” capacity will be felt in the shower as exhausted well before 40 gallons of use.

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BIT-NETRaptor t1_jcv2di5 wrote

Do you in fact know that there will be mixing, or did you guess? I thought so too until I watched a Technology Connections video on electric water heaters.

https://i.imgur.com/hoqaigM.png

It mixes much less than you think.

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scouter t1_jcv5eex wrote

True, I was guessing, but I have always had a gas water heater. The stratification should not come as a surprise as I am familiar with sharp thermoclines in the ocean but I did not make the connection.

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JohnProof t1_jcuvt4f wrote

That's where the 2 thermostats and dip tube come into play:

If the dip tube is working, it's putting the cold water into the bottom of the tank which helps stratify hot water at the top where it gets drawn off. And the lower thermostat will kick on to heat that cold makeup water to help maintain the tank temperature.

Once too much hot water is used and the lower element can't keep up, then it shuts off and the upper element kicks on to re-heat water near the draw-off as quickly as possible.

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