I take long hot showers after bike rides, so I’m used to the feeling of the water going cold in a shower. What’s interesting to me is that this happens fairly quickly, seemingly toward the end of the supply of water that was hot when you began showering.
But i’ve been thinking about boilers and it’s not clear to me how this works. It occurred to me that maybe the tank only begins refilling with cool water from the pipes once it runs low — but the tanks in most places I have lived are in the basement and don’t have obvious pumps attached. If the tank weren’t full I don’t think you’d be able to maintain constant water pressure upstairs. At the same time, the hot water seems to stay hot for 30+ minutes, even as the tank is presumably refilling itself with cold city water. How does this work? Thanks!
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The water in the tank is always hot. When you turn on the hot water, you get that hot water. Once that water is used up, you’ve run out of hot water, and you need to wait for the tank to heat up again.
Heating water takes a lot of energy, so it’s a very slow process. Generally, there are two heaters, one on the bottom and one in the middle. When you start using hot water, cold water enters the bottom, and the bottom heater turns on. Now that water doesn’t get hot, but it does gets warm. Once you have used enough hot water that the warm water reaches the second heater, that turns on to get that warm water to become hot water. This gives you a hot water capacity of about 150% the volume of the tank before its too difficult to keep up the demand.
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