why can’t you get hot water from a faucet when the freshwater supply is turned off, even though there’s a water heater full of water being heated and at the ready?
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Vacuum sealed causes the lock due to nothing following the water you are using.
Think of it as a drinking straw. Stick it in water then put your finger on the top to seal the straw. The water will not fall out when you lift your finger.
It’s a safety device installed to prevent fires, if no water is incoming the device won’t let water leave. If all the water were drained out of the tank the flame still on underneath would damage the tank and could spread.
I imagine the boiler has an automatic shutoff to prevent draining the boiler without fresh water coming in.
Or the heater doesn’t have a tank, and heats source water directly in real time.
Or is what you’re saying even correct?
Seriously, this isn’t an ELI5 question. This isn’t a complex problem to be answered simply. This is a simple question with a simple answer, but with a lot of missing information that you need to ask whoever services your water heater.
A hot water heater is not a tank. Think of it more as a big bulge in a pipe. The water in the water heater is pressurized by the incoming cold water and then as a result hot water exits the heater when someone opens a hot water tap.
You can actually drain your hot water heater (through the little valve near the bottom) but that requires turning off the incoming cold water, and also opening a hot water tap somewhere in the house to release the suction.
Because the water heater is usually in the basement, and water flows from high places to low places.
So normally what happens is the fresh water pipe that goes into the water heated (so the water can be heated up) has pressure and pushes out the hot water that’s in the heater tank. Without fresh water pressure, water will just flow from high (your faucet on the main floor) to low (the water heater in the basement).
That’s the point of [water towers](https://upload.wikimedia.org/wikipedia/commons/d/df/Plymouth_Township_Watertower_002.jpg) by the way, to supply water to the city. Because it flows from the high tower to the ground-level houses. And they periodically refill the tower using pumps.
Here, the water heaters are on the roof (more than 90% use solar heating), and we do have water when there is a shortage
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