In homes that I've visited that have a conventional boiler setup (where hot water is stored in a tank usually contained within the airing cupboard), I've noticed that after the hot water has been used continuously for several minutes, the tank appears to then take several minutes more to refill. As an example, if a shower is used for 15 minutes, the sound of the tank being refilled is audible for around 10 minutes afterwards.

Taking this into account, does this then mean that the tank is typically filled slower than hot water is taken from it? If a shower was to stay on continuously for several hours, for example, could the hot water tank then run out of water completely (including cold water that is yet to be heated)?

  • 1
    In systems I’m familiar with, the pressure of the cold water supply provides the pressure to push the hot water out through the piping to the tap. So it would be fundamentally impossible for it to behave as you surmise.
    – prl
    Sep 21, 2019 at 20:25
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    Is it possible the sound you’re hearing is the sound of the water being heated, rather than the tank filling?
    – prl
    Sep 21, 2019 at 20:27
  • Thank you for your thoughts; in this case, this is impossible because my boiler isn't set to heat the water at the times this has been observed.
    – elliott94
    Sep 21, 2019 at 20:40
  • 1
    you cannot drain water from the tank unless you let in something else, either air or water ... does the tank allow air to enter?
    – jsotola
    Sep 21, 2019 at 21:52

3 Answers 3


When you take hot water from the tank cold water flows into the tank to replace the hot at the same rate.

The trick employed by tank designers is that hot water is lighter than cold water and floats on top. so the cold water is introduced at the bottom of the tank and does not disturb the pool of hot water floating on top. thus you can draw hot water from the tank until the tank is full of cold water.

As an example, if a shower is used for 15 minutes, the sound of the tank being refilled is audible for around 10 minutes afterwards.

what you are hearing is not the hot water tank filling, but instead the header tank of the low pressure hot water system filling, this is a tank of about the same volume as the hot water tank located somewhere above it, perhaps in the attic space.

so, water flows though a float operated valve into the header tank, and on demand flows by gravity to underneath the hot water tank, it then enters the tank, is heated and comes through a pipe at the top and flows to the outlets.

The float valve starts gradually so when the header tank begins to empty the flow rate is quite low, but increases as tank empties ans the float drops, after the shower the tank is still catching up with the water used.

  • This makes sense - thank you. So just to clarify - what's the relation to the header tank and the main hot water tank?
    – elliott94
    Sep 22, 2019 at 10:26
  • The header tanks is the low pressure supply for the hot water tank.
    – Jasen
    Sep 22, 2019 at 10:37

The tank fills at the same rate the water exits this is simple physics. However the tank starts heating water when some is removed. Since tanked water heaters are limited to about 5kw on average they have a limited ability to heat new fresh cold water. This is where tankless water heaters have an advantage they can supply hot water for long periods of time (I used to fill my 600 gallon hot tub at 103f every few months when I cleaned it. A 40 gallon tank well it has 40 gallons of much hotter water but in the US this is normally a 240v split phase circuit at 30 amps. The tankless electric can be 60, 90 or the largest I have installed 120a , this required a huge service upgrade to a true 400a service since the home was all electric and could supply 119 deg f water all day long where a tanked heater has ~20 minutes of hot water then the temp starts to drop because the heating elements only can supply 5Kw of heat but the pressure remains the same on a tanked system where a tankless system may reduce the flow to maintain the temperature.


The only way that could happen is with an unpressurized tank with a float valve, as used in toilets. Otherwise, as water left the tank, it would collapse from the air pressure, or the air pressure would prevent water from leaving the faucet.

That said, I've never heard of such an unpressurized system. Perhaps it might be used in an area where water-flow from a well is very slow, allowing a cistern to fill for later use.

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