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)?
Best Answer
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.
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.