It depends how the immersion heater is plumbed to the storage cylinder, with the Willis system the hot water is fed into the top of the storage cylinder and slowly the amount of hot water held in the tank increases, this means you get hot water available minutes after switching on, but it seems only the Irish have the ability to understand and install this system, rest of UK don't seem to understand how it works.
So in the main the immersion heater is placed at some point in the cylinder and will heat the water above it, in some units baffles are fitted to reduce the thermals in the using, common with wall mounted kitchen boilers, but most water heaters don't boil the water, and having water at boiling point would be considered dangerous from taps, with the special units for making tea, as many know when going into a works canteen it will sudden go cold, but without the baffles the transition is more gentle as noted when running a bath, there are normally quite a few gallons of warm water before it runs cold, even when using a system boiler to heat the domestic hot water which is not running when filling the bath, so not simply the water being heated while it is being run off.
Water is most dense at 4°C above and below that it expands and so becomes lighter by volume so both ice and hot water rise to the top. Bottom of deep lakes stays at 4°C and water naturally gets a gradient of temperature. We use this, so the hot coil for the input and output of heat from the cylinder are set at different heights. With at the bottom the solar coil, then the central heating coil, then the bath immersion heater and finally the hand basin immersion heater, and if the immersion heater thermostats are set correctly then they will not switch on when the central heating is running.
However the times given,
- intervals between users remain same; say 5 minutes of each other
would point to a large property, in which case there is likely a circulation pump, this ensures the water in the pipe stays hot, and the return point into the cylinder would determine how much water is maintained at the same temperature. Often the pump is timed, and does not run in the dead of night.
Also there are two ways to use the cylinder, traditional method it using the water from the cylinder, but you can also use the water from the hot coil, this allows the stored water to be low pressure supplied from header tank, but the DHW is at mains pressure, the coil needs to be rather large to transfer heat fast enough, but it means there is no movement of the water in the cylinder due to a tap being opened. It is becoming more common, and is the method used to combine different forms of heating, so solar, solid fuel, LPG and electric can all work central heating and DHW only seen one house with it actually fitted, and it did work well, was my brother-in-laws house, and he spend a lot of time in Germany and his Welsh home would be maintained at 12°C solely using solar power while he was away, and geofencing would detect his eminent return and auto turn up the heating.
Only problem was the installation cost and weight of water cylinders, so really only an option when fitted to new build so added to mortgage. For safety you want thermo-syphon rather than pumped from the wood burner, so cylinders need to be on an upper floor, which has to stand the weight, so in real terms only as original equipment.
