The hot water tank is a copper cylinder with holes in it, which tends to live all nice and quiet in the airing cupboard.
The holes are usually one at the side and one at the top into which copper pipework is connected by means of fittings, and connects the cylinder to the hot water heating system from the boiler. There will also be a third big hole into which the immersion heater element is fitted.
The immersion heater is the electrical means of heating the tap water either to speed up the water heating from the boiler, or to heat the water if the boiler has failed. If you have a combination boiler, then don’t worry looking for the cylinder as you won’t have one.
Hot Water Tank
Cylinders corrode and leak usually at the bottom after about 20 years or so, and can leak from any of the fittings. Sometimes they drip water for years with the floorboards soaking it up.
But when they leak more seriously, it will be a continuous leak with the cold water storage tank (found up in the loft) feeding the water to the cylinder. This is a huge problem when the storage tank keeps filling up from the incoming cold mains from the street, until the main stop cock to the house is switched off, or the cylinder isolating wheel valve is switched off. However, these may not have been turned off for around 20 years, and often break if forced.
The answer is to isolate the cylinder, disconnect the fittings (make sure the electrical supply to the immersion element is turned off) and drain it down by use of a hosepipe leading outside. The boiler also needs to be turned off.
Then old cylinder out, and new in! Reverse the above and you are great for another 20 years or so!