Immersion Heater (Clean)

Usually the hot water in your home sits in a cylinder that is fitted with one or more electrical heating elements. These are fitted to heat the cold water that is fed from the mains. These elements can be a victim of something called scale. The scale reduces the efficiency of the element.
There is a routine to clean the deposits from the elements, but, if you are not confident to take this on, then, you need to call a professional plumbing service like Wandsworth Plumbers.
Isolate the Power to the Heater.

Drain the cylinder. Connect a rubber hose to the drain valve at the bottom of the water heater, and open the drain valve and a hot water tap. When the cylinder is empty, using a screwdriver, undo and remove the access cover for the element. It is possible that you may need to move some insulation in order to see the element.
Disconnect the cabling and remove the element(s) as there could be more than one fitted. The element(s) are normally held by a series of bolts, or they have a threaded base that screws straight into the cylinder.
Remove and clean the element, using a solution of sodium carbonate and water or vinegar mixed with water. (2 tablespoons of vinegar or 2 tablespoons of sodium carbonate in 1 quart of hot water) and a cleaning pad. A Wandsworth Plumber can install heating elements. Should the element be corroded you will need to fit a replacement.

If your water is cooler than normal, erratic hot water, or a smaller than normal supply of hot water are all signs of an element that is damaged.

Re-install the cleaned or new element(s).
Reconnect the wiring. Re-fill the cylinder by closing the drain valve and turning on the cold water supply valve.