Water Heater Element

Water hardness decides the material and the service interval

Scale is what kills water heater elements, not electricity. It grows fastest on the hottest surface in the tank, which is the element, and once a layer has formed the element has to run hotter to push the same heat through it. That makes more scale. The loop ends with a failed element and a cylinder full of debris.

Where the water is hard, choose Incoloy over copper, keep the watt density at the lower end of the range by using a longer element, and put descaling in the maintenance calendar rather than waiting for a failure. A cylinder that takes noticeably longer to heat than it did last year is telling you the schedule is already overdue.

Look after the anode and the tank looks after the element

Every steel or enamelled cylinder has a sacrificial anode, and its whole purpose is to corrode so the tank does not. Once it has dissolved, the tank starts to rust, the water carries that rust, and it settles onto the element. Elements fitted into a corroding cylinder fail again and again, and each time it looks like an element problem.

Pull the anode when the element is out. It costs very little and it is the only moment when checking it is easy.

Fit the element low, and leave the sludge somewhere to go

Sediment collects at the bottom of a cylinder. An element sitting right on that layer heats sludge instead of water, and the sheath under it runs far hotter than the water around it. A few centimetres of clearance above the floor keeps the element in clean water and leaves a place for the debris to sit until the cylinder is drained.

The gasket, not the thread, does the sealing

Most cylinder bosses are a parallel thread with a flat face, and the seal is the gasket against that face. Winding extra tape onto the thread does not help and can split the boss. Use a new gasket, make sure the face is clean and unpitched, and tighten enough to compress the gasket evenly. If a boss face is pitted, that is a cylinder problem and no gasket will hide it for long.

Earth it properly and use an RCD

The element is a metal object in water, connected to the mains, in a room where people are wet. The earth connection at the terminal head is not decorative. Make it off, tighten it, and confirm it. Fit an RCD on the circuit, and if it trips, treat it as a real fault — measure the insulation resistance from the terminals to the plug before switching anything back on. A wet magnesium oxide core reads low, and it will trip again.

Two elements are often better than one

On a large cylinder, two smaller elements at different heights beat one big one. The lower element does the main heating, the upper one can be switched alone for a quick, small volume of hot water without heating the whole tank. Each runs at a lower watt density, so both scale more slowly, and a failure leaves you with half a system rather than none.