Types of Water Heaters and How to Choose a Heating Element

Domestic water heaters can be sorted many ways: storage or instantaneous, vertical or horizontal, enamelled or stainless tank. For anyone shopping for a replacement element, one distinction matters more than the rest: how the element meets the water. That gives two types. In a wet, or open, heater the element tube sits in the water. In a dry, or closed, heater the element sits in a pocket and stays dry.

Wet heaters

This is the most common and the cheapest layout. Heat passes straight into the water, so an 80-litre tank on a 1.5 to 2 kW element warms from cold in roughly two to three hours. The price is scale. The tube runs above 100 degrees, hardness salts settle on it first, and in hard water an element lasts two or three years. Replacing it means draining the tank and removing the flange. Next to the element on the same flange sits a magnesium anode that takes the corrosion instead of the tank; it should be changed every year or two, or the tank starts to rust.

Dry heaters

Here a pocket wall separates the element from the water. It heats more slowly and costs more, but the element can be swapped in ten minutes without draining, the electrical safety is better, and the element lasts far longer. Tanks of 80 litres and up usually carry two elements, and both can run together when speed matters. For a house on a borehole with very hard water, a dry heater often works out cheaper over its life despite the higher shelf price.

What to match when choosing an element

Whichever layout you have, the new element has to agree with the old one on a few points.

• Mounting is either threaded, where the element screws into the tank, most often on a 1¼-inch thread (about 42 mm outside diameter) and less often 1 or 1½ inch, or flanged, where a round flange bolts to the tank; for flanges the diameter, typically 48, 64, 72 or 92 mm, and the number of holes must match.
• If the old flange carried an anode, the new one needs a threaded socket for it, usually M4 or M6.
• The tube shape, whether straight, U-shaped, horseshoe or ring, should repeat the old one, since a bent element will not always pass through an opening made for a straight one.
• Copper tube is cheaper and conducts heat better; stainless steel holds up longer in aggressive water, and a stainless tank should get a stainless element to avoid a galvanic pair.
• Take the wattage and voltage from the old element's label; fitting a more powerful element is only sensible if the wiring and thermostat are rated for it.

Before ordering, unscrew the old element, measure the heated length, the tube diameter and the thread or flange, and photograph the terminals. With those figures a stock element can usually be matched; where it cannot, DAVIGO will make one to a drawing or to your sample.