Choosing the Right Heating Element Power

Element power is chosen against three things at once: how much water there is, how quickly it has to be hot, and what the wiring will carry. Sizing on the first alone is the usual mistake. Too much power in a small tank boils at the tube and scales up fast; too little and the tank takes half a day. The figures below are the ones we check orders against at the factory, and every one of them is rounded.

Storage water heaters

One kilowatt raises 100 litres of water by roughly 8 to 9 degrees an hour, ignoring losses through the tank wall. That gives the familiar pairings.

• A 30 to 50-litre tank takes a 1.0 to 1.2 kW element and reaches 60 degrees from cold in about two hours.
• An 80-litre tank takes 1.5 kW and needs 2.5 to 3 hours.
• A 100 to 120-litre tank takes 2.0 kW and a little over three hours.
• A 150 to 200-litre tank takes 2.5 to 3.0 kW, or two 1.5 kW elements that can run together or singly.

Go to the lower figure if the heater runs overnight on a cheap tariff, and to the upper one if it stands in a cold cellar or hot water is needed in a hurry.

Wiring and the breaker

At 220 V a 2 kW element draws about 9 amps and a 3 kW element almost 14. A standard socket on 1.5 mm² copper is rated for 16 A, so 3 kW is the ceiling for a plug-in heater, and only if nothing else shares the circuit. Anything larger wants its own 2.5 mm² cable from a 16 to 20 A breaker, and from about 5 to 6 kW a three-phase supply. Aluminium wiring in older houses cuts those limits by roughly a third.

Elements for radiators

Here the starting point is the room, not the tank. A room with ordinary windows and a 2.7 m ceiling needs about 100 W per square metre, so 15 m² calls for 1.5 kW. The second check is the water volume: allow about 1 kW for every 15 litres in the system. A cast-iron section holds 0.6 to 1.5 litres and gives out about 180 W; an aluminium section holds about 0.2 litres. A ten-section cast-iron radiator therefore wants a 1.5 to 2 kW element, and it must have a thermostat, because water in a sealed radiator will otherwise boil.

Shape and length

The tube is bent to suit the vessel: straight, U-shaped, horseshoe, ring, kettle form. Shape sets the heat-transfer area, and the length of the heated section sets the surface loading: the same wattage on a shorter tube runs hotter and dies sooner. A replacement should therefore repeat the old length, shape and bend angle. Before ordering, take from the old element the wattage and voltage on its label, the length from flange or nut to the tip, the tube diameter, the thread size or flange diameter, and the number and type of terminals. With those DAVIGO can match a stock element or make a custom one to your drawing.