Boilers & Pump
Immersion Heater (a 3 kW element in the cylinder)
In short
An immersion heater is an electric element that screws into the hot water cylinder. Its terminals: L, N, E.
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
What it is
An IMMERSION HEATER is an electric ELEMENT that screws into the hot water cylinder. In its head are its own rod THERMOSTAT (stat) and a safety CUT-OUT. It is DrawWarm's own design, built from what the makers' instructions say.
A 3 kilowatt (3 kW) element draws about 13 amps (13 A): far past the heating controls' 3 A fuse. So it has a circuit of its own from the CONSUMER UNIT, the home's fuse box.
It is switched by a DOUBLE-POLE switch, with at least 3 millimetres (3 mm) contact separation in both poles. That is a 20 A double-pole switch, or a double-pole fused spur rated for it.
It is wired in HEAT-RESISTANT FLEX, of at least 1.5 square millimetres (1.5 mm²), rated for 85 to 90 °C. It MUST be earthed.
Its thermostat feels the same water as the heating system's cylinder thermostat, but the two are never wired together. The immersion heater never switches the boiler, and the cylinder thermostat never switches the immersion heater.
So a cylinder can have two supplies, or more: some have two elements on two circuits. Isolate EVERY supply before any work on the cylinder: here, the heating controls' spur and the immersion heater's own switch.
Its cap is drawn off here, so you can see inside. On site the cap is back on before it is switched on, and nothing covers it.
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Terminals
| Terminal | What it is for |
|---|---|
| L | The live (L) in (brown), from the immersion heater's own double-pole switch. Inside, it runs through the cut-out and the thermostat to the element. |
| N | The neutral (N) in (blue): the element's other end, through the cut-out's second pole. |
| E | The earth (E) stud on the brass boss (green-and-yellow, on its nut and washer). The element's sheath and the cylinder are earthed through it, so it must be connected. |
What the meter reads
Its own supply on, and the water below its setting: the thermostat (stat) calls and the element heats. The water is the Conditions card's Cylinder water.
While it heats: 230 volts (230 V) across L and N, and about 13 amps (13 A). Its status plate says HEATING.
The thermostat comes set at 60 °C, and goes from 10 to 70 °C: press - or + beside the knob. Joule sets it at 60 to 65 °C, and Heatrod no higher than 60 °C. That is hot enough against LEGIONELLA, but not so hot that it scalds or scales the element.
It switches 2 degrees either side of its setting: a 4 degree DIFFERENTIAL. Set at 60 °C, it is satisfied at 62 °C, and calls again below 58 °C.
In the Sandbox the cylinder warms while it heats, and stops just past its setting. In a lesson the water holds still.
Its CUT-OUT opens at 80 °C, whatever the thermostat says, and stays open. The plate says CUT-OUT, and the small red RESET stands proud. The reset closes it only once the water is below 80 °C.
On site, only a competent person resets it. Isolate every supply first and lock it off: the reset is under the cap, beside live screws. Let the cylinder cool, and find the cause.
With no neutral, it does not heat. But while the thermostat calls, its N terminal reads 230 V to earth through the element, similar to a heater on site.
Dead, on site, a good 3 kilowatt (3 kW) element reads about 18 to 19 ohms across L and N. The thermostat must be calling. In this simulator it reads 17.6 ohms there: 3 kW at 230 V. It reads OL with the thermostat satisfied or the cut-out tripped.
Where it connects
Lessons that use it
- An immersion heater on its own circuit
Intermediate · Protection and extras
Practise it in the simulator.
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