How to Turn On an Immersion Heater and Use the Switch

Where the immersion heater switch is

An immersion heater is fed from its own circuit at the consumer unit, through a dedicated switch close to the cylinder. In most London homes that means on the wall just outside the airing cupboard, inside the cupboard itself, or in the landing or bathroom next to it. In flats with the cylinder in a hall cupboard, the switch is often above the cupboard door.

Look for a switch that is bigger than a light switch, with a rocker marked on and off, and a flex coming out of the bottom or side of the faceplate that disappears towards the cylinder. Many are labelled "water heater" or "immersion". The small red light is the neon, which glows when the switch is on and power is reaching it.

If you cannot find a switch anywhere near the cylinder, check the consumer unit. Some installations have the immersion on a labelled breaker, and some off-peak setups have a separate small consumer unit or a timer box by the meter. If the cylinder is heated only by the boiler and has no immersion at all, there will be no switch to find. Our guide to what is an immersion heater explains how to tell whether yours has one.

The different switches, and what each one means

Immersion switches come in a handful of types. They all do the same basic job, but the markings can be confusing the first time you look at them.

• A plain double pole switch with a neon: on is on, off is off. The most common type.
• A Bath and Sink switch: a second rocker picks which element runs. Sink heats the top of the cylinder only, which is quicker and enough for washing up or a shower. Bath heats from the bottom and warms the whole tank.
• A boost switch: runs the immersion for a set time, often 30, 60 or 120 minutes, then turns itself off. Good for homes where the boiler normally heats the water.
• A timer: a dial with push-in tabs or a digital display. The immersion only runs in the hours you set. Common in electric-only flats and on Economy 7.
• A smart controller: the same idea as a timer, run from a phone app.

How to turn it on, step by step

Before you switch on, make sure the cylinder is full of water. An immersion element must never run in an empty or part-drained tank, because it burns out in seconds. If you have just had the cylinder drained, or you have turned the water off because of a leak, leave the immersion off until the supply is back on and hot taps run with a steady flow.

• Check the cylinder has water: open a hot tap for a few seconds and make sure it runs normally.
• Flip the immersion switch to on. On a Bath and Sink switch, set the second rocker first, then switch on.
• Check the neon light is lit. If it is not, see the section below on a switch that will not come on.
• On a timer, either set a time window that covers now or press the boost or advance button.
• Wait. Feel the pipe coming out of the top of the cylinder after 20 to 30 minutes. If it is getting warm, the element is working.

How long should an immersion heater be on for?

It depends on the size of the cylinder and how cold the water is to begin with. As a rough guide, the top of the tank gives you enough for a shower or washing up in well under an hour, and a full cylinder from cold takes a couple of hours, sometimes more for a large tank. Our guide on how long does hot water take to heat up has the detail by cylinder size.

Leaving the switch on for longer than that does not overheat the water, because the thermostat on the immersion switches the element off once the water reaches its setting, normally around 60 degrees. What it does do is keep topping the tank up all day to replace the heat it loses through its insulation. If a gas boiler normally heats your water, that is electricity you are paying for twice over.

The simple rule: if the boiler is working, leave the immersion off and use it only as a backup. If the immersion is how your water is always heated, put it on a timer so it runs before you need the water, not around the clock. The immersion heater timer guide explains which type suits your home, and immersion heater running cost works through what an hour of heating costs.

Using the immersion when the boiler breaks down

This is the main reason people go looking for the switch. The boiler has stopped, the heating is cold, and there is no hot water. If you have a cylinder with an immersion, you can still have hot water while you wait for the boiler to be fixed.

Turn the immersion on, use Sink first if you have the choice so there is something hot within the hour, then switch to Bath overnight or when you need a full tank. When the boiler is repaired, turn the immersion off again. It is surprisingly easy to forget, and an immersion left on for weeks after a boiler repair is one of the more expensive mistakes on an electricity bill.

If the boiler is working but the cylinder still is not heating from it, the problem is often in the controls rather than the boiler itself. Our guide to no hot water but the heating works goes through that side of things, and no heating or hot water covers the case where both have stopped at once.

The switch is on but there is no hot water

If the neon is off, check the consumer unit for a tripped breaker, and any fused spur feeding the switch for a blown fuse. If the breaker trips again the moment you switch the immersion on, stop there and leave it off. That is usually an element that has failed to earth, and it needs testing, not resetting. Our guide on why an immersion heater keeps tripping explains the causes.

If the neon is lit but the water stays cold for hours, the element or its thermostat has probably failed, or the safety cut-out on the immersion has tripped. Our guide on how to reset an immersion heater explains the cut-out, and immersion heater not working walks through how the element and thermostat are tested.

On Economy 7 and similar setups, the bottom element may only get power during the cheap night hours. No hot water in the afternoon is not necessarily a fault. Try the boost or the Sink setting to see whether the top element heats.

When the switch itself is a warning sign

An immersion switch carries a heavy load for hours at a time. A faceplate that is slightly warm to the touch after a long run can be normal. These are not normal, and are a reason to switch the circuit off at the consumer unit and leave it off:

• A switch that is too hot to keep your hand on
• Brown or black scorch marks on the faceplate or around the flex
• A burning or hot plastic smell near the switch or the airing cupboard
• Crackling, buzzing or a flickering neon when the immersion is running
• A flex that looks melted, hardened or cracked where it meets the switch or the cylinder

Scorching or a burning smell at the switch means a loose or overheated connection. Turn the immersion circuit off at the consumer unit and do not use it until the switch and wiring have been checked. The switch and circuit are electrical work; the element, thermostat and cylinder are what we deal with.

Getting the immersion checked

If the immersion will not heat, keeps tripping, or the cylinder is leaking around the element, call us on 020 4652 5111 or send an enquiry with a photo of the cylinder label and the switch. We have 20 engineers across London working on hot water systems every day, including G3 qualified engineers for unvented cylinders, and the photo lets us bring the right element and thermostat for your cylinder. Our rate is on the pricing page and agreed before we attend. For what the visit involves, see our immersion heater repair service.