The overlooked plug in your airing cupboard that quietly adds £80 a year to your electricity bill

Emily Turner • December 10, 2025 08:18

You open the electricity bill, blink twice, and do the usual mental dance. Tumble dryer? Maybe. Teenagers and their 40‑minute showers? Probably. But something still doesn’t add up. The numbers creep up every month, even though you swear nothing much has changed.

A few days later, you’re hunting for towels and hear it: a faint click from the airing cupboard, followed by the soft hum of something waking up behind the hot water cylinder. A small red light glows on a lonely wall switch or plug that nobody can quite remember turning on.

It looks harmless. It’s been there for years. It doesn’t run, whirr or flash like the big appliances. Yet that forgettable switch could be quietly leaking £80 a year – sometimes more – straight into thin air.

Most homes with a hot water cylinder have one. Few people really know what it does.

The silent appliance in the hot-water cupboard

Behind the towels and bedsheets, many British homes hide an immersion heater: a metal element buried inside the hot water cylinder, rather like a giant kettle. It usually has its own fused switch or socket, often with a little neon light that comes on when it’s powered.

In theory, the immersion is a backup. If your boiler fails or you’re on an off‑peak electricity tariff, you can use it to heat water. In practice, it often ends up left on permanently, quietly cycling on and off all day to keep a big tank of water hot “just in case”.

It doesn’t roar like a shower pump or shake the floor like a washing machine. It just sits there, sipping power in the background.

Because it’s hidden, it behaves like a subscription you forgot you signed up for.

Over time, that “invisible” sip becomes a serious line on your bill, especially now unit prices are high and many homes are trying to cut back.

How £80 disappears without you noticing

An immersion heater typically runs at about 2–3 kW. That doesn’t mean it’s drawing that power all day, but every time the cylinder cools a bit, the thermostat clicks and the element fires up again to top the heat back up.

Even if you barely use hot water, a poorly insulated cylinder can still lose heat into the cupboard and surrounding rooms. The immersion quietly reheats it, over and over. You pay for the leaks as well as the baths.

Here’s what that “background heat” can look like in money:

A typical older cylinder can lose around 1 kWh of heat per day just sitting there.
At a unit rate of about 22p per kWh (rough mid‑2024 price), that’s £0.22 per day.
Over a year, that’s roughly £80 – before you even factor in the water you deliberately heat and use.

In a well-insulated home, or if the immersion is only ever used as a short boost, the cost drops. But in many properties, especially with:

thin or missing cylinder jackets
a warm airing cupboard that feels toasty even in summer
an immersion left “on” 24/7 out of habit

…the annual waste can easily creep into three figures.

The problem isn’t using the immersion. It’s forgetting that it’s on.

The worst part? A lot of households don’t actually rely on that airing‑cupboard plug at all – their gas boiler already heats the cylinder, and the immersion is just a forgotten backup sitting permanently live.

When you should not just switch it off

Before you march to the cupboard and flip every switch, it’s worth understanding what you’ve actually got. The airing‑cupboard plug or switched spur may be:

the only way your home heats water (common in all‑electric flats)
a backup to your gas or oil boiler
a “boost” heater set to run during cheap off‑peak hours (Economy 7/10)

If you rely entirely on electricity for hot water, the immersion heater is doing the heavy lifting. Turning it off without a plan just means cold showers. In that case, the aim is to control when it runs, not to cut it out completely.

There’s also a safety angle. Hot‑water systems are usually designed to reach at least 60°C once a week to reduce the risk of bacteria such as legionella. If you dramatically change how you heat water, especially in older systems, it’s wise to:

keep regular full‑heat cycles
speak to a qualified plumber if you’re unsure how your system works

Save money by using it smarter, not by guessing and hoping.

If you have a combi boiler with no hot water cylinder, this article doesn’t apply. No cylinder generally means no immersion, so the mystery plug in the cupboard might be feeding something else entirely.

How to spot and tame the airing-cupboard plug

Most people inherit their hot‑water system from a previous owner or landlord and never get a guided tour. A few calm minutes can change that.

Start with a simple check:

Find the cylinder. Usually a tall, round tank in a cupboard, often with pipes at the top and bottom.
Look for a cable and switch. The immersion’s electrical feed is normally:
- a fused switch with a red rocker and sometimes a neon light, or
- a single socket with a chunky plug and flex disappearing into the cylinder.
Check if it’s on. Is the neon glowing? Is the switch in the “on” position all the time?

If you also have a boiler that clearly heats water for your taps and shower, ask yourself who actually needs that immersion running all day. In many boiler‑plus‑cylinder homes, it’s only meant for emergencies.

You can gently test the setup on a day when hot water demand is low:

turn the immersion off at the switch
leave your normal boiler programme as it is
see if you still get hot water from the taps and shower as usual

If nothing changes, you’ve probably just identified a long‑forgotten backup that’s been quietly costing you money.

Simple settings that save money every month

Once you know what the airing‑cupboard plug does, you have options. They don’t require rewiring the house or buying a new boiler – just a bit of attention.

For most households, one of these three approaches will make sense:

Backup only:
If your boiler already heats the cylinder, leave the immersion off as default. Turn it on only if the boiler fails, and turn it off again afterwards.
Timed on cheap rate:
If you’re on Economy 7/10 or similar, use a timer (often a small clock next to the switch, or a plug‑in timer if it’s a socket) so the immersion only runs during off‑peak hours. One or two well‑chosen heat cycles can cover a day’s hot water.
Short daily boost:
In all‑electric homes without off‑peak tariffs, run the immersion in short bursts: for example, 30–60 minutes before the main shower rush, then off. You’ll quickly learn how long it takes to heat enough for your household.

A few small tweaks make each kWh go further:

Fit or upgrade a cylinder jacket if the tank feels warm to the touch. Extra insulation can slash standing losses.
Check that the thermostat on the cylinder or immersion is not set higher than needed (usually around 60°C is enough for most homes).
Make it a habit to glance at the switch when you grab towels. A two‑second check prevents months of “accidental always‑on”.

A weekly 10‑second glance at one switch can save more than a streaming subscription.

If you rent, it’s still worth asking the landlord or managing agent to explain the system. Many are happy to reduce needless consumption too – it lowers wear on equipment as well as bills.

A quiet corner that deserves more attention

The airing cupboard rarely makes it onto home‑energy checklists. We worry about window seals, draught excluders and tumble dryers, but ignore the small red light that hums away day and night behind the linen.

Treat that cupboard like a tiny plant room. Know which switch does what, when it should be on, and when it can safely be off. Write a small label on the wall if it helps the next person who moves in.

You don’t need apps, spreadsheets or smart meters to cut this particular cost. You just need to remember that not all plugs are equal – and that one forgotten switch can be worth £80 a year.

FAQ:

How do I know if my airing-cupboard switch is an immersion heater?
Look for a cable running from the switch or socket directly into the side of the hot water cylinder. If there’s a neon indicator and the label says “immersion” or shows a little water symbol, that’s a strong clue. When in doubt, a qualified electrician or plumber can confirm.

Is it safe to just turn the immersion off?
If your boiler already heats your water and you only use the immersion as backup, switching it off at the fused spur is normally fine. If the immersion is your only source of hot water, you should use a timer rather than switching it off permanently, and make sure the system still reaches safe temperatures regularly.

Can a smart plug control my immersion heater?
Many immersions draw too much current for standard smart plugs, and they often need a fused spur rather than a plug socket. In most cases, a purpose‑built immersion timer or work by a qualified electrician is safer than a DIY smart plug solution.

What if I have a combi boiler and no cylinder?
Then you probably don’t have an immersion heater at all. Any mysterious airing‑cupboard socket may feed something else (like a pump or extractor), so don’t switch it off permanently without checking what it powers.