On this page
- What an immersion heater actually is
- Why most gas-heated homes still have one fitted
- Using it as a genuine backup when your boiler breaks down
- Running cost: why it is a backup, not a replacement
- Basic checks you can safely do yourself
- When to call a qualified engineer instead
- Keeping it in working order even when you do not need it
- Book with Gasmaster
- Frequently asked questions
Quick answer
An immersion heater is an electric heating element fitted inside a hot water cylinder that heats water independently of the boiler. Most UK homes with a cylinder have one fitted as standard, even where gas is the main heat source, and it is a genuinely useful backup for temporary hot water if the boiler breaks down, though it costs more to run than gas.
Key takeaways
- Immersion heaters are fitted in hot water cylinders, so combi boiler homes without a cylinder usually do not have one.
- They give you a safe, workable backup for hot water while you wait for a boiler repair.
- Electricity typically costs considerably more per unit than gas in the UK, so an immersion heater is a backup, not a replacement.
- A tripped reset button on the thermostat is safe to check yourself; anything electrical near water beyond that is a job for a qualified person.
- Running the immersion heater occasionally, even when you do not need it, helps stop the element and thermostat seizing up.
If your boiler has ever broken down in the middle of winter, you will know how quickly cold showers stop being funny. What a lot of homeowners do not realise is that many of them already have a backup sitting quietly in the airing cupboard, ready to be switched on.
What an immersion heater actually is
An immersion heater is an electric heating element that sits inside a hot water cylinder, submerged directly in the water it heats, hence the name. It is wired to the mains electricity supply and controlled by its own thermostat, completely independent of the boiler and central heating system. You will usually find the switch either on the airing cupboard wall next to the cylinder or, on newer installations, on the cylinder itself.
Why most gas-heated homes still have one fitted
It might seem odd for a gas-heated home to have an electric water heater built in, but it is standard practice in the UK wherever a hot water cylinder is used, which covers system boilers and regular (conventional) boilers. It is fitted as a backup by default during installation, largely for exactly the situation this article is about: giving homeowners a fallback if the boiler is ever out of action. Combi boilers heat water on demand with no cylinder, so if your home has a combi, you will not have an immersion heater to fall back on.
Using it as a genuine backup when your boiler breaks down
If your boiler stops working and you are waiting for an engineer, switching on the immersion heater is a sensible short-term fix. It will heat the water in the cylinder using mains electricity, giving you hot water for washing, showers and washing up while the boiler repair is arranged. Most immersion heaters take roughly an hour or two to heat a full cylinder from cold, so it is worth switching it on as soon as you know you will need it rather than waiting until the last minute.
It is a genuinely useful stopgap, particularly if you are facing a wait for parts or an emergency call-out at a busy time of year, but it should not become a long-term arrangement.
Running cost: why it is a backup, not a replacement
This is the part homeowners most often get caught out by. As a widely quoted rule of thumb in the UK energy market, electricity typically costs considerably more per unit than gas, often several times as much depending on your tariff. That means heating a full cylinder of water via the immersion element regularly, rather than the boiler, can push your electricity bill up noticeably if it becomes your main method of heating water rather than an occasional fallback. Use it to get you through until the boiler is fixed, then switch back once repairs are done.
Basic checks you can safely do yourself
If your immersion heater does not seem to be working, there are a couple of things you can check without any electrical or plumbing knowledge:
- Confirm the switch is actually turned on, sometimes at both the wall switch and an isolator switch nearby
- Check your consumer unit (fuse box) to see if a relevant circuit has tripped
- Look for a small reset button on the thermostat itself, often behind a cover plate, which can trip if the water gets too hot
If none of these resolve it, stop there.
When to call a qualified engineer instead
An immersion heater involves mains electricity submerged in water, and that combination is not one to experiment with. Faults beyond a simple reset, such as a tripping circuit that will not stay reset, water around the electrical connections, or a thermostat that needs replacing, should always be handled by a qualified engineer or electrician. This is not an area where a confident DIY attempt is worth the risk.
Keeping it in working order even when you do not need it
An immersion heater that sits unused for years can seize up, with the thermostat sticking or the element scaling up, particularly in hard water areas like much of Staffordshire. Running it for an hour every month or two, even when your boiler is working perfectly, keeps it ready to actually work as a backup when you need it. It is a small habit that avoids discovering it has failed at the exact moment you needed it.
Book with Gasmaster
If your boiler has broken down and the immersion heater is getting you through, get in touch and we will get an engineer out to diagnose and fix the real problem. Gasmaster is Gas Safe registered (220932), based in Stoke-on-Trent and covering Trentham, Biddulph and Stone. Book a boiler repair or service online and we will have you back on gas-heated hot water as quickly as possible.
Frequently asked questions
Can I use the immersion heater and boiler hot water at the same time?
Generally yes, both heat the same cylinder of water, so using the immersion heater does not interfere with the boiler and either can be used independently or together.
Why is my immersion heater not heating the water at all?
Common causes include a tripped circuit, the switch not being on, or a tripped reset button on the thermostat. If checking these does not fix it, the element or thermostat itself may have failed and needs a qualified engineer.
Is it normal for a combi boiler home to not have an immersion heater?
Yes. Combi boilers heat water directly on demand with no storage cylinder, so there is nothing for an immersion heater to be fitted into. If your boiler ever fails, a combi home does not have this backup option.
How long does it take an immersion heater to heat a full cylinder?
It varies by cylinder size and element wattage, but most UK cylinders take somewhere around one to two hours to heat from cold using the immersion element alone.
Is it expensive to run an immersion heater as a permanent hot water solution?
It can be, since electricity typically costs more per unit than gas in the UK. It works well as a temporary backup but is not usually cost-effective as your everyday hot water method if you have a working gas boiler.
Sources
- Ofgem
- Energy Saving Trust
- Which?
- Gas Safe Register (gassaferegister.co.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
