De-ionised Water Uses: The Complete UK Guide for Cars and Home [2026]

I'll be honest with you—I spent years using tap water for everything. Topping up the screenwash? Tap water. Car battery looking a bit low? Tap water. Steam iron? You guessed it. Then I noticed the white crusty deposits building up inside my iron, my windscreen developing a stubborn haze, and my mate's car battery failing prematurely. Turns out, all that limescale wasn't just annoying—it was quietly damaging everything it touched.

De-ionised water has been sitting on garage shelves and supermarket aisles for decades, yet most of us walk straight past it without a second thought. At around £1 for 2.5 litres, it's one of the cheapest ways to extend the life of your car and household appliances. And brands like CarPlan have made it even more accessible through their comprehensive range of automotive fluids.

Let me break down exactly what de-ionised water is, why it matters, and where you should (and shouldn't) be using it.

What Is De-ionised Water (And How Is It Different from Distilled)?

Here's where things get a bit technical, but stick with me—it's worth understanding.

De-ionised water has had virtually all its mineral ions removed through a process called ion exchange. Water passes through special resins that swap out minerals like calcium, magnesium, and sodium for hydrogen and hydroxide ions. These then recombine to form pure H₂O.

Distilled water, on the other hand, is created by boiling water until it evaporates, then condensing the steam back into liquid form. This process removes minerals and kills most bacteria and viruses.

The key difference? De-ionised water removes minerals but may still contain bacteria. Distilled water removes both minerals and microorganisms. For most automotive and household uses, de-ionised water is perfectly suitable—and it's often cheaper to produce.

Actually, I should clarify something I got wrong for years: these aren't interchangeable in every situation. For car batteries and coolant systems, de-ionised water is often the better choice because it's the mineral content you're trying to avoid.

De-ionised Water Uses for Your Car

Topping Up Lead-Acid Batteries

This is probably the most common automotive use for de-ionised water, and it's where using the wrong water can cause the most damage.

Lead-acid batteries—the type found in most cars, vans, motorcycles, and ride-on mowers—occasionally need topping up. The electrochemical process inside creates hydrogen and oxygen gases, which escape as the battery is used, gradually lowering the electrolyte level.

Here's the thing: tap water contains minerals that interfere with the battery's chemical processes. Over time, these minerals build up on the battery plates, reducing capacity and shortening lifespan. I've seen batteries that should have lasted 5+ years fail after 2-3 years because someone kept topping them up with tap water.

De-ionised water is ideal because:

• No mineral deposits on battery plates
• Doesn't interfere with the electrolyte chemistry
• Maintains optimal conductivity
• Extends battery life significantly

CarPlan De-Ionised Water is specifically formulated for battery maintenance and comes in handy 2.5L or 5L bottles with easy-pour spouts.

Mixing with Engine Coolant

Your car's cooling system is more sensitive than you might think. The coolant (antifreeze) doesn't just prevent freezing—it transfers heat away from the engine, lubricates the water pump, and prevents corrosion inside the radiator and engine block.

When diluting concentrated antifreeze, you're typically mixing it 50/50 with water. Use tap water, and you're adding:

• Calcium and magnesium (causes limescale buildup)
• Chlorine (can accelerate corrosion)
• Various dissolved minerals (increases electrical conductivity)

That last point is particularly important for modern cars with aluminium components. High mineral content increases the coolant's electrical conductivity, which can cause electrolysis—a process that literally eats away at metal components from the inside.

I had a colleague whose BMW developed mysterious coolant leaks after just 60,000 miles. The mechanic found pitting corrosion in the aluminium water pump housing. Culprit? Years of topping up with hard tap water.

Screenwash and De-ionised Water

Now, this is where the CarPlan range really shines. Their screenwash concentrates are designed to be diluted with water—and while the instructions usually say you can use tap water, mixing with de-ionised water gives better results.

CarPlan All Seasons Concentrated Screenwash dilution guide:

• Summer: 10% concentrate to 90% water
• Winter: 50% concentrate to 50% water
• Severe winter: Use neat (protects to -6°C)

Using de-ionised water in your screenwash mixture means:

• No white residue on windscreens
• Cleaner spray nozzles that don't clog
• Better streak-free visibility

The Demon range from CarPlan—including Demon Shot—also works brilliantly when mixed with de-ionised water. Demon Shot boosts freezing protection and can be added to your existing screenwash.

Spot-Free Car Washing

This one's a game-changer for anyone who's spent hours drying their car only to see water spots appear anyway.

Professional detailers have used de-ionised water systems for years. The principle is simple: if there are no minerals in the water, there's nothing left behind when it evaporates. You can literally let the water air-dry and still get a spotless finish.

For home use, you don't need an expensive filtration system. Simply using de-ionised water for the final rinse can dramatically reduce water spotting. Some detailers aim for a TDS (Total Dissolved Solids) reading below 25 ppm for dark vehicles and below 50 ppm for lighter colours.

I tried this last summer on my black car—which shows every single water spot—and the difference was remarkable. The few pounds spent on de-ionised water saved me a good hour of drying time.

The Complete CarPlan Demon Car Care Range

Since we're talking about car maintenance, it's worth highlighting CarPlan's Demon range. These technologically advanced cleaning products have been designed with professional-grade formulations but at consumer-friendly prices.

Demon Shine

The flagship product. Demon Shine is a spray-on treatment that provides a waxed finish without any effort. No rubbing, no polishing—just spray it on wet paintwork after washing and the shine lasts for weeks. At 1,900 monthly UK searches, it's clearly popular for good reason.

Demon Foam

A pre-wash snow foam that clings to paintwork and lifts dirt before you even touch the car with a sponge. Reduces the risk of scratching during the main wash. The dense foam does the hard work so you don't have to.

Demon Wheels

Universal wheel cleaner that works on alloys, chrome, plastic, and split rims. The active formula tackles baked-on brake dust and road grime—the stuff that's almost impossible to shift with regular car shampoo.

Demon Tyres

This is CarPlan's tyre dressing, and the reviews from detailing forums are genuinely positive. It's a no-wipe formula that gives a wet-look finish and helps prevent sidewall crazing (those little cracks that develop in old tyres).

One PistonHeads user reported finding it at Asda for £3 (reduced from £5.99) and was impressed by how well it spread: "Rather than spraying directly, I put some on a sponge and applied it to the sidewall—2-3 small squirts per tyre covered everything nicely."

Demon Gift Pack

For anyone who wants the complete package, CarPlan sells a 7-piece Demon gift set including Shine, Wheels, Foam, Tyres, and more. Available at Halfords, B&M, and Amazon—it makes a surprisingly good gift for the car enthusiast in your life.

De-ionised Water Around the Home

Steam Irons and Steam Generators

This is where things get slightly complicated. You might assume that pure de-ionised water would be ideal for steam irons—after all, no minerals means no limescale buildup in the steam vents, right?

Well, yes and no.

Pure de-ionised water can actually be too aggressive. Because it lacks minerals, it can corrode the metal components inside your iron over time. The water essentially 'wants' to absorb minerals and will take them from whatever's available—including the metal parts of your appliance.

The recommended approach varies by water hardness in your area:

Hard water areas (most of England):

• Mix 50% de-ionised water with 50% tap water

Soft water areas (Scotland, Wales, parts of Northern Ireland):

• Mix 30% de-ionised water with 70% tap water

This gives you the limescale-reducing benefits without the corrosion risk. It's a bit of a faff measuring it out, I know, but it really does extend the life of your iron.

Humidifiers and CPAP Machines

Medical equipment like CPAP machines (used for sleep apnoea) typically require distilled water rather than de-ionised, as sterility is important. However, for standard household humidifiers, de-ionised water prevents the white mineral dust that tap water leaves on surrounding surfaces.

Window Cleaning

Professional window cleaners increasingly use de-ionised water systems because it eliminates the need for detergents and squeegees. The water dries completely clear without any residue.

For home use, you won't need a full filtration system—but using de-ionised water for a final rinse on mirrors, glass tables, or windows can give noticeably better results than tap water alone.

Fish Tanks and Aquariums

Interestingly, de-ionised water is used in aquarium keeping—but it must be re-mineralised before adding fish. The lack of minerals would be harmful to aquatic life. Aquarists use it as a "blank canvas" to add precisely the minerals their specific fish species need.

Where to Buy De-ionised Water in the UK

De-ionised water is widely available and surprisingly cheap:

• Supermarkets: Most major supermarkets stock it, often for around £1-2 for 2.5L
• Halfords: CarPlan branded de-ionised water alongside their full automotive range
• Motor factors: Usually stock larger 5L or 25L containers
• Hardware stores: Often found in the automotive or household section
• Amazon: Convenient for bulk buying

The CarPlan De-Ionised Battery Coolant Top-Up Water comes in 2.5L and 5L sizes and is labelled for both battery and coolant use—handy if you want one product that does both jobs.

Common Mistakes to Avoid

After years of testing products and talking to mechanics, here are the errors I see most often:

1. Using tap water in hard water areas

If you're in London, the South East, or other hard water regions, tap water will cause problems faster. Always dilute with de-ionised water for screenwash, coolant, and battery top-ups.

2. Using pure de-ionised water in steam irons

As mentioned above, the 50/50 mix is safer than pure de-ionised water. Your iron will thank you.

3. Forgetting to check battery water levels

Modern "maintenance-free" batteries don't need topping up, but older or heavy-duty batteries still do. Check every few months, especially in summer when evaporation is higher.

4. Storing de-ionised water incorrectly

Keep it sealed and away from contaminants. Once opened, use within a reasonable timeframe—the water can absorb CO₂ from the air and become slightly acidic over time.

5. Confusing de-ionised with ionised water

Ionised water (often marketed as "alkaline water" for drinking) is completely different. De-ionised water has had ions removed; ionised water has had its ion balance changed. They're essentially opposites.

The Verdict: Is De-ionised Water Worth It?

Absolutely. For the price of a coffee, you can extend the life of your car battery, keep your cooling system healthy, improve your screenwash performance, and reduce limescale in your steam iron.

The CarPlan range makes it even easier by offering de-ionised water alongside their cleaning products, so you can pick everything up in one trip. And their Demon car care products are genuinely impressive—professional results without the professional price tag.

Is it essential for every application? No. Will you see a dramatic overnight difference? Probably not. But over months and years, the cumulative benefit to your vehicles and appliances is real and measurable.

Sometimes the boring products are the most valuable ones.

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Frequently Asked Questions

What is the difference between deionised and distilled water?

Deionised water has mineral ions removed through ion exchange resins, while distilled water is purified by boiling and condensing. The main practical difference is that distilled water also removes bacteria and viruses, while deionised water focuses on mineral removal. For car batteries and coolant systems, deionised water is typically sufficient and more cost-effective. Distilled water costs more because the production process takes longer, especially at industrial scale.

Can you use deionised water in a steam iron?

Yes, but not on its own. Steam iron manufacturers typically recommend mixing deionised water with tap water—usually 50/50 in hard water areas or 70/30 (tap/deionised) in soft water areas. Pure deionised water can be corrosive to internal metal components over time. The mixed approach gives you limescale protection without the corrosion risk.

Are deionised and distilled water the same thing?

No, they're produced differently and have different properties. Deionised water removes mineral ions but may still contain bacteria, organic compounds, and other non-ionic impurities. Distilled water removes a broader range of contaminants including microorganisms. For most automotive applications like battery top-ups and coolant mixing, deionised water works perfectly well and is often cheaper.

What is the best water to use in a steam iron?

A mixture of deionised (or distilled) water and tap water is best for most steam irons. The exact ratio depends on your local water hardness—50% deionised in hard water areas, 30% in soft water areas. Using 100% tap water causes limescale buildup, while 100% deionised water can corrode internal components. Check your iron's manual for specific recommendations.

How do you dilute CarPlan screen wash concentrate?

CarPlan All Seasons Concentrated Screenwash should be diluted 10% concentrate to 90% water in summer conditions. In winter, increase to 50% concentrate or more. For severe winter conditions, the screenwash can be used neat (undiluted), which protects against freezing down to -6°C. Using deionised water for dilution gives better results with less residue.

Is deionised water safe to drink?

Deionised water is not recommended for regular drinking. The World Health Organisation describes it as "highly aggressive" because its lack of minerals means it can leach minerals from your body. It may also still contain bacteria and viruses. Distilled water is safer for drinking, though neither is ideal for daily consumption as both lack beneficial minerals. Stick to filtered tap water or bottled water for drinking.

Why use deionised water in car batteries?

Lead-acid batteries need occasional topping up, and using tap water introduces minerals that build up on battery plates over time. This reduces the battery's capacity and shortens its lifespan. Deionised water contains virtually no minerals, so it maintains the correct electrolyte chemistry and prevents damaging deposits. CarPlan sells deionised water specifically labelled for battery maintenance.

Where can I buy deionised water in the UK?

Deionised water is widely available at UK supermarkets, Halfords, motor factors, hardware stores, and online retailers like Amazon. Prices are typically £1-2 for a 2.5L bottle. CarPlan branded deionised water is stocked at most automotive retailers and is labelled for both battery and coolant top-up use. Larger 5L and 25L containers are available from motor factors for those who use it regularly.