Coolant — most drivers still call it antifreeze — does three jobs at once. It carries heat away from the engine, it stops the cooling system freezing on a sharp Pennine frost, and it protects the inside of your radiator, heater matrix, water pump and cylinder head from corrosion. That third job is the one people forget, and it is the one that empties your wallet. A cooling system is a patchwork of aluminium, cast iron, copper, brass, steel and rubber seals. Left unprotected, coolant turns acidic and begins eating those metals from the inside out. The result is a weeping radiator, a noisy water pump, a heater that blows cold, and — after one bad overheating episode on the M1 or the climb up to Ringinglow — a warped cylinder head.

Know Your Coolant Chemistry

Coolant is not one product. There are three families of chemistry in common use, and they do not play nicely together.

• IAT (Inorganic Acid Technology) — the traditional green or blue coolant. It uses silicates and phosphates that coat metal surfaces almost immediately, which makes it very effective at protecting older cast-iron engines and copper-brass radiators. The downside is that the protective layer depletes, so IAT coolants typically need changing every two to three years.
• OAT (Organic Acid Technology) — usually pink, red, orange or purple. Instead of coating surfaces, OAT additives work chemically to inhibit corrosion, and they last far longer — often five to ten years or 100,000 miles. OAT is what most modern cars use, but it is not kind to older systems with copper and brass, and it can attack some silicone gaskets.
• HOAT (Hybrid Organic Acid Technology) — a halfway house, blending a small amount of silicate with organic additives. Common in vehicles from the mid-2000s onwards, it offers long life with quicker initial protection.

Colour Is Not a Specification

This is the single most expensive misunderstanding in DIY car care. There is no legal or industry-wide colour code. One maker's pink is another maker's red, and a green coolant on the shelf may be an IAT, a HOAT, or something entirely different. Buying by colour is guesswork. Buying by specification is not.

The specification is printed on the bottle — often a manufacturer standard, sometimes a British Standard such as BS 6580 for traditional IAT coolant. If the bottle does not state the standard clearly, put it back on the shelf. Vague claims like "suitable for all vehicles" or "universal" are a warning sign, because no single coolant genuinely suits every engine ever built.

What Your Handbook Actually Says

Your owner's handbook is the authority here, and it takes thirty seconds to check. Look for a coolant specification in the servicing or capacities section — something like "use only coolant meeting specification X" or "silicate-free OAT coolant required". Note the total system capacity too, because you will need it if you ever do a full change.

If the handbook has long since vanished from the glovebox, a dealer parts desk can confirm the correct specification from your registration number, and most motor factors can do the same. Buying a litre of the right stuff costs far less than a new radiator.

Mixing, Topping Up and Local Conditions

Coolants of different chemistries react with each other. Mix IAT with OAT and the silicates can drop out of suspension, forming a gritty sludge that clogs the heater matrix and blocks narrow radiator cores. If you only need a small top-up and you genuinely cannot identify what is already in the system, topping up with deionised or distilled water is the safer short-term fix — then get the system properly flushed and refilled.

Sheffield tap water is unusually soft, drawn largely from Peak District reservoirs, so it is kinder than hard-water areas when used for dilution. Even so, deionised water is the better habit: it contains no dissolved minerals to scale up inside the radiator.

On mix ratios, a 50:50 mix of concentrate and water gives protection down to roughly -37°C, which is more than enough for South Yorkshire. Premixed coolant is already diluted and is the easiest option for most drivers — just do not dilute it further. Check the level only when the engine is cold, and never open a hot expansion cap.

Buying Advice and Change Intervals

When you buy, match the specification, not the colour. Prefer premixed for top-ups and concentrate if you plan a full change, so you can control the ratio. Look for a clear date or batch marking, and store it sealed and out of sunlight.

Change intervals matter as much as chemistry. Traditional IAT coolant should be renewed every two to three years. Long-life OAT and HOAT coolants typically last five to ten years, but that assumes the system has never been topped up with the wrong product or run low. If your coolant looks rusty, cloudy, or has an oily film on top, have it checked — that is a symptom, not a cosmetic issue.

Finally, keep an eye on the level itself. A cooling system that needs topping up regularly has a leak somewhere, whether it is a perished hose, a tired pressure cap or a failing water pump. Catching it early on a cold Sunday morning in the driveway is far cheaper than catching it on the Sheffield Parkway with the temperature gauge climbing.