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John Devonald has some tips on a topic rarely considered.

This short article is really one I should have written before winter set in, but I am still getting queries from people about issues they are having with their cooling systems. Here are a few points and tips to, hopefully, help anyone who is unsure as to what it entails.

On most canal boats, the cooling system is a closed loop with a water pump that moves coolant through the engine to cool it and then into the skin-tank to reduce the temperature of the coolant before recirculating it back to the engine. Tap-offs are used to supply the coolant to heat a calorifier in some cases. It is basically the same as a car but with the skin-tank taking the place of the radiator.

What liquid is used in this closed system is important: it must not freeze in cold weather or there could be a catastrophic engine failure from the expansion properties when water freezes – split hoses at a minimum and possibly even a cracked engine block in the very worst case. It has to transfer heat effectively to cool the engine and it also has to inhibit corrosion in the engine passageways. Surprisingly, I haven’t come across ‘special’ boat antifreeze at three times the cost of anything else, but I’m sure someone somewhere must have thought of it!

So, on to what is available and the differences. There are three main types of coolant in use: Inorganic Additive Technology (IAT), Organic Acid Technology (OAT) and Hybrid Organic Acid Technology (HOAT) and the basic rule is ‘never any of them shall mix’ as there is the chance of sludge forming that will inhibit water flow and cooling. I would be very surprised if anyone has HOAT coolant in their boat; it’s relatively new and, the last time I looked, expensive. You cannot totally rely on colours to differentiate between IAT and OAT, but nearly all the time IAT is a blue/green colour and OAT is a red/orange colour. I have to admit I have mixed coolants with nothing to go on except colour in the past but it’s not really recommended.

If you have had your coolant changed during an engine service, you should have details of what was put into your system. The simplest rule is the same as oil; put the same product in to top it up. This eliminates any of the issues that have been highlighted by mixing coolants.

Of course, that is in an ideal world. In my present boat, I have no idea of the brand, just the colour it is – so where do I go from here? The coolant colour in my boat is blue. There is also a half-empty container of IAT coolant in the engine room and my Webasto heating system has blue antifreeze, so I’m sure enough to risk topping-up with that if I need to. If I had any doubts, there is a choice of two things I could do. If I had time I would change the coolant completely. I will do so soon anyway as the IAT (blue) coolant is only good for two years, whereas the OAT (red) is good for five. If I need to top-up in the meantime, then, for a small amount, distilled or deionised water will do. The cooling system on a narrowboat with a skin-tank is pretty big and you aren’t going to dilute it too much from a small top-up. The other way is to use a coolant that is compatible with both. I have used a brand called ‘Prestone’ which says it can be used with any coolant without issue. It’s widely available from Tool Station and Amazon, to name just a couple of sources.

A couple of other points to finish: if you can get it, buy concentrated coolant that you dilute with deionised water to the correct ratio as this will work out a lot cheaper than buying premixed at nigh on the same price per litre. A useful addition to your toolkit is an antifreeze tester. This is a bit like a battery hydrometer and will give you an idea of the protection the coolant is giving. Available for few pounds upwards from Amazon and auto-shops, it will give you a bit of peace of mind over winter.