Anyone making a recommendation is going to be wrong - sooner or later
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Newbies reading this and not knowing the location, weather, etc might still be clueless and possibly make mistakes. You don't say whether your engine is in a frost-free environment. Furthermore there is no detail as to type of cooling system. No mention of operation cycles either. 24/7 running may onlyneed frost protection when the engine is off through failure or service - and if in a frost-risk environment.
Soft water, inhibitor and a bucket may be ideal for occasional use. For a thermosyphon system you could drain the head and block into the bucket and return the bucket contents to the cooling tank as appropriate. OK until you forget
Radiator system - tghtwad if you will not invest in the small amont of antifreexe needed. It will have inhibitors in it. How much protection do you need? -10 C freeze is probably as much as (or more than?) you will ever need in an enclosed building in the southern UK unless you are going away for an extended period (the freeze-solids will not damage the castings for several more degrees of frost).
CHP system - use a heat exchanger and the same applies as radiator.
These engines ran OK for years without all the new fangled additives. Simple common sense prevailed. They de-scaled as required, used rainwater as coolant top-up and drained them down as necessary.
So my advice is: Design the system around your requirements to be safe and frost-damage free. Replacing cracked blocks is a d--n sight more expensive than buying some antifreeze.
There are various frost protection systems (frost-stats and appropriate heating elements, circulation systems, etc) to counter freezing conditions. Depends whether you are on the grid or not, of course. This is not apparent, or otherwise, from your posting so may be irrelevant.
Like I said a bit earlier, look at your system and do it right. That way you will have only yourself to blame when it goes wrong.
RAB
