But your trying to crack a nut with a sledgehammer!
So your anti-freeze idea is to protect against those breakdowns of boiler stat failure or pump failure (where your heating is broken down) on those 'how many nights a year' that there is a potential for the system to freeze, and at what cost....200+ euro every 4/5 years??????
Against one or two stats (at a 30 euro cost) that are there to protect the system, at a very low cost of fuel that this would take (it's not like that system will kick in for long to prevent a freeze) and will only use the protection when it is required (on how many nights a year?) when the system
is not already in use
After all, the fact that all decent outside oil modules have frost stats fitted as standard, and practically every gas boiler manufacturer now has frost protection as an on board built-in feature, but hey, all us designers must be wrong eh! We have just worked in the industry for years, looking at all the data, considering failures, breakdowns, ect and attempting to design them out.
If anti-freeze was a viable consideration do you not think this would already be considered a standard practice, as with inhibiting system water, chemical flushing etc.
There is also further considerations, the chemicals you are touting, have not been tested in conjunction with any appliances to my knowledge, so the materials used, from metal, to seals, to plastics etc maybe effected in what manner? If a gasket breaks down as a result an floods a house, or an entire apartment block who is liable?
Lets say there is a failure in the dhw cylinder coil or a non-return valve sticks open, the there is contamination into the domestic water, how would person (or baby/child) be at risk from the anti-freeze?
The method i propose here, is considered an acceptable standard that provides a reasonable means for protection against a domestic heating system freezing in our winter.............the choice is yours