Andy Hall wrote:
As a sanity check in a typical house you will be between 50 and
150litres.

Work out the Alphi-11 on that basis. It doesn't matter too much if
you go to higher concentration than 25%. I have mine at about 35%.

Why not drain off a rough 25% - say 25L if Andy is right - add this amount
of antifreeze to the header tank and run the system for a few months until
it's thoroughly mixed. (It's not going to freeze over the summer...).

Then take a small sample from the drain point and put it in the fridge
ice-box or freezer.It shouldn't freeze at all in the former, and should stay
liquid or at worst slushy in the latter.

If this antifreeze is glycol- based you might also get a good idea of its
freezing point using a hygrometer designed for car radiators.


--
LSR