Electrolyzers of this type have been operated in Canada and Alaska for over a decade with absolutely no issue about freezing.
Think of the electrolyzer kind of like a reverse battery. Your battery doesn’t freeze because the water is mixed with electrolyte. It’s the same with the HyZor, the water is mixed with electrolyte and will not freeze even below – 40°C.
That said, electrolyzers (like batteries) become very inefficient when cold. So it’s a good idea to heat them. The HyZor is designed to operate efficiently at engine coolant temperatures and we have (in our HyZor Resources and in the HyZor Technology book) full instructions on how to heat the HyZor.