A 25% mix (verified with a refractometer in experienced hands) will protect from "burst" down to zero Fahrenheit or so. If you plan on shutting down the heat system and/or pumps here in frosty Minnesota you will want a 30% solution affording you burst protection down to -20°F, our mean extreme.

Push it up a little and you can melt snow!

Comparison of freeze protection vs. burst protection for different volumes of Propylene Glycol http://www.solar365.com/sites/default/files/field/insertable_image/propylene_glycol_mix_percentages.png