Freezing, well, you could run your circulating pumps full time when your boiler is off. Better protection would be glycol, but expensive. My neighbor ran out of wood this year, so it does happen, he did say, because of the total mass of heating medium, the boiler and the system held heat for several days after he ran out of wood. The outdoor boiler is a secondary heat source for him, so, really no inconvenience, however, the cost of wood fuel is beginning to set in. Weil Mclain HE boilers, one of my favorites!!!!