[SIKSUKR]

You did exactly the right thing short of the electic resistence mentioned by Brk.Here's the rub though.If this is the first time that you have had this zone freeze by turning down the temp than you have another thing to think about.You mentioned the pipes are located in the wall.This would make them closer to the outside cold air.In the time it took for your space to cool down to the set temp,that pipe froze.That's pretty fast which tells me that zone is doing a lot of heating to the outside as well as your house.It may well be worth your effort in fuel savings along with eliminating this potential problem to get at those pipes and either move them inside the heated space or insulate them.