Heat System wont hold Pressure
2 years ago i built a house on a slab and installed 1/2" pex (******* brand, if thats what it is, i dont recall) tubing to specification, per my plumber friend, in my slab. As a cheap heat system i bought a residentual hot water heater and he helped me with pumps, controls etc. it seemed to work fine. last winter i noticed i had to add water alot. now this winter i go to start it up and i cant get it to fill up and hold pressure. i run it up to 30# and come back in 10 minutes and it goes to zero. i valve off the system supply lines and it holds. i have straight water in the system. its not cold enough yet for them to be frozen. i'm afraid to ask this question...didnt sleep at all last night....but is it possible that some where in one of my runs a line froze last winter and burst and now i cant fill the system???? if so, now what am i going to do??