I have forced hot air and my thermostat keeps shutting off so no heat is coming out of vents, thermostat is brand new and battteries were changed. But my gas fireplace works. Was wondering what it could be and if my pipes would freeze with just the fireplace running

Well,not knowing the lay-out of your house,its hard to say.Is the plumbing in the room where the fireplace is? Is the fireplace under the flooring where the plumbing is? Most fireplaces only heat the room where they are and heat rises they don`t do a very good job of heating the floor or underneath the flooring where the plumbing is located