I remember seeing this episode when I was a child, around 1985, and at that time, it was the scariest thing I had ever seen.
Here is the plot (or at least what I remember of it, so this may not be 100% accurate): one morning, a family wakes up. They soon realize that they are trapped inside the house. Something is preventing them from opening the main door, I believe it was some kind of metallic wall outside the whole house. The father somehow tries to dig under the door (maybe through the floor ?), then realizes that the barrier is also under the house. I remember that the temperature rises toward the end of the episode, while something is slowing filling the fireplace..