High 20s for a few hours is enough to do damage. But, the good news is you only have to protect the lines you don't want damaged.
You ask if a space heater will work, should I leave the furnace on, etc. That depends on where the lines are located at and whether or not the space around them will stay above freezing. How am I supposed to determine all that from 2000 miles away? Can you get heat into all the places the water lines run? what about the pump?
You either have to keep the lines from freezing or protect them. You can protect them by blowing all the water out or displacing the water with antifreeze.
What do the people around you do?