What is the minimum temperature for house heating system to keep pipes from freezing
This depends on the particular house - both how the plumbing is laid out (are water pipes located in exterior walls, or not?) and the relationship between heat delivery and plumbing. 

It may also depend on the heating system - i.e. hot water baseboards often have pipes that run in poorly insulated spaces at the edges of the house, and can themselves freeze if the house is too cool for too long.

Typical recommendations range from 45 to 55 F

One solution to avoid spending much on heating for a non-occupied house is to drain the plumbing and put non-toxic antifreeze in the traps so that freezing won't bother the pipes.