furnace blower at night constantly runs?

My parents have a central boiler. During the day it runs ok. At night it runs constantly all night. We know that it colder at night, but my sister and brother in laws doesn't do the that.
My father had it installed but it runs uphill to the house coming into the basement. He ran all copper tubing. He has a garage heater too and we noticed its ran the opposite way though it. I dont know if that would cause the problem.
Just thinking if the tube bundle is too small for the furnace/house. We are really baffled on this!