We bought this house 5 years ago and from the beginning every spring at the end of March/beginning of April our oil furnace is blowing cold air at night only, fan running constantly whole night. Everything works fine during the day and around midnight cold air is starting to be pushed even tough temperature is set to 72 F. Fan is running constantly blowing cold air and around 6 am temperature inside goes to 65 F and after 6/7 am it starts to blow hot air again. This happen every year for about a month. We have no problems with A/C during summer or heat during winter.