I just purchased a newly built home about six months ago. I have noticed that during the night I have to turn the top floor thermostat on 79 degreess to keep it warm. When the heat shuts off a light draft slowly developes until it becomes conciderably cooler then the heat kicks back on. The first floor termostat is kept around 73 degrees. I have had the builders come out and they say everything is fine but something is clearly not right. What should I look for and what are my options?