The furnace only seems to blow warm air every 3rd time it runs. Is this normal or do I have a problem. The temp. outside went to 0F and the frurnace would only heat the house to 56degrees. Even though the thermostat was set on 80degrees. When the outside temp came back up to 30F. The furnace heated the house to 70degrees. Is this all normal or do I have a problem? Also I was just down beside the furnace and watched it run twice with the upper door off and could not see a flame through the sight glass."