Furnace blows hot air for about 3-4 min. (burners all firing nicely) then burners go out but air continues to blow (cold) for 3-4 min. All registers are open and the pressure is VERY strong (small house - 6 ports) cycle repeats about every 10 min. or so. Also, what would the black plastic bucket full of water (20gal. Or so and half full) and connected to furnace be? Appreciate any info. Thanks