frost

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when temperature drops around 20 deg.and lower ,i get frost in some spots of my basement walls. they are of concrete ,some block in the entrance.can't find any cracks . it seems to start at ground leavel and go up,distance from ground to sill is 24" .no shrubs near foundation, basement has 7' ceiling ,furnace in basement,w/extra ventless on inside wall.sump pit is open bottom 24"deep.no other water problems.where do i look ?