How Cold Does It Have To Be Forto Crack Block How Can I Stop Cold Air From Coming In Through My Forced Air Vents That Are Behind My Fireplace Hearth?

How can I stop cold air from coming in through my forced air vents that are behind my fireplace hearth? - how cold does it have to be forto crack block

Two holes in my 6-leakage of air in my family room are from my fireplace at home. It seems that the management of the work first, then firplace was built over it. Thus, the left, the 3x8 rectangles sideways on each side of the chimney for ventilation. But this room is always cold and it was until today, I realized that the coal is in the air by two holes. But the holes in the other in this room are very good. This piece was made a aiddtion to my home. I have no idea when. However, an air conditioner has separated from the house.