metal heatform to brick/masonry seal

I have a superior metal heatform in my fireplace. I have the hot air outlet connected to my house a/c-heater ducts with a blower to blow the hot air through the ducts. I can't use it because the metal firebox is not sealed to the brick facing and air blows into the firebox causing smoke to be blown into the room. I've tried wood burning stove caulk but it wont adhear to the brick, does anyone have any ideas as to what to use. Thanks for any help