I gave an old home and the fireplace on the outside shows cracks and shifted brick. I tried to use the fireplace and it was fine but eventually smokes started coming in the house. I know there are cracks but I'm looking for an inexpensive way to seal cracks on the inside so I can use the fireplace as I can't afford to fix it properly. Yes, the vent is open - I checked. I'm worried about using a spray sealant and actually really messing up my fireplace. Any suggestions?