Old unused fireplaces are a source of cold air in the home. The vents are necessary to provide air flow into a fireplace for proper combustion of the wood. The vents serve no function when a fireplace isn't being used. They may be covered as part of your energy-saving plan for your home.
Measure the length and width of your vent hole and add 1 to 2 inches to each side of the measurement. Write this modified measurement on a piece of paper.
Purchase two or more sheets of metal tin. Using a carpenters square, scratch out the rectangular area of the same measurements you wrote down in Step 1. Cut the rectangular cover out of the tin sheet with a pair of tin snips.
Apply a heavy bead of caulk around the edge of the vent you desire to cover. If you only desire a temporary seasonal cover, use foam window sealing strips instead. Place the tin over the top of the your chosen seal and hold it in position.
Drill holes 1/4-inch from the edges of the tin. Drill one hole in each corner and one hole between the corner holes on each side, drilling all holes with a properly sized bit that matches the size of masonry screws you will use to mount the panel. While drilling, squeeze lightweight machine oil onto the bit to prevent overheating and damage to the bit.
Screw the panel down tightly over top of the underlying seal with your masonry screws, creating a tight seal to prevent moisture and air leakage.
Metal sheet tin should always be used for your vent covers, as it is lightweight and contains a smooth surface which seals well. Alternatively, you may use plastic; keep in mind that plastic expands and contracts according to the varying air temperatures coming into contact with it through the vent. This may cause the plastic to pull from the screws, tearing the plastic. If your budget allows for using commercially fabricated vent covers, feel free to use decorative or glass covers. Consider covering the outlet of your vent on the exterior of your home by screwing down screen-door screen or another piece of your sheet tin over the opening to prevent insects and animals from building nests in the vent pipe. You may use standard wood screws for this purpose. Apply a bead of caulk around each screw head to prevent water from leaking through.
Never seal fireplace vents if you intend to use the fireplace. Doing so will cause improper combustion of the burning wood and may cause carbon monoxide poisoning within your home which can be fatal. If you desire only to block your fireplace vents temporarily during warmer weather, use an inflatable polyurethane plug in your chimney and vent systems.