If your fireplace does not have a damper, or if the one it has is inefficient, one way to ensure the fireplace will not cost you far more heat than it ever will produce is to install an after-market damper. Even then, you will lose heat when the damper is open. The City of Phoenix (Arizona) official website points out on its fire safety page that even when a fire is blazing, you will lose as much of your home's heat from a fireplace opening as you would from the same size hole in your wall.
Two basic types of damper are made for aftermarket applications. A throat damper is essentially a balloon that inflates just above the hearth to seal off the chimney. A chimney-top damper attaches to the outside top of your chimney and is operated at the hearth. Which damper will work best for you will depend on how much you plan to use your fireplace.
Decide if you want to use a throat-seal or top-mount damper. If you decide to use a top-mount damper, check your chimney top for damage. Inspect it for cracks and missing bricks or mortar. Repair if necessary. If you have chosen to use the throat-seal damper, skip to Step 7.
Place the ladder against the house in a secure position. Put on the safety harness, place the cleaning rag in the damper kit box and securely tie one end of the rope to the box. Run the other end of the rope through a strap in the safety harness without actually tying it.
Climb onto the roof. Be sure to have an assistant on hand during the climb in case you should fall and to make sure the rope does not get tangled or caught while you are climbing. Once you are safely on the roof, pull the damper kit up and tie the rope to the safety harness and around the chimney immediately, with no more than 3 feet of slack. If you are not absolutely certain your chimney is sound, however, do not work around it.
Attach the control cable or chain to the damper and drop it down the chimney. It may help to have your assistant tell you if the cable has reached the fireplace. Clean the top of the chimney liner with the cleaning rag and apply the adhesive. Set the damper in place according the manufacturer's instructions.
Attach the control-cable mounting bracket to the firebox, using the electric drill and the masonry bit. Run the cable though the bracket and attach the handle.
Test the damper to be sure it will open and close properly.
To install a throat-seal damper, place it in the chimney opening just above the flue and inflate it according to the instructions. Check to be sure the removal-reminder banner is readily visible.
Consider adding a substantially airtight see-through fire screen or fire door to the front of your fireplace. You will be able to close it to prevent much of the heat loss that can occur when the damper has to be left open while the fire is cooling.
If you are not absolutely certain your chimney is sound, get it inspected and repaired. Do not climb onto the roof to work around it. If you have any issues with heights, either find someone else to install the top-mount damper kit or use the throat-seal damper. If you choose to use the throat-seal damper, remember to remove it before you start a fire.