Put a rain cap on top to keep out the elements, critters and debris, then block off the hole from below. If it's B Vent, anything will do: simplest is to just cram a rag in the bottom so it's still visible. With masonry, close it up with bricks and mortar, or put debris in it and plaster it over with refractory cement.
You'll have to baste tin on the flue pipe into the hole and spray foam/mortar it first and maybe stick a contrasting color of tile to the mortar, like dark blue glass or something and you'll want to drain the fluids out of the Intruder.
If it is a thimble opening, you can block the opening with solid mineral wool insulation, cut to the size of the opening and fit snugly. This will stop the downward drafting. Add a thimble cap over the insulation and it will look fine. It also will stop cold air from entering the house through the flue.
Sealing the top and bottom of the old flue with a can of expanding polyurethane foam, before closing with a well-fitting metal cap, will stop any warm or cold air infiltration, preventing all issues with condensation and ice build up inside the unused unit.
I have found the best way to seal an unused fireplace flue is to install a piece of slate or thin stone on top of the unused flue tile using masonry caulk to adhere the stone. This way, if the fireplace is ever to be used in the future, the piece of stone can be removed easily without causing any damage to the flue.
You could go for an inflatable draught excluder, which will fit snugly inside your chimney. Just make sure that it has a small vent, so air can pass through. Or you might prefer a draught excluder made for the specific size of your chimney.
Slide the cap over the flue or insert into flue depending on design of cap. Leave at least 6 inches of space between top of cap and flue edge. The cap should come with self-tapping screws. Insert screws into holes on cap and tighten with screwdriver.
Another option when capping a chimney that is no longer in use is to use a disused chimney cap, which is simply a terracotta or metal disk that fits over the top of the chimney pot easily and quickly.
Cut a piece of sheet metal in a rectangular shape slightly narrower than the hole diameter and a few inches taller than the hole. Put a long skinny bolt in the center to use as a handle. Insert the metal and move it until it is flush against the back blocking the hole.
This can be done in a number of ways. But remember, if you block it completely and make it airtight at the bottom, there needs to be an air gap on top of your chimney. You could remove the fireplace and block up the opening with bricks and plaster.
There are many reasons your flue may have become blocked. Blockages can be caused by old birds' nests, dead birds, leaves or general debris. If your home is surrounded by trees, leaves and other debris, such as conkers or pinecones, can fall into your flue.
Suitable materials should adhere to the flue and the building fabric and must set firm. Sand & cement is preferred, but expanding foam, silicone sealant and other similar products are suitable. Non-setting compound is not a suitable material for sealing the annular space.
Place a piece of tall furniture in front of the wall vent to hide it. Alternatively, hang small pictures, awards, or floating shelves in front of the air vent. For a more permanent solution, install a drywall vent cover that matches the design of your home.
Select the correct diameter for the vent hole being sealed. The smaller end will fit in and the plug will go about 1/2 way in. Several firm hits with a hammer will drive the plug in tight. The aluminum or zinc, being soft metals, will form to the hole and create a tight wedge seal.
Use vent deflectors or magnetic covers to redirect airflow or block it completely. You can find these at any home renovation store.
A cap is a simple device that blocks your chimney at the top rather than through the fireplace to prevent air leaks. They are a plastic or metal covers that go over the chimney pot to seal it, capping it off more permanently than hearth covers, inflatable and felted wool flue blockers.
Flex Seal will work on roofs, gutters, skylights, windowsills, flashings, downspouts, foundations, awnings, chimneys, vent pipes, RV's, campers, trailers.
Therefore, the best option would be stainless steel due to its durability, but its costs are a little higher. On the other hand, copper is the most durable and the best option if you prefer aesthetic value. One of the easiest ways to install a chimney cap is using a Chimney C Cap, which regulates heat loss.
If the chimney flue isn't too wide, you should be able to seal it by simply stuffing some wool-type insulation into the opening. This will allow some air to pass through (good ventilation) while preventing uncontrolled loss of warm air.
Purchase a cap that fits the flue, and decide whether you want a temporary or permanent model. Then, slide or insert the cap onto the flue and secure it with a screwdriver. Be sure to avoid over-tightening the screws to prevent chimney damage.