Buying guide
Wool pad or inflatable?
A wool pad is a thick felted block that you push into the opening of the flue, where it is held by its own shape. It is breathable, so moisture can still escape, and it is the better long-term choice for a chimney you do not use. An inflatable balloon is useful when the opening above the fireplace narrows then widens, because it can be pushed through a tight throat and inflated to fill a larger space behind it.
Either way, the chimney is not sealed solid. A chimney needs some air movement to stay dry, so choose a product designed to block the draught without trapping moisture.
Measuring a chimney opening
Measure the opening from front to back and side to side at the narrowest point you can reach, and write both numbers down. Sizes are given in inches in this range, such as 9 x 16 or 13 x 13, and the correct product is just a little larger than the opening so that it is snug. A round flue is measured by its diameter.
Getting the size right is the difference between a good seal and a pad that drops out of place, so measure twice.
Safety: never light a fire with it in
Take the excluder out before you light a fire, stove or any other appliance that uses the chimney, and use the supplied reminder tag where there is one. If your chimney serves a gas fire, boiler or other fuel-burning appliance, do not block it, and ask a qualified engineer if you are unsure. Also remember that some rooms need permanent ventilation for appliances, so never block an air vent to save a little heat.