Chimney Balloons and Draft Stoppers: Do They Work?

Why an idle chimney makes a room cold

Think of an open chimney as a tall tube connecting your living room to the sky. In winter, the warm air in the house is lighter than the cold air outside, and it rises and escapes up any open path, the same stack effect that makes a chimney draw when a fire is lit. Cold outdoor air then falls down the flue or leaks in elsewhere to replace it.

A good throat damper slows that exchange, but many in older houses do not seal at all. Cast-iron dampers in older brick houses rust, warp, lose their handles or get stuck half open. Some were removed years ago to make room for gas logs. Behind a fireplace that has been cold for a decade, the result is a steady draft at ankle height and, on windy days, a faint smell of soot drifting into the room.

Balloons, pillows, plugs and top dampers compared

All of these products block the flue at some point below the top. They differ in where they sit, how well they adapt to an odd opening, and how easy they are to take out.

Measuring and fitting a plug

A plug only works if it fills the opening. With the damper open, measure the throat or the flue just above it in both directions, and look at the shape. Many old fireplaces have a long, narrow throat or an irregular damper frame, and a few have a smoke shelf that changes the shape a few inches up. A balloon that is too small slides upward or lets air pass around it. One that is too large will not seal and may tear on rough mortar.

Brush loose soot off the area first, wear eye protection and gloves, and use a flashlight to see what you are pressing against. Do not push anything past a broken or missing damper into a flue you cannot see. If tile pieces, bricks or nesting material come down while you work, stop and have the chimney looked at before fitting anything.

When a plug is the wrong tool

Lighting a fire under a balloon or pillow can melt or burn it and fill the room with smoke. Even a brief fire of paper and kindling is enough. That risk is the reason most makers supply a warning tag to hang in the firebox or on the damper handle.

The bigger hazard is a shared flue. In plenty of pre-war houses, a boiler, water heater or furnace vents into the same chimney as a fireplace, sometimes into the same flue. Plugging that flue can block exhaust from an appliance that runs automatically, day and night, and send carbon monoxide into the house. The same goes for a fireplace with gas logs or a standing pilot. If you are not sure what else uses the chimney, find out before plugging anything. Keep CO alarms working on each floor; an alarm, or anyone feeling ill, means leaving the house first and phoning 911 or the gas company from outside.

Moisture and smells behind a blocked flue

A plug stops air from the room, not water from the sky. If the chimney has no cap, rain and snow still fall down the flue and can collect on top of the balloon or pillow, along with leaves and soot. Over a winter that can mean a wet, sour-smelling plug and staining on the firebox walls when it comes out. After a wind-driven nor'easter rain, it is worth checking.

Old creosote in an unswept flue can also smell stronger in humid weather. A plug often reduces that smell in the room, but it does not remove the source. If a smoky or musty odor is the main reason you want a plug, a sweep and a cap may solve more of the problem.

Drafts a chimney plug will not fix

Not every cold draft near a fireplace comes down the flue. Air also leaks through gaps where the hearth meets the floor, around the mantel and surround, through an ash dump door in the firebox floor and the cleanout door in the basement, and through glass doors with worn or missing gaskets. An exterior chimney can also leave the chimney breast itself feeling cold, which a plug does nothing about.

A simple test on a windy day helps tell them apart. Hold the back of your hand, or a thin strip of tissue paper, near each gap. If the air moves at the throat, the flue is the source. If it moves at the hearth edge or around the surround, a plug will not help, and sealing those gaps will.

Living with a plug through the winter

A plug works well as part of a small routine that everyone in the house knows about:

• Hang the warning tag where anyone reaching for matches will see it
• Tell guests, sitters and house cleaners the fireplace is blocked
• Check the plug after storms and cold snaps for slipping or deflation
• Look for soot, damp or debris piled on top when you remove it
• Take it out and store it before any fire, then refit it once the ashes are cold

Fixes that last longer than a plug

If you keep reaching for a plug, the damper is the real issue. A rusted or warped throat damper can often be repaired or replaced from inside the firebox. A top-sealing damper seals the flue at its top with a gasketed lid operated by a cable down to the firebox. It also keeps rain, birds and squirrels out, and it keeps the flue warmer, which helps the fireplace draft better when you do light it.

Glass doors with good gaskets across the fireplace opening are another option for a fireplace you still use. They cut the air loss when the fireplace is cold, and closed doors limit how much heated room air goes up the flue as a fire dies down. Follow the fireplace and door instructions on whether the doors should be open or closed while a fire burns.

For a fireplace you will not use again, the chimney can be capped and the fireplace closed off properly, with the flue still ventilated so moisture does not build up inside. Which option fits depends on the flue size, the damper that is there now and whether anything else vents into the chimney. We can look at it and tell you which suits your house.