How to Stop Cold Air Coming From the Fireplace

Why a cold fireplace leaks winter air

Every chimney is a hole in the house that runs straight to the sky. Between fires, the damper is the main thing separating the room from the outdoors. Most masonry fireplaces in the region's older homes have a cast iron throat damper, a hinged plate just above the firebox. It was designed to control draft during a fire, not to make an airtight seal, and after years of heat cycles and rust many of them barely close at all.

Coal-era fireplaces in brownstones and rowhouses often have no damper of any kind. In those homes the flue is simply open, all winter, every winter.

How air moves through an idle chimney

Two forces are at work, and they can push air either way. When the house is warm and the flue is relatively warm too, heated room air rises up the chimney and escapes, and cold air leaks in somewhere else, around windows and doors. You may not feel that at the fireplace, but you pay for it in heat.

The opposite happens when the chimney itself is cold. A chimney built on an exterior wall is a tall column of brick exposed to winter on three sides. The air inside it cools, becomes heavier than the room air, and sinks down the flue and out through the fireplace opening. That is the steady, cold breath people describe, strongest on still, freezing nights.

The house can also pull air down the flue. Kitchen range hoods, bathroom fans and clothes dryers all push air out of the house, and the replacement air comes in through the easiest opening. In a tight, renovated home, an unsealed fireplace is often that opening. Wind adds gusts on top, especially across an uncapped flue.

Finding where the cold comes from

On a cold, breezy day, with the damper closed, hold the back of your hand or a lit stick of incense close to the damper plate. Air moving through means the damper is leaking. Then check the edges of the firebox and the face of the fireplace, where gaps between the brick and the plaster or the surround can let cold air out of the chimney cavity itself.

Look up with a flashlight. Daylight around the damper plate, a plate that hangs at an angle, rust flakes on the firebox floor or a handle that turns without moving anything all point to a damper that needs work.

• Damper not fully closed, or the handle slipping
• Damper plate rusted, warped, cracked or missing
• No damper at all, common in old coal fireplaces
• Gaps around the surround, the hearth or the mantel
• Air vents or gaps around a gas fireplace or insert
• No cap, letting wind push straight down the flue

Fixing or replacing the damper

The first fix costs nothing. Make sure the damper is fully closed, and clear any soot, debris or rust scale that keeps the plate from seating. Sometimes the handle or linkage has worn and only needs adjusting, or a chunk of fallen mortar is propping the plate open.

When the plate or frame has rusted through or warped, the throat damper can be repaired or replaced, though a new throat damper is still a metal plate on a metal frame and seals only so well. Where the old damper is badly damaged, or the fireplace never had one, a top-sealing damper is often the stronger fix.

Top-sealing dampers

Mounted on the flue at the crown, a top-sealing damper is a spring-loaded lid with a gasket around its edge. A stainless cable runs down the flue to a handle in the firebox. Pull it closed and the whole flue is sealed at the top, so the column of air inside stays close to room temperature rather than filling with cold outdoor air. Many models also work as a cap, with mesh sides that keep out rain and animals when the lid is open.

Because it closes the top, a top-sealing damper also keeps rain, birds and wind out of the flue between fires. It is installed from the roof and has to be sized to the flue, and it only goes on a flue that serves the fireplace alone, not on one shared with a boiler or water heater, since those flues have to stay open.

Glass doors, plugs and covers

Glass fireplace doors cut drafts at the room side. They are not airtight, since the frames have gaps for air during a fire, but combined with a damper that closes properly they make a noticeable difference.

For a fireplace used only a few times a winter, an inflatable or foam flue plug pushed up into the throat stops a lot of air. It has to be removed before any fire, so leave a clear reminder hanging into the firebox. Insulated covers that close the opening from the room do a similar job for fireplaces that are rarely lit.

Gas fireplaces and inserts

Some gas fireplaces and wood or gas inserts have openings that bring in outside air for combustion, or let room air circulate around the firebox. Those openings can let cold air in when the unit is off. Read the manufacturer's instructions before blocking anything, because some openings must stay clear for the appliance to work safely.

Gaps around an insert's surround panel, or a missing seal where the liner meets its top plate, also let cold air through. Those are sealed during a service visit, along with a check of the liner and its cap.

When to call someone

If the damper will not close, the handle is broken, the plate has rusted through, or you feel cold air with the damper shut, the damper needs repair or replacement. The same goes for cold air that comes with a musty smell or water stains, which usually means the flue is also taking rain.

A visit covers the damper, the cap and crown, the firebox and the surround, and the fix is matched to how you use the fireplace: a throat damper repair for a fireplace burned often, a top-sealing damper for one used now and then, or a properly vented seal for one that will stay decorative. You get upfront pricing for the option you choose.