What to Do With a Chimney You No Longer Use

Check what still uses it

Before anything else, find out whether the chimney is truly unused. A boiler or furnace may have been replaced with a unit that vents through the wall, but the gas water heater may still be piped into the old flue. A fireplace upstairs may share the stack with a flue serving the basement. In attached houses, a neighbor's flue can run in the same stack. Closing a flue that still carries exhaust can send carbon monoxide into a home.

Look in the basement for any pipe running into the chimney, follow it to its appliance, and look at the roof from the ground to count the flue tops. If the count of flues does not match what you know is in the house, ask before closing anything.

Why an unused chimney still needs care

A chimney in use is warmed and dried every time the appliance or fireplace runs. An unused chimney is not. It stays cold and damp all winter, rain soaking into the top dries slowly, and freeze-thaw cycles work on the crown and mortar without any warmth to help. Open flues collect rain, leaves and nests; the damper rusts; and moisture trapped in the flue can come through the plaster as damp patches and stains.

That is why a forgotten chimney is often in worse condition than one used every day, and why doing nothing is rarely the cheapest option over time.

Option one: keep it ready

If you might use the fireplace again, or sell to someone who will, keep the chimney sound. That means a cap on each flue, a crown that sheds water, flashing that seals, mortar that is intact and a damper that closes. A yearly look still makes sense, since NFPA 211 calls for yearly inspection of chimneys and the crown, flashing and mortar wear whether or not the flue is used.

Keeping a chimney in shape is far easier than bringing back one that sat open for years of weather. A working fireplace is also a feature many buyers look for in older homes.

Option two: cap and close it

For a chimney you will not use, a full-width cover over the top keeps rain, birds and squirrels out. Many covers for abandoned flues have vented sides or a small opening so a little air still moves through. Sealing a flue tight at both the top and the bottom can trap moisture in the masonry, which shows up as damp patches and peeling paint on the chimney breast.

Inside, the fireplace can be closed with a fitted panel, ideally with a small vent, or with a sealing damper, and the basement cleanout door kept shut. Old connector openings in the basement are closed with masonry or a proper thimble cover, not a pie plate or tape.

• Unused chimney cap or cover sized to the whole crown or to each flue
• Crown sealed or repaired so water does not soak in from the top
• Flashing checked, since the roof joint still leaks if it fails
• Fireplace opening closed inside, damper shut or flue plugged
• Old pipe openings closed properly with masonry or a fitted cover

Option three: take it down

A chimney that serves nothing and keeps leaking or crumbling can be taken down, either just below the roof or further. The roof deck and roofing are then closed over so no trace of the opening is left to leak. Removing the stack inside the house as well is a much bigger job that involves floors, walls and possibly structural changes, and is usually reserved for renovations.

Removal makes more sense when the stack needs major repair and nobody wants a fireplace. Shared stacks need the neighbor's agreement, and permit rules vary, so check locally before any demolition.

• Keep it ready if you may use it again or want it for resale: cap, crown, flashing, damper and a yearly check
• Cap and close it if there are no plans to use it and it is sound: vented cover, crown repair, sealed openings
• Take it down if it is unused and needs major repair: removal below the roof, roof closed over, permits as required

Rowhouses and shared stacks

In rowhouses, brownstones and twins, the chimney you no longer use may be part of a stack that still serves the house next door. Capping or sealing the wrong flue can block a neighbor's boiler exhaust, and taking down your half of the stack can weaken what remains or expose the neighbor's flues to weather.

Before closing or removing anything, each flue in the stack is traced to its fireplace or appliance, and the plan is agreed with the neighbor. A full-width cap that covers every flue, with each one still free to vent, is often the simplest way to protect a shared stack when only some of its flues are in use.

Using an unused chimney as a chase

Sometimes an unused chimney finds a new job. Some high-efficiency boilers and water heaters allow their plastic vent and intake pipes to run up an unused chimney instead of exiting the wall, if the maker's instructions permit it. Cable, plumbing or a new liner for a different appliance can also use the space. Each use has its own requirements, so plan it before the chimney is closed or removed.

What an assessment covers

A technician confirms what each flue serves, checks the crown, cap, flashing and upper brick from the roof, looks at the damper and firebox, and opens the basement cleanout. The answer is a recommendation for one of the three options, with the work for each listed so you can compare. Expect it in writing, along with anything urgent, such as loose brick at the top.

If you are planning a roof replacement, schedule the chimney decision first. Removing a stack or rebuilding its flashing is far simpler while the roof is open.

Should you cap an unused chimney?

In most cases, yes. An open flue is a funnel for rain, which rusts the damper, soaks the brick and can lead to damp inside, and it invites animals to nest. A cap or cover is one of the simplest ways to protect a chimney you are not using, and it is often the first step whatever you decide later.

Choose a cap made for the purpose: stainless or copper, sized to the flue or the whole crown, and fastened so wind and animals cannot lift it.