By Johnathan Mason, Ace Chimney Service.
Older houses often have chimneys serving appliances that no longer exist — a coal stove, a wood range in the kitchen, an oil burner, a second-floor parlour stove. The appliance left decades ago. The flue, and the round opening in the wall where the pipe entered, frequently stayed. Those openings are called thimbles, and they are one of the more overlooked hazards in older Marengo homes.
Why an unused flue is not a harmless flue
An abandoned flue still collects water if the top is open, still deteriorates internally, and can still be shared — sometimes unknowingly — with a live appliance in the same stack. Old chimneys frequently contain several flues in one structure, and if the dividing masonry between them has failed, exhaust from a working furnace can cross into the abandoned one and find its way back into the house.
The thimble problem
A thimble is the sleeve where a stove pipe passed through the wall into the chimney. When the appliance was removed, these were often covered with a decorative metal plate, a piece of tin, wallpaper, or simply plastered over. None of that is a proper closure. A covered thimble can hide an open path between a live flue and the room, and a plastered-over one can conceal deterioration nobody has looked at since the Truman administration.
How to close one properly
An unused thimble should be sealed with masonry — brick and mortar filling the opening — not a plate, not foam, not drywall. If the flue itself is being permanently abandoned it should be sealed at the top as well, while still allowing the masonry to breathe, and the top protected so water stays out of the stack.
What we look for in older stacks
How many flues are actually in the chimney and what each one serves. Whether the dividing masonry between them is intact. Whether an old flue is still connected to anything. Water damage from decades of an uncapped opening. Nesting material and debris. Deteriorated tile, or in the oldest chimneys, no tile at all — unlined brick, which was standard construction long ago and is not acceptable for a modern appliance. That is where relining usually enters the conversation.
The case for looking now
If you have bought an older house, or you have lived in one for years and never had the full chimney examined rather than just the fireplace, this is the sort of thing a proper chimney inspection exists to find. It is not alarming work — it is usually a straightforward masonry closure and peace of mind.
Ace Chimney Service has 29 years in the trade, is insured and bonded, and gives the price up front. Seven days a week, 7 AM to 7 PM, serving Crystal Lake and McHenry County. Call (815) 320-0883.
Frequently asked questions
What is a chimney thimble?
The sleeved opening in the chimney wall where a stove or furnace pipe connected. Many remain after the appliance is long gone.
Is a metal cover over a thimble acceptable?
No. A permanently unused thimble should be filled with masonry, not covered with a plate or plastered over.
Can an unused flue affect a working one?
Yes, if the masonry dividing them has deteriorated. Gases can cross between flues in the same stack.
Should an abandoned chimney be capped?
Sealing the top and protecting the crown keeps water out, which is what destroys a disused stack. It should still be able to breathe.
My old chimney has no clay tile — is that a problem?
Unlined masonry was common in very old construction and is not suitable for modern appliances. Relining is the usual remedy.
Can I take an old chimney down instead?
Sometimes, but it is structural work and the roof and framing have to be addressed properly afterwards.
How do I know how many flues my chimney has?
Count the openings at the top, and have it confirmed from inside. Multi-flue stacks are common in older homes.
Does an unused chimney still need inspecting?
Yes, particularly if it shares a structure with a live flue or if water has been getting into it.
Published September 20, 2026