A Chimney Nobody Uses
An unused chimney still leaks, still deteriorates, still lets animals in and still loses heat, and the one thing not to do is nothing.
Short answer
Four options: maintain it, cap and seal it while keeping it ventilated, take it down to the roof line and make the roof good, or remove it entirely. The one that goes wrong is sealing it completely top and bottom, which traps moisture in the structure.
Why it still matters
The crown still cracks and the masonry still absorbs water, so it keeps deteriorating whether or not anything burns in it.
It still leaks into the house through all the usual routes.
Animals still get in and nest, which is a smell and a mess regardless of use.
It is still a heat loss path: a throat damper that does not seal is an open hole from the living room to the roof.
A deteriorating stack above the roof is a falling-masonry hazard and a roof leak in waiting.
And at sale it is a line on the home inspection report whether it is used or not.
Do not seal it airtight
Sealing an unused flue completely at the top and the bottom traps moisture inside the masonry, which cannot then dry in either direction. The result is accelerated deterioration and frequently damp on the chimney breast inside the house. Any sealing arrangement has to leave the flue ventilated.
The four options
Maintain it: crown, cap, pointing, flashing, and an inspection every couple of years. Appropriate if there is any prospect of future use or if the chimney is part of the character of the house.
Cap and close while keeping it vented: a top-sealing damper or a sealed cap with a vent, and the fireplace opening closed off with a vented cover. Stops the heat loss and the animals, keeps the flue dry.
Take it down to the roof line: remove the stack above the roof, make the roof good with proper framing and sheathing, and leave the chimney below. Removes the maintenance and the hazard, and is frequently the pragmatic answer.
Remove it entirely: a bigger job involving the structure, the roof, the ceilings and the floors, and the openings it leaves. Sometimes the right answer during a renovation.
The second and third are the most common choices on a Toronto house.
And in each case, consider the structural role, because some chimneys are doing something.
The practical considerations
Does any appliance still vent into it — a water heater is the one people forget, and it is a safety matter.
Is the chimney structural or supporting anything.
Is it shared with a neighbour in a semi-detached or row house, which is extremely common in Toronto and which makes removal a party wall question.
Heritage designation, which restricts removal or alteration in several Toronto neighbourhoods.
Permits, which taking a chimney down generally needs where it is structural or involves the roof.
And future use, because putting a chimney back is far more expensive than keeping one.
Common questions
Can I just seal an unused chimney?
Not airtight. It has to stay ventilated or moisture is trapped in the masonry.
Is taking it down to the roof line a good option?
Frequently the pragmatic answer. It removes the maintenance and the hazard.
What do people forget to check?
Whether a water heater still vents into it.