A chimney liner is sized from the appliance it serves, not from the hole in the chimney. The inputs are the appliance's rated BTU input, the manufacturer's published venting requirements, the total height of the chimney, and the length and geometry of the connector between appliance and flue. Get those right and the appliance drafts as designed. Get them wrong in either direction and you have a chimney that appears to be at fault when the actual problem is the size of the pipe inside it.
Why oversized is a genuine fault
People assume a bigger flue is a safety margin. It is the opposite. Exhaust gases rise because they are hot and buoyant. Spread that same volume of gas into a much larger cross-section and it slows, loses heat to the flue walls faster, and arrives at the top cooler than it should. Two things follow: the draft weakens, and the cooler gas condenses moisture and combustion products on the flue wall.
On a wood stove that shows up as poor draw, a smell in the room and creosote building faster than the owner expects. On a gas or oil appliance it shows up as condensation, and eventually as damaged mortar joints inside the chimney.
Why undersized is also a fault
Undersized is more obvious — the flue cannot carry the volume, so exhaust backs up. It is far less common, because the usual situation is a modern appliance connected to a flue built for something much bigger.
The Connecticut version of this problem
The single most common sizing mismatch we find in Connecticut homes is a wood stove or an insert fitted into a masonry fireplace flue. That flue was built for an open fire, which needs a large cross-section because it is drawing a whole room's worth of air. A modern stove is a sealed appliance drawing a fraction of that. The flue can be four or five times the area the stove needs.
That is why fitting a stove properly means running a correctly sized stainless liner down the flue rather than simply connecting the stove and hoping. It is not an upsell; it is how the appliance is meant to be vented.
The other variables
Height
Taller chimneys generate more draft, all else equal, because there is more column of warm air to do the work. A short chimney on a ranch house and a tall one on a three-story colonial serving the same appliance are not the same calculation.
The connector run
Every foot of horizontal connector and every elbow adds resistance. Manufacturer venting tables account for this explicitly, which is one of the reasons the tables rather than a rule of thumb are what an installation should be built on.
Where the chimney runs
An exterior masonry chimney on a north wall stays cold. A liner in that chimney needs insulating, otherwise it never reaches temperature and you get the cool-flue problems all over again despite having sized it correctly. ABC Chimney Solutions insulates wherever the run is cold or external.
What this means when you are getting quotes
If someone quotes a liner without asking what appliance it is serving, what its input rating is, how tall the chimney is and how the connector runs, they are guessing. Those questions are the job.
Where a stainless liner is not the right answer, the alternatives are a cast-in-place liner, which also adds strength to the surrounding masonry, or clay tile relining where the original build allows it. The relining hub sets out where each one fits.
Frequently asked
Can I use my existing fireplace flue for a wood stove?
Rarely as it stands. A fireplace flue is built for an open fire and is normally several times larger than a stove needs. ABC Chimney Solutions sizes a stainless liner to the stove so the flue is matched to the appliance. Learn more about this service.
Does a liner have to be insulated?
Not always, but ABC Chimney Solutions insulates where the chimney runs on an exterior wall or is otherwise cold, because an uninsulated liner in a cold chimney will not reach temperature and will draft and stay clean less well. Learn more about this service.
What if my chimney is not tall enough?
Chimney height is part of the sizing calculation and sometimes part of the problem. Where height is the limiting factor, ABC Chimney Solutions will say so rather than fitting a liner that cannot draft properly. Learn more about this service.