The Liner
An unlined or deteriorated flue is the most common significant finding on an older Toronto chimney, and the appliance it serves decides what goes in.
Short answer
A liner is needed where there is none, where the existing clay tiles are cracked or missing, where the flue is the wrong size for the appliance, or where a new appliance has been installed. The liner material and size are set by the appliance — wood, gas and oil all have different requirements.
When it is needed
No liner at all, which on a pre-war Toronto chimney is common — the brick was the flue.
Cracked, spalled or missing clay tiles, which a camera inspection establishes.
A flue sized wrong for the appliance, which is the usual situation when a modern appliance has been fitted into an old chimney.
A new appliance installation, where the liner is part of the install rather than an extra.
After a chimney fire, which cracks tiles and is a standard reason for relining.
And where the flue is shared or has been shared, which is a specific and unacceptable arrangement for most appliances.
Why the size matters
A flue much larger than the appliance needs will not warm up, so the draft is poor and the flue gases cool and condense. On wood that means rapid creosote build-up; on gas it means condensation attacking the masonry. An oversized flue is a real problem and relining it to the correct size frequently solves a draft complaint as well.
By appliance
Wood stove or insert: an insulated stainless liner sized to the appliance outlet, which is the standard and what a WETT inspection will expect.
Open wood fireplace: clay tile or a stainless liner, sized to the fireplace opening ratio.
Gas appliance venting into a masonry chimney: a liner appropriate to the appliance category, because modern higher-efficiency gas produces condensate that attacks masonry.
A high-efficiency gas furnace or boiler: generally vents directly through a wall in plastic pipe rather than up the chimney at all, which frequently leaves an orphaned water heater.
Oil: stainless of an appropriate grade, and the acidic condensate makes the material choice specific.
And in every case, the manufacturer instructions for that appliance govern.
The orphaned water heater
Replacing a mid-efficiency furnace with a high-efficiency one takes the furnace off the chimney and leaves the gas water heater venting up a flue sized for both.
That flue is now far oversized for the water heater alone, so it does not warm up and the exhaust condenses.
That condensate attacks the masonry and the liner from inside, and in a bad case the water heater does not vent properly at all.
It is a genuine safety issue, which is a carbon monoxide risk rather than only a masonry one.
The usual answers are a correctly sized liner for the water heater, or moving the water heater to a direct-vent or power-vent unit.
And it should be addressed at the time of the furnace replacement rather than discovered years later, which is frequently not what happens.
Common questions
Does my chimney need a liner?
If there is none, if the tiles are cracked, or if the flue is the wrong size for the appliance.
Why is an oversized flue a problem?
It does not warm up, so the draft is poor and the flue gases condense.
What happens when the furnace is replaced?
The water heater is left on an oversized flue, which is a genuine safety issue.