The Orphaned Water Heater
This is the most common unaddressed safety issue in a Toronto basement, and it is created by an energy upgrade.
Short answer
Swapping a mid-efficiency furnace for a condensing one moves the furnace off the masonry chimney and vents it through a wall. The gas water heater is left venting up a flue sized for both appliances, which is now far too large. It does not warm up, the exhaust condenses, and the venting can fail.
What goes wrong
The flue was sized for the combined output of the furnace and the water heater.
With only the water heater on it, there is not enough heat to establish and maintain a draft.
The flue stays cold, the exhaust cools, and the water vapour in it condenses inside the chimney.
That condensate is mildly acidic and it attacks the clay tiles and the mortar from inside.
Deteriorating tiles then shed into the flue, which can partly block it.
And in the worst case the water heater does not vent properly at all, which spills combustion products into the basement.
Why it matters
A water heater that cannot establish a draft spills flue gases into the basement, which is a carbon monoxide exposure. The water heater is also in a basement that is frequently depressurized by the dryer, the bathroom fans and now a high-efficiency furnace drawing combustion air. It is a genuine safety matter rather than an efficiency one.
The options
Reline the chimney with a correctly sized liner for the water heater alone, insulated, which is the standard answer where the water heater stays.
Replace the water heater with a direct-vent or power-vent unit that vents through the wall, which takes the chimney out of the equation entirely.
Replace it with a tankless unit, which also vents through the wall.
Where a new furnace is being installed, address this at the same time rather than afterwards, which is cheaper and is when the installer should raise it.
And in any case, test the draft on the water heater after any change to the mechanical room, which is a simple check.
Checking your own
Did the furnace get replaced with a high-efficiency unit venting out of a side wall in plastic pipe.
Is the gas water heater still connected to the masonry chimney.
If both are yes, the flue is almost certainly oversized for the water heater alone.
Look for rust and staining on the water heater draft hood and the vent connector, which indicates condensation and spillage.
Look for staining or efflorescence on the chimney in the basement.
And get a carbon monoxide alarm in the basement if there is not one, which in Ontario is required where there are fuel-burning appliances.
Common questions
What does orphaned mean here?
A gas water heater left venting up a chimney sized for it plus a furnace that has since been replaced.
Is it dangerous?
It can be. Poor draft means combustion products spilling into the basement.
What are the options?
A correctly sized insulated liner, or a direct-vent or tankless water heater venting through the wall.