Sized for the coldest night
Ontario's Building Code sizes heating across Ottawa for a January design temperature of −25 °C, and −26 °C in Orléans. A furnace has to keep the whole house comfortable on that night.
Too small, and it runs flat out and still falls behind. Too big, and it starts and stops all day, which wears out parts and leaves the rooms far from the furnace cold. The right size comes from the house: its floor area, insulation, windows and air leaks. The label on the old furnace doesn't tell you that, because the house may have changed since then, and the old furnace may never have been the right size.
Why a new furnace can change the venting
Since the end of 2009, almost every gas furnace made for a Canadian home has had to be a high-efficiency, condensing model: 90% efficient at first, 95% since mid-2019. These furnaces vent through plastic pipe out the side of the house instead of up the chimney.
That matters in older neighbourhoods, from Nepean's postwar streets to Blackburn Hamlet. Where the old furnace shared a chimney with the gas water heater, taking the furnace off leaves the water heater venting alone into a chimney that's now too big for it. That can cause condensation and poor draft. The usual fixes are a chimney liner or giving the water heater its own vent. The technician will tell you about it before the job starts, not after.
The new vent pipes also need to clear windows, doors, the gas meter and the depth snow usually reaches against that wall.
A furnace, a heat pump, or both
Many homeowners now pair a new furnace with a cold-climate heat pump. The heat pump heats through fall and spring and cools in summer, and the furnace takes over on the coldest days. If your air conditioner is also near the end, it's worth comparing. See heat pumps.