Blog · Guelph
Repair or Replace? A Straight Answer for Guelph Furnaces
Age, repair cost and efficiency decide this — not the salesperson's quota.
Last updated September 2, 2026 · Reviewed by the Guelph Furnace Installation team
The three numbers
Age. Under 12 years with a single failed part: repair. Over 15: replacement almost always wins on total cost over the next decade. In between, it depends on the part.
Repair cost. If the repair is more than a third of a new furnace, replace. A cracked heat exchanger is a replace, full stop — it's a CO risk.
Efficiency. Most Guelph furnaces from the 1990s–2000s are 80% mid-efficiency units. A 96%+ condensing furnace uses noticeably less gas; Natural Resources Canada rates condensing units at 90–98% AFUE.
The chimney problem
A condensing furnace vents through PVC out a side wall, which leaves your natural-draft water heater alone on the old metal chimney. That's a backdraft and deterioration risk, and code requires it to be addressed — a chimney liner ($700–$1,400) or a power-vent/tankless water heater. Any quote that doesn't mention this is incomplete.
Sizing
Insist on a heat-loss calculation, not a copy of the old nameplate. Oversized furnaces short-cycle, run louder and die younger. In many Guelph homes the right size is smaller than what was there.