Repair or Replace? How to Decide on an Aging Furnace

The cracked heat exchanger: no debate

The heat exchanger is the metal barrier between the burning gas and the air you breathe. When it cracks, combustion products including carbon monoxide can enter the airstream and be distributed through the house by the blower.

It cannot be safely patched. On virtually every residential furnace the exchanger costs more than the furnace is worth, and the labour to replace one is substantial. So a confirmed crack means replacement.

If we find one we will show it to you — on camera, in place — before we say anything about price. The furnace gets red-tagged, which means it is shut down and the gas utility is notified as required, and we will work hard to get you heat back the same day. We would rather lose the job to someone else's quote than leave a cracked exchanger running because the conversation was uncomfortable.

Age and the one-third rule

A gas furnace in this climate typically lasts 15–20 years. It runs far more hours here than in a mild climate, so the shorter end of the national range is realistic for the Twin Ports.

Under 15 years: repair is usually right. The equipment has life left and the repair is an investment in it.

15–18 years: judgement. Use the one-third rule — if the repair costs more than about a third of a replacement, replacement is usually the better money, because you are buying a large repair on equipment that will need the next one soon.

Past 18–20 years: lean replacement, especially if this is the second or third repair in a few seasons. At that point you are on the failure curve where components start going in sequence, and each individual repair looks reasonable while the total does not.

Also worth weighing: parts availability. Once a model's control boards and gas valves are no longer manufactured, a cheap repair becomes an expensive scavenger hunt, and we will tell you when you are at that point.

The arguments people underweight

Efficiency. Replacing an 80% furnace with a 96% unit cuts roughly 16% off the gas you burn for the same heat. Over a 9,000+ heating-degree-day season that is a real annual number, and it accrues for twenty years. It rarely justifies replacing a healthy furnace on its own, but it changes the arithmetic on a marginal decision.

Sizing. Most furnaces here are oversized, frequently by 30–50%, because each replacement matched the previous nameplate. Replacing is the only opportunity you get to fix that, and a correctly sized furnace short cycles less, lasts longer and heats more evenly. If your current furnace short cycles and has cold rooms, replacement solves a comfort problem that no repair will.

Bundling. If you are also due a water heater, considering air conditioning, or have an orphaned water heater venting problem, doing it together saves a second round of labour and lets us get the whole system right at once.

Timing. A furnace replaced in September is a planned purchase with options and rebate paperwork done properly. The same furnace replaced on the coldest night in January is an emergency with whatever is on the truck. If yours is on the edge, decide in the fall.