A furnace that has run for 25 winters is near the end of its expected life, but it is not automatically finished. The useful question is not how old it is but what failed, what the rest of the furnace looks like, and whether the house around it has changed. Here is how to think it through.
Age sets the odds, the failed part sets the answer
Most gas furnaces are designed for roughly two decades of service. At 25 years, three things work against a repair:
- Wear is spread across the whole unit. Fixing the igniter today does not stop the inducer bearing, the blower motor or the control board from going next winter.
- Replacement parts get harder to find. Some original parts are discontinued, and a universal substitute may need adapting.
- The manufacturer's parts warranty ended long ago, so every part is paid for in full.
That said, a small repair on a furnace with a sound heat exchanger and clean combustion can be a reasonable way to get through a season while you plan a replacement properly. A rushed replacement in January, sized from the old label, is often a worse outcome than a planned one in the fall.
A workable rule: if the repair is a small fraction of what a replacement would cost and the heat exchanger passes inspection, repairing is defensible. If the repair is a large fraction, or it is the second or third significant repair in a couple of years, put the money toward replacement.
The most expensive things to fix on a furnace
Costs are not given here because they vary by model and part availability, but the order is fairly consistent.
- **Heat exchanger.** This is the metal chamber that separates the burner flame and combustion gases from the air blown through your house. On many furnaces, replacing it means taking most of the unit apart, and on a 25-year-old furnace the part may not be available at all. It is almost always the most expensive repair and, at this age, the one that usually tips the decision to replacement.
- **Secondary heat exchanger on a condensing furnace.** High-efficiency furnaces have a second exchanger that pulls extra heat from the exhaust and produces acidic condensate. When it corrodes or leaks, the repair is similar in scale to the primary one.
- **Blower motor.** Variable-speed motors with their own electronics cost considerably more than older single-speed motors.
- **Control board.** The board runs the ignition sequence and safety checks. Older boards can be hard to source.
- **Inducer motor.** This small fan pulls combustion gases through the furnace and out the vent. It is a mid-range repair, but on an old furnace a failing inducer is often a sign of other wear.
Igniters, flame sensors, pressure switches and limit switches sit at the low end. These are the parts that most often cause a furnace to run without heating, which is covered in why a furnace runs but does not heat the house.
Why a cracked heat exchanger is not a wait-and-see repair
A crack or hole in the heat exchanger can let combustion gases, including carbon monoxide, into the air the blower circulates. Carbon monoxide has no smell or colour. The Ontario Fire Code requires a working carbon monoxide alarm next to each sleeping area of a home with a fuel-burning appliance, a fireplace or an attached garage. If your furnace is 25 years old, check those alarms now, test them, and replace any past their expiry date.
A technician looks for a failed heat exchanger by visual inspection with a light or camera, by watching the burner flame when the blower starts (a flame that lifts or wavers when the fan comes on suggests air leaking through a crack), and by measuring carbon monoxide in the flue gas and at the supply registers. If the exchanger has failed on a furnace this old, replacement is the normal recommendation, and the furnace should not be left running in the meantime.
Your house changes the decision
Grimsby has three broad generations of housing, and each shapes what replacing a 25-year-old furnace involves.
- **Grimsby Beach and Main Street East and West.** These older wood-frame houses were often heated by wood or coal first and later converted. Ductwork was squeezed in afterwards and walls may have little insulation. If the current furnace vents into an old masonry chimney, a replacement needs the chimney and liner assessed, because modern equipment produces cooler, wetter exhaust that an unlined chimney may not handle.
- **Lakemount Terrace in the north end.** These 1950s houses usually began with oil or coal forced-air systems later converted to gas. Undersized ducts and weak return air are common. A new variable-speed furnace pushed through restricted returns runs loud and wears early, so the return side may need work as part of a replacement. If the new furnace vents through the side wall, the gas water heater can be left alone in a chimney sized for both, where it may not draft properly. That has to be addressed at the same time.
- **The west end, around the Winston Road Neighbourhood.** Newer homes here typically have condensing furnaces with plastic side-wall vents and heat-recovery ventilation. The oldest of these are now reaching replacement age. Replacement is usually simpler, but vent terminations, condensate drainage and the ventilator need checking.
The lake also matters. Lake Ontario moderates winters along this shore, so the furnace in a well-insulated Grimsby house often has more capacity than it needs. A replacement should be sized from a heat-loss calculation and a duct assessment, not by matching the old unit's rating. An oversized furnace short-cycles, which wastes fuel and wears parts. The same mild winters are why a heat pump or hybrid system is worth comparing at replacement time; see heat pumps, rebates and the cheapest way to heat a Grimsby home.
If your house is older, check the Town of Grimsby's heritage register before planning a new side-wall vent or other exterior change. The Town keeps a public register of properties with heritage value that are not formally designated.
What furnace maintenance actually involves
Maintenance is how you find out whether a 25-year-old furnace is worth keeping, before it fails in a cold snap. A thorough visit should include:
- **Combustion analysis.** Measuring carbon monoxide, oxygen and flue temperature in the exhaust shows whether the burners are firing cleanly.
- **Gas pressure.** Inlet and manifold pressure are measured and compared with the rating plate. Wrong pressure causes poor heating, sooting or overheating.
- **Temperature rise.** The difference between return and supply air temperature, checked against the range printed on the furnace. Too high suggests restricted airflow; too low can point to under-firing.
- **Static pressure.** Measures how hard the blower is working against the ductwork, which shows up undersized returns in older houses.
- **Heat exchanger inspection.** Visual and camera inspection where access allows.
- **Flame sensor and igniter.** The flame sensor signal is measured in microamps; a weak reading means it needs cleaning or replacement before it starts causing lockouts.
- **Safety controls.** Pressure switch, limit switch and rollout switches are tested to confirm they shut the furnace down when they should.
- **Venting.** Draft on chimney-vented furnaces; on condensing furnaces, the side-wall terminations are checked for blockage, ice and correct clearances.
- **Condensate system.** The trap and drain line on a condensing furnace are cleaned and checked. A clogged or frozen trap backs water into the pressure switch tubing and shuts the furnace down.
- **Filters.** The furnace filter, and the HRV filters where the house has a ventilator.
On an old furnace, the results of these measurements are what a repair-or-replace decision should rest on.
Who may do the work
Ontario Regulation 212/01 governs who may install and service gas appliances, and installations follow the CSA B149.1 Natural Gas and Propane Installation Code. Work is performed by TSSA-certified gas technicians. Whatever you decide, ask for the measurements and a written price for both options before agreeing to either.
If your furnace has reached this point, our furnace installation and replacement page explains how a replacement is assessed and sized.