When to Service Your Furnace, and the Symptoms That Mean Do Not Wait

Every seasonal checklist says to service the furnace, including ours. None say what the service consists of, so most homeowners cannot tell a thorough visit from a fifteen-minute filter change, or a nuisance noise from a reason to shut the thing off. This article covers both, and how yearly service relates to how long a furnace lasts.

What annual service actually involves

A proper annual service on a gas furnace is a safety inspection as much as a tune-up, and it has to be done by a licensed gas technician. In Ontario that means a TSSA-registered contractor; other provinces have their own authority. The parts that matter most are the parts you must not open. A technician should be doing roughly this.

• Inspecting the heat exchanger for cracks or corrosion. It separates combustion gases from the air you breathe, and a failure here is the carbon monoxide risk the whole inspection exists to catch.
• Checking the venting and draft, including the flue, connections and outside terminations, and confirming the furnace is not spilling combustion gases into the house.
• Cleaning the burners and checking the flame pattern.
• Measuring combustion, including gas pressure and a flue carbon monoxide reading, with instruments rather than by eye.
• Testing the safety controls: limit switch, flame sensor, rollout switch and pressure switch, the devices that shut the furnace down before a problem becomes dangerous.
• Cleaning the blower and checking its electrical draw, where a struggling motor shows up first.
• Clearing the condensate trap and drain on a high-efficiency unit, which blocks with sediment and causes nuisance shutdowns.
• Checking the thermostat, filter and airflow, and telling you what filter the unit takes.

Your own part between visits matters more than people expect: change the filter on schedule, keep supply and return vents unblocked, keep stored items clear of the furnace, and keep the outside intake and exhaust terminations clear of snow all winter. A buried exhaust is a real carbon monoxide hazard, and in a heavy snow year the most likely one in the house.

When to book it

Late summer to early fall, before the first cold week. Two reasons. Availability: a technician is easy to book in September and hard during a January cold snap, when every emergency call in the city is ahead of you. And consequence: a worn part found in September is a scheduled repair, while the same part in January is a night without heat.

Once a year, which is what most manufacturers specify. Annual service is often a condition of the warranty and a claim can ask for proof, so keep the invoice. A new furnace is not exempt: the early years are when you want the installation verified and the record started. If you have a heat pump with gas backup, both halves need attention.

The symptoms that mean do not wait

Some of these are emergencies. Those come first.

A carbon monoxide alarm sounding. Get everyone outside into fresh air, then call 911 or your gas utility's emergency line from outside. Do not go back in to investigate, and do not silence the alarm and go back to bed. Carbon monoxide has no smell and its early symptoms resemble the flu.

Symptoms of exposure: headache, dizziness, nausea, confusion or unusual fatigue, especially when more than one person or a pet feels it at once, and especially when it improves outside. Treat it as carbon monoxide, get out, and call for help.

A smell of gas, often described as rotten eggs. Leave without using switches, appliances or anything that could spark, and call your gas utility's emergency line from outside.

Soot, scorching or melted material around the furnace or vent, a yellow flickering burner flame where it should be blue and steady, or a rusted or disconnected vent pipe. Shut the furnace down if you can do so safely, and have a licensed technician inspect it before it runs again.

These are service calls rather than emergencies, but should not wait for next season.

• A furnace that repeatedly locks out or needs resetting.
• A bang or boom on startup, which can mean delayed ignition.
• Short cycling: switching on and off every few minutes.
• A burning smell persisting past the season's first run.
• Water pooling around a high-efficiency unit.
• A blower that never shuts off, or a breaker that keeps tripping.
• Rooms that have stopped getting heat.

Two rules matter more than any single symptom. Never bypass or disable a safety control, and never tape over or remove the battery from a carbon monoxide alarm: a furnace that keeps shutting itself down is doing its job. And never attempt work on gas piping, burners, the heat exchanger or the venting. A suspected cracked heat exchanger is not a DIY diagnosis or a patchable part.

Carbon monoxide alarms are required near sleeping areas in many provinces. Test them monthly and replace each unit by the end-of-life date printed on it, because the sensors expire whether or not the alarm has sounded.

An annual service is not a substitute for a qualified inspection, and neither is an article. If something about your furnace worries you, have a licensed technician look at it.

How servicing relates to the 15 to 25 year service life

InterNACHI's life expectancy chart puts a gas furnace at 15 to 25 years. The range is the honest answer rather than a hedge: a furnace retired at 15 and one still running at 25 are both ordinary. The figures assume regular recommended maintenance and normal wear, and InterNACHI states plainly that they are guidelines rather than guarantees.

Where in that range your furnace lands comes down to maintenance, installation quality, whether the unit was sized correctly, and how hard it has worked. Servicing does not stretch the range indefinitely. It keeps you inside it rather than below it, and it tells you the condition of the heat exchanger, usually the component that ends a furnace's life. So the figure changes what you do, at two points.

At the low end, around 15 years, switch from reacting to planning. Ask the technician each year for a straight read on the unit's condition. Treat a large repair quote at that age as a replacement decision, and plan any replacement for the shoulder season, when you can choose calmly instead of buying whatever can be installed tomorrow.

Past the high end, "it still works" describes today and says nothing about February. A furnace there is on borrowed time by definition, and the sensible posture is to have decided in advance what you will do when it stops.

All of this depends on knowing the furnace's age, which many homeowners do not. The label usually carries a date code, and the model and serial numbers let the manufacturer or your technician confirm it. Photograph the label once, record the install year and every service visit, and the age stops being a guess. That is what a homeowner record is for.

If you do not know when the furnace was installed, the data plate usually carries a date code. How to find a furnace's age from its serial number covers reading one, and the home system lifespan calculator reads that year against the published range.

Frequently asked questions

How often does a furnace need servicing? Once a year, by a licensed technician, which is what most manufacturers specify. Filter changes happen more often and are not a substitute.

What is the best month to book? Late summer or early fall, when technicians are available and anything found becomes a planned repair rather than a cold-weather emergency.

Does skipping service void my warranty? It can. Many manufacturers make annual maintenance a condition of coverage and ask for records at claim time, so keep the invoices.

My furnace is 20 years old and works fine. Should I replace it? Not automatically, but it is inside the published 15 to 25 year range, so plan rather than wait. Ask your technician for a condition assessment and let a major repair quote make the decision.

Is changing the filter the same as servicing? No. The filter is housekeeping; the service is a combustion and safety inspection that needs instruments and a licence.

---

The furnace is the one system whose failure arrives in the worst week of the year and is genuinely a safety matter. An annual visit in September, a known install date, and a record of what has been done is the whole discipline. See how Habyn helps homeowners.