How often should you have furnace maintenance?

The short answer: once a year, every year, ideally in September or early October before the first cold snap. That is what every major furnace manufacturer specifies in its installation manual, what most parts warranties require, and what keeps a gas furnace burning cleanly through a Hamilton winter.

The longer answer is that "maintenance" means two different things. There is the annual professional service, which needs the panels off, test instruments and a licensed gas technician. And there is the small amount of upkeep a homeowner should do every month or two. Both matter, and skipping either is how a furnace ends up failing on the coldest night of January.

Why once a year is the right interval

A gas furnace runs for roughly 1,500 to 2,000 hours over an Ontario heating season. In that time the burners collect dust and rust scale, the flame sensor picks up a film of silica that stops it reading the flame, the condensate drain on a high-efficiency unit grows a layer of sludge, and the blower motor loads up with whatever got past the filter. None of it is dramatic on its own. Together it is why a furnace that was fine last March will not light in November.

An annual visit resets all of that before the season starts. It also catches the things that are dangerous rather than merely inconvenient: a heat exchanger that has started to crack, a flue joint that has come loose, a gas pressure that has drifted out of specification.

Three practical reasons the interval matters:

• Warranty. Most manufacturers require proof of annual professional maintenance to honour the parts warranty on the heat exchanger and the control board. A ten-year warranty with no service records is often a ten-year warranty in name only.
• Fuel cost. A burner that is cleaned and adjusted every year runs close to its rated efficiency. One that is not drifts downward a little each season, and the difference shows up on the gas bill.
• Timing. A fall tune-up means any part that needs replacing is ordered in October, not during a February cold snap when every heating company in the Hamilton area is booked solid.

What a proper annual tune-up includes

Not every "furnace check" is the same. A 15-minute visit that changes the filter and looks at the flame is not maintenance. A full annual service on a gas furnace should cover all of the following, and you are entitled to ask which of them were done.

• Burners removed or brushed clean, and the flame checked for a steady blue burn with no yellow tipping or lifting.
• Flame sensor cleaned and its signal measured. A dirty flame sensor is the single most common cause of a furnace that lights and then shuts off after a few seconds.
• Hot-surface igniter inspected for cracks and its resistance checked.
• Heat exchanger inspected for cracks, rust-through or corrosion, with a carbon monoxide reading taken in the supply air.
• Combustion analysis on the flue gases: oxygen, carbon monoxide and flue temperature. This is the test that proves the furnace is burning cleanly and efficiently, and it is the one most often skipped.
• Gas pressure measured at the manifold and adjusted to the rating plate.
• Every safety control tested: the high-limit switch, the rollout switches, the pressure switch and the door switch.
• Venting checked end to end. On a high-efficiency furnace that means the PVC intake and exhaust, their slope and their terminations outside; on an older mid-efficiency unit it means the metal flue and the chimney liner.
• Condensate trap and drain line flushed and the neutralizer checked, on condensing furnaces.
• Blower wheel inspected and cleaned if loaded, motor amperage measured, and the capacitor tested.
• Filter replaced and the filter cabinet checked for bypass.
• Thermostat operation and calibration confirmed, including any heat-pump or humidifier controls wired into it.
• A written report of what was found, what was done and anything that will need attention before next year.

In the Hamilton area, any of that work on the gas side has to be carried out by a technician holding a Technical Standards and Safety Authority (TSSA) gas certificate. Ask to see it if you have not used the company before.

What you should do between visits

The professional visit is once a year. These take a few minutes and make the difference between a furnace that reaches that visit in good shape and one that does not.

• Check the filter every month during the heating season and replace it when it looks grey. A one-inch filter rarely lasts more than two or three months; a four-inch media filter usually lasts a season. A blocked filter overheats the heat exchanger and is the most common reason a furnace trips its limit switch.
• Keep the area around the furnace clear. A metre of clearance, nothing stored against the cabinet, nothing leaning on the venting.
• Look at the outdoor vent terminations after a snowfall and after high wind. A drift or a bird nest over the intake will shut a high-efficiency furnace down.
• Replace the thermostat batteries every fall if it takes them. A dead thermostat looks exactly like a dead furnace.
• Listen. A new rattle, a bang on start-up or a smell of burning dust that does not clear in the first day of the season are all reasons to call before the visit, not after.

Signs a furnace is overdue

If the furnace has not been serviced in more than a year, or you do not know when it last was, watch for these. Any one of them is a reason to book a visit rather than wait for the fall.

• Short cycling: the furnace lights, runs for a few minutes, shuts down, and repeats.
• Cold air from the vents while the blower is running.
• A yellow or flickering burner flame instead of a steady blue one.
• Rising gas bills with no change in the weather or the thermostat setting.
• A furnace that is more than fifteen years old and has never had a combustion test.

How long the visit takes and what to ask for

A thorough annual service on one furnace takes about an hour, a little longer on a high-efficiency unit with a condensate system to flush. Book it for September or early October, ask for a written list of what was checked and the combustion readings, and keep the report with your warranty paperwork. If the technician recommends a repair, ask for the price in writing before the work starts.

Maintenance plans, where the company books the annual visit for you and gives plan members priority when the weather turns, are worth asking about if you would rather not think about it each fall.