Annual service on a gas furnace is worth having, and what is delivered under that heading varies enormously.
The measurements that make it real
Combustion analysis — carbon monoxide, oxygen and flue gas temperature, taken with an analyser in the flue. This is the single most informative test on a gas appliance and the one most often skipped.
Manifold gas pressure, measured with a manometer and compared to the manufacturer's specification for the fuel.
Temperature rise across the heat exchanger, compared to the range on the rating plate.
Static pressure across the air handler, which reveals a duct or filter restriction long before it causes a complaint.
Amp draw on the blower and inducer motors, which trends upward as a motor approaches failure.
An invoice showing these figures is a record. An invoice saying "system operating normally" is not.
The physical work
Clean the burners and the burner compartment.
Clean the flame sensor, which is the most common cause of a furnace that lights and then shuts down after a few seconds.
Inspect the heat exchanger, visually and with a camera where access allows.
Check the igniter for resistance and for cracking.
Inspect and clean the condensate trap and line on a condensing furnace.
Check the inducer and its pressure switch tubing, which collects moisture and blocks.
Clean the blower wheel where needed. A dirty blower wheel loses a surprising amount of airflow, and it is rarely inspected because it takes effort to reach.
Check the flue and the terminations.
What the homeowner does
Change the filter on schedule, using the type the system was set up for. This is the single highest-value maintenance action and it does not require a technician.
Keep the returns and registers clear of furniture and rugs.
Look at the vent terminations after a snowfall.
Test the carbon monoxide alarms.
Timing
Before the heating season, not during it. A furnace serviced in September has its problems found in September, when parts are available and a technician can be scheduled.
The same call in January is an emergency call at emergency prices.
What it does and does not save
It does not restore efficiency that was never there. A correctly running furnace does not drift far from its rating.
It does prevent failures, which is the real return — a failing igniter or a blocked condensate trap found in autumn is a small job.
It catches the safety items, which is the reason a combustion analysis matters more than anything else on the list.
It preserves the warranty, since most manufacturers require documented annual service.
Service contracts
Most contractors offer an annual agreement covering one or two visits plus priority scheduling and a discount on repairs.
Whether it is worth it depends on the price against the cost of the same visits bought separately. The priority scheduling has real value in a cold snap, when every contractor in the region is booked.
What to check before signing: what specifically is included, whether the visits include the measurements above, and whether the discount applies to parts, labour or both.
The record is the point
Keep the invoices with the readings on them.
Combustion figures and static pressure trended across several years show a furnace deteriorating well before it fails, and they are exactly what a second contractor needs when a large repair is eventually proposed.
Where a tune-up is oversold
A service visit that ends with a recommendation to replace the furnace, with no readings attached, deserves a second opinion.
The two findings that most often appear this way are a cracked heat exchanger and a "failing" inducer or board. Both are real faults and both are verifiable — a camera image, a combustion analysis, an amp draw. Ask for the evidence rather than disputing the conclusion.
Chemical cleaners and additives
There is nothing to add to a gas furnace. Coil cleaners belong on the cooling side, and anything sold as a fuel treatment or an efficiency additive for a residential gas appliance is not doing anything the burner needs.
The things that actually restore performance are mechanical: clean burners, a clean flame sensor, a clean blower wheel and correct gas pressure.