When to book a furnace tune-up before winter, and what the tech checks

The yearly heating service is one of the few house jobs you don’t do yourself — you hire it. So the job is really the booking: doing it in the fall rather than the first cold week, and knowing what belongs on the technician’s list.

Written by the developer of Gable · Published 13 September 2026

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It’s a safety check, not only an efficiency one

Inside a gas or oil furnace, the heat exchanger is the metal wall between the burner flame and the air blown through your rooms: combustion gases on one side, house air on the other. If that wall cracks or corrodes through, the two sides meet and carbon monoxide can be carried into the supply air. You can’t see it from the outside, and the furnace keeps making heat while it happens.

That is why the job is on Gable’s list at all. The library’s reason for it reads: “The burner, heat exchanger and safety controls are checked before the first cold night. A cracked heat exchanger leaks carbon monoxide; a service catches it.” ENERGY STAR’s maintenance checklist names the same items as things to expect from a contractor, because a dirty burner or a cracked exchanger affects safety as well as efficiency.

For scale, hedged as the US figures they are: CDC’s MMWR QuickStats counted 2,244 deaths from unintentional carbon monoxide poisoning in the United States over 2010–2015, the highest count each year in the winter months. That covers every unintentional source, vehicles and generators included, not furnaces alone; the seasonal shape is the part worth reading. A service is no substitute for alarms, or the reverse: see how often to test and replace smoke and CO alarms.

Gable’s library puts the cost of skipping it as “A $100–200 service vs an emergency call in January and a real CO risk.”

When should you schedule a furnace tune-up?

In the fall, before the heating season starts. ENERGY STAR’s reasoning is scheduling, not weather: contractors get busy once summer and winter come. Book in September or October and the visit happens on your calendar. Wait for the first genuinely cold night and you are calling in the same week as everyone whose furnace just failed.

Gable’s library gives the task a September–November window and a 12-month interval, and the app mirrors library windows by six months for the southern hemisphere, so a house in Australia or Chile gets the same job in March–May.

Two smaller jobs sit in the same window and are worth lining up with the visit: Gable schedules the thermostat check — fresh batteries, and a look at last year’s schedule — for September–November too, and the air filter every three months year round.

What does a furnace tune-up include?

From ENERGY STAR’s maintenance checklist, what Gable’s library tells you to ask for, and what contractors themselves list for a tune-up:

• Gas or oil connections, and gas pressure. Connections that don’t operate properly are a fire hazard in their own right.
• Burner combustion, measured. This is the combustion analysis: an instrument in the flue reading carbon monoxide, oxygen and flue temperature against what the manufacturer specifies. It is the difference between “the flame looks fine” and a number.
• The heat exchanger. Visually, and with a camera or mirror where the design allows one in.
• Safety controls, and the start-up and shut-down cycle. The system should start, run and shut off properly on command.
• Blower components, cleaned and adjusted. Airflow problems cost efficiency even when the burner side is perfect.
• The flue and venting, and on a high-efficiency condensing furnace, the condensate drain.
• The air filter, inspected and changed.

What to ask for when you book

1. Say “combustion analysis” and “heat-exchanger check” on the phone. Both are in Gable’s instruction for this task, and both are things a cheap seasonal “tune-up” special can quietly leave out.
2. Ask for the readings — a printout, an emailed report, or a photo of the analyser screen. Next year’s numbers only mean something if you have this year’s.
3. Ask them to change the filter, and to tell you the size they fitted.
4. Keep the invoice. Several manufacturers make annual professional maintenance a condition of the parts warranty and can refuse a claim with no service record. It varies by brand and model, so read your own warranty paperwork.
5. Check they’re licensed or registered for gas work where you live — a licensed HVAC contractor in the US, a Gas Safe registered engineer in the UK.

What you can do yourself before the visit

Nothing here involves opening the burner compartment or touching a gas line. That part is the technician’s job, and it stays their job.

• Change the filter, or at least look at it, so the tech isn’t diagnosing airflow through a grey mat. See how often to change a furnace filter.
• Clear a working space. Boxes and paint tins migrate toward the furnace over a summer, and flammable things stored against it are a problem either way.
• Vacuum the supply and return vents. Gable puts this one on a six-month cycle.
• Put fresh batteries in the thermostat and check last winter’s schedule still matches this winter’s life.
• Test every smoke and CO alarm. Gable asks for that monthly, and the month before the heating starts is the one that counts.

Boilers and radiators

Gable offers this task to a house with a boiler and radiators as well as to one with a forced-air furnace — two separate answers in the app’s house profile, and either adds the pre-winter service. The idea is the same, and an annual service is the standard recommendation in the UK.

If you rent out a property, one distinction matters: a gas safety check and a service are not the same thing. Sources disagree on whether a landlord must also have an annual service, but the safety check by a Gas Safe registered engineer is the legal duty. Check the current rules rather than this page.

Bleeding the radiators and checking boiler pressure is a separate, owner-level job, which Gable schedules for September–October.

When not to wait for the appointment

• You smell gas. Get everyone out, then call your gas utility’s emergency line or emergency services from outside. Don’t hunt for the source or start flipping switches.
• A CO alarm sounds. Treat it as real: everyone, pets included, out into fresh air, and call emergency services or the gas utility from outside. Don’t go back in to investigate.
• Soot, scorch marks, or a burner flame gone yellow and wavering where it used to be blue. Stop using the system and get a licensed technician out rather than waiting for the calendar.
• No heat in a cold snap, with pipes at risk. The emergency call this post exists to help you avoid — see also winterizing outdoor faucets.

How often should a furnace be serviced?

Every 12 months is what Gable’s library schedules, and yearly professional maintenance for a furnace or heat pump is the standard advice in US federal energy guidance too. The cooling side is its own appointment, which Gable puts in a March–May window for houses with air conditioning or a heat pump. A house with both gets two visits a year; the fall one has the heat exchanger in it.

Book it before the cold, not during it

Tell Gable your house has a furnace, or a boiler and radiators, and the pre-winter service joins your plan in its September–November window. It reminds you, and when you log the visit you can record what it cost, so next autumn’s quote has something to compare against. No account needed.