The First Cold Night: 7 Things to Check Before You Turn the Heat On

TL;DR: Before the first cold night, check your furnace filter (replace if it's been 60-90 days), test every carbon monoxide detector, clear vents and registers of 6+ months of dust, and run the system for 10 minutes while you're home and awake to catch smells or sounds. Most furnace fires and carbon monoxide incidents happen in the first two weeks of heating season, not mid-winter.

_Last reviewed: October 2026 · 7 min read_

Your furnace has been sitting untouched since April. Flip it on without a quick check first, and you're trusting a six-month-old system to fire up safely on the coldest night so far this year. Here's what to look at before you commit to the first full night of heat.

Okoniq Property Hub logs your furnace filter changes and seasonal startup checks so you're not guessing when the last service happened.

How do you know your furnace filter is still good?

Pull it out and hold it up to a light. If you can't see light through it, it's done, and running the furnace with a clogged filter restricts airflow, strains the blower motor, and can cause the heat exchanger to overheat. A standard 1-inch filter needs changing every 60-90 days during active heating season; a 4-inch media filter can often go 6-12 months.

If you're not sure which type you have or how often to swap it, the difference between 1-inch and 4-inch filters explains the lifespan gap and why thicker filters cost more upfront but less over a year. Either way, write the install date on the filter frame with a marker. It's the fastest way to stop guessing in January.

Are your carbon monoxide detectors actually working?

Press the test button on every unit and confirm it beeps within 5 seconds. Carbon monoxide incidents spike in the first few weeks of heating season because that's when furnaces, which can leak CO through a cracked heat exchanger, get used for the first time in months. A detector with a dead battery is the same as no detector.

Detectors also expire. Most CO units are rated for 5-7 years, and the date is stamped on the back. If yours is older than that, replace it rather than testing it. These are the rooms that need a carbon monoxide detector most — bedrooms and any room with a fuel-burning appliance top the list, and one detector per floor is the minimum, not the goal.

What should you check at the vents and registers before heat kicks on?

Walk every room and make sure nothing is blocking a supply or return vent. Furniture, rugs, boxes, and curtains pushed over a register for six months restrict airflow and can cause uneven heating or, in bad cases, overheat the system trying to compensate. Open all registers at least partway, even in rooms you don't use often, since closing too many can raise pressure in the ductwork and reduce efficiency across the house.

While you're at it, check the attic access panel or vent for dust buildup and confirm nothing's nesting up there, since rodents and birds sometimes move into attic spaces over the summer and block vents you don't normally check. Poor attic airflow also makes your whole heating system work harder in winter. If you've noticed ice damming or frost on the underside of your roof deck last winter, signs your attic ventilation is failing your roof is worth a look before the first freeze.

Should you run the furnace before the first real cold night?

Yes, run it for 10-15 minutes on a day when you're home, awake, and can stand near the unit to listen and smell. A brief burning-dust smell in the first few minutes is normal and comes from dust settling on the heat exchanger over summer. A smell that persists past 10 minutes, or anything sharp and electrical, is not normal and means you shut it off and call a technician.

Listen for banging, screeching, or rattling, which usually points to a loose panel, a dying blower motor, or a belt issue — all cheaper to fix in October than to diagnose during a January cold snap when every HVAC company in town is booked solid. If the furnace is part of a system you haven't fully mapped out yet, knowing where your home's core systems are — shutoff valves, breaker panel, furnace switch — saves real time in an emergency.

What else is worth checking before winter really sets in?

Check your thermostat batteries and settings now, not during a cold snap. A dead thermostat battery in January means no heat until you find batteries at 11pm. Program a reasonable schedule — 68°F while home, 62°F overnight — since that alone can meaningfully lower a winter heating bill. For more ways to trim costs once the system is running, 20 ways to lower your utility bills covers settings and habits that add up fast.

Also check your furnace's service record. If it hasn't had a professional tune-up in over a year, schedule one before the coldest months, not during them. A $120-180 fall tune-up is far cheaper than an emergency repair call on a Sunday night in January.

FAQ

Why does my furnace smell bad the first time I turn it on?

Dust that settled on the heat exchanger over summer burns off in the first few minutes, producing a mild, dusty smell that clears within 10-15 minutes. If the smell is sharp, chemical, or lasts longer than 15 minutes, turn the system off and call a technician.

How often should I change my furnace filter in winter?

Every 60-90 days for a standard 1-inch filter during active heating use, or every 6-12 months for a thicker 4-inch media filter. Check it monthly during the coldest stretch since heavy use clogs filters faster.

Is it normal for the furnace to make noise the first time it runs each season?

A brief clicking or ticking sound as metal parts expand with heat is normal. Banging, screeching, or a loud rattle is not and usually means a loose panel, worn belt, or motor issue that should be checked before you rely on the system overnight.

Do I need to test my carbon monoxide detector before winter even if it tested fine in spring?

Yes. Batteries drain over six months of inactivity, and detectors have a 5-7 year lifespan printed on the unit. Test every detector before the first cold night regardless of when you last checked it.

What temperature should I set my thermostat to when I first turn the heat on?

Start around 68°F while home and 62°F overnight or away, then adjust based on comfort. Jumping straight to a high temperature doesn't heat the house faster, it just runs the furnace longer than needed.

This is educational information, not HVAC or safety advice. Call a licensed heating technician if your furnace smells unusual, makes persistent noise, or if any carbon monoxide detector sounds an alarm.