When the furnace runs for the first time each fall, four things are worth checking: the furnace filter (weak airflow), the humidifier pad (dry air and static), the weather stripping (drafts at doors and windows) and the thermostat (constant adjusting). Each check takes a few minutes and needs no tools. Here is what each sign means, what to look at and when to replace it.
Why does the air from my vents feel weaker when the heat comes on?
Check your furnace filter first. Once the furnace is running every day, a filter that made it through the summer can clog quickly. A dirty filter restricts airflow, so less warm air reaches the rooms furthest from the furnace.
- Slide the filter out and hold it up to a light. If you can barely see light through it, or it looks grey and matted, replace it.
- Note the size printed on the frame (for example 16x25x1) and the arrow showing airflow direction. The arrow points toward the furnace.
- Most 1-inch filters need changing every 1 to 3 months during the heating season; thicker 4- and 5-inch media filters last longer. Our guide on how often to change your furnace filter covers the details.
Not sure which number on the frame matters? Our guide to nominal vs actual filter sizes explains it. Furnace filters are listed by size, and odd sizes can be made to measure.
Why does my house feel so dry once the furnace is running?
Check your humidifier pad. Heating dries out indoor air. If you have a whole-home humidifier on your furnace, its water panel (pad) is what holds the water the air passes through. A pad crusted with minerals can't hold much water, so the humidifier adds less moisture even when it's running.
- Turn off the humidifier's water and power, open the cover and look at the pad. Hard, white or brown mineral build-up means it's time for a new one.
- Most manufacturers recommend replacing the pad at least once a heating season, and more often with hard water.
- Your humidifier's model number (for example Aprilaire 600) tells you which pad fits.
Replacement pads are organised by humidifier brand and model under humidifier filters and pads. For why indoor humidity matters in winter, see the risks of dry indoor air.
How do I find and stop drafts around doors and windows?
Check your weather stripping. Weather stripping wears out, flattens and cracks over time, leaving small gaps that let cold air in and warm air out.
- On a windy day, hold your hand or a lit incense stick near the edges of doors and windows. Moving air or wavering smoke shows where the gap is.
- Close a door on a sheet of paper. If it slides out easily, the seal isn't doing its job.
- Door sweeps, foam tape and V-strip seal different kinds of gaps, so measure the gap first to choose the right type.
The main weather stripping types are door sweeps for the gap under a door, foam tape for uneven gaps, and V-strip for the sides of sliding windows and doors.
Should I switch to a programmable or smart thermostat this fall?
Consider a programmable or smart thermostat. Fall temperatures swing a lot from morning to afternoon. A programmable thermostat follows a schedule you set, and a smart thermostat can be adjusted from your phone, so you're not changing the setting by hand several times a day.
- Before replacing a thermostat, take a photo of the wires behind your current thermostat. Many Wi-Fi models need a C (common) wire.
- Thermostats for furnaces and central air are low-voltage. They're not for electric baseboard heaters, which need line-voltage thermostats.
- If the wiring doesn't match or you're unsure, have a licensed HVAC technician install it.
Product pages for smart thermostats and other thermostats list the wiring each model needs.
Fall furnace checklist at a glance
| What you notice | What to check | What to do |
|---|---|---|
| Weak airflow from vents | Furnace filter | Replace if dirty; note the size and airflow arrow |
| Dry air, static shocks | Humidifier pad | Replace if crusted with minerals |
| Drafts at doors and windows | Weather stripping | Find the gap, then reseal |
| Constant thermostat changes | Thermostat | Set a schedule or upgrade to programmable |
Common questions about the first cold snap
How often should I change my furnace filter in the winter?
For a standard 1-inch filter, check it monthly and replace it every 1 to 3 months during the heating season. Homes with pets, renovations or allergies usually need the shorter end of that range. Thicker 4- and 5-inch media filters last longer; check the manufacturer's guidance or the product page for your filter.
When should I replace my humidifier pad?
At least once every heating season, usually in the fall when you turn the humidifier back on. Replace it sooner if it's hard or covered in mineral scale.
Why does my house feel colder even though the furnace is running?
A clogged furnace filter can restrict airflow so less heat reaches your rooms, and worn weather stripping lets warm air escape. Check both before calling for service.
