What should I do before turning on my furnace for the first time in the fall?
Put in a new filter, test the carbon monoxide alarms, clear around the furnace and the outdoor vents, then switch to Heat on a mild day and watch one full cycle. A dusty smell for a few cycles is normal. An electrical smell, rotten eggs or repeated failed starts are not.
Key points
- Do the first start on a mild afternoon in September or October, not on the first cold night.
- New filter, clear space around the furnace and clear PVC vents outside before you switch to Heat.
- A dusty, toasty smell that fades within a few cycles is normal; hot plastic, exhaust or rotten eggs is not.
- Swap the humidifier pad, turn its water back on and open the bypass damper for winter.
- Test your CO alarms and book the tune-up before the first cold week.
Before the first cold night, put in a new filter, test your carbon monoxide alarms, and clear the space around the furnace and the vent pipes outside. Then switch the thermostat to Heat on a mild afternoon and watch one full cycle. If you have central air, the blower ran all summer, but the burners, igniter and flame sensor have sat idle since spring, and a 55-degree afternoon is the easy time to find out one of them has a problem.
A dusty smell that fades over a few cycles is normal. Most of this post is about telling normal from not. For the wider list of fall chores, see our fall furnace checklist.
Before you switch to Heat
- Filter. If the air conditioner ran all summer, the filter has been working the whole time and is probably due. Put in a new one with the arrow pointing toward the furnace, and write the date on the edge.
- Space around the furnace. Summer storage creeps toward the furnace. Keep boxes back to the clearances on the furnace label, and move paint, gasoline, solvents and laundry chemicals away from the furnace and water heater. Make sure nothing blocks the return grille.
- Switch and door. The furnace switch, which looks like a light switch on or near the furnace, should be on, and the lower door should be firmly seated. The door presses a safety switch, and the furnace won't run without it.
- Vents outside. On a high-efficiency furnace, check the two white PVC pipes for leaves, wasp or bird nests, and anything new crowding them, like a deck, a shrub or a window-well cover. On an older furnace that vents into the chimney, look at the metal flue pipe for gaps or rust at the joints, and keep storage away from it, because it gets hot.
- Condensate drain. If there's a condensate pump, make sure it's plugged in, then pour in a cup of water and watch it kick on and empty.
- Gas. The gas to a furnace normally stays on all year. If the handle on the furnace's gas line sits crosswise to the pipe, someone shut it off. Don't turn it back on yourself; have a technician open it and check the furnace.
Switch the thermostat over
Change the mode from Cool to Heat and set the temperature a few degrees above the room, so the furnace gets a real call. Put in fresh batteries while you're there if the thermostat uses them.
Check the fan setting too. If you ran the fan on On all summer to keep air moving, switch it to Auto. With the fan on On, the blower keeps running between heating cycles, and once the burners shut off, the air coming out of the registers feels cool.
Auto mode, where the thermostat picks heating or cooling on its own, is handy in September if the heat and cool setpoints sit a few degrees apart. If you have a heat pump, leave Emergency Heat for emergencies. That setting shuts out the heat pump and runs only the backup heat.
What should you see, hear and smell on the first furnace cycle?
Stand near the furnace for the first start. On a typical gas furnace, the inducer fan whirs to life, the igniter glows or ticks, the burners light with a soft whoosh, and the main blower starts after a short delay. The first air out of the registers can feel cool for a few seconds while the ducts warm up, and you may hear ticking or popping in the ductwork as the metal expands.
Then comes the smell. Dust settles on the burners and heat exchanger over the summer, and it burns off during the first few cycles with a dusty, toasty smell. Open a window and let it run. It should get weaker with each cycle.
| What you notice | Normal? | What to do |
|---|---|---|
| Dusty, toasty smell for the first few cycles | Yes | Open a window and let it burn off |
| Ticking or popping in the ducts as they warm | Yes | Nothing |
| Cool air for a few seconds before the warm air | Yes | Nothing |
| Steam from the PVC pipe outside on a cold morning | Yes | Keep snow away from the pipes this winter |
| Dusty smell still strong the next day | No | Turn it off and have it checked |
| Hot plastic, electrical or fishy smell | No | Turn it off at the furnace switch and call |
| Rotten eggs | No, that's gas | Leave the house and call your gas utility or 911 from outside |
| A bang or loud whoomp when the burners light | No | Turn it off and call |
| Burners light, go out after a few seconds, and repeat | No | Often a dirty flame sensor |
| Inducer runs, burners never light, then it quits | No | Check the vent pipes and the condensate drain |
| Squealing or grinding from the blower | No | Turn it off and call |
| Water under the furnace | No | Call |
If your furnace has a window onto the burners, the flames should be steady and mostly blue. Lazy yellow flames, or flames that curl out of the burner opening, mean shut it off and call. Our guide to furnace smells goes further into what each smell points to. If the burners keep lighting and dropping out, cleaning the flame sensor is the usual fix.
Humidifier: new pad, water on, damper open
A whole-home humidifier sits dry all summer, and it needs a few minutes of attention before the heating season.
- Replace the pad. Aprilaire calls it a water panel. Last winter's pad is usually crusted with scale from the water, and a crusted pad puts very little moisture into the air.
- Turn the water back on at the small valve on the supply line that feeds the humidifier.
- Open the bypass damper. Bypass models have a damper in the round bypass duct. It goes open for the heating season and closed for summer.
- Check the drain. Flow-through models send the water that doesn't evaporate down a small drain line. Make sure it still runs freely to the floor drain or the condensate pump.
- Set the humidity by the weather. Roughly 25–35% in ordinary cold, and lower in deep cold. If the windows fog or frost on the inside, turn it down.
Our whole-home humidifier guide covers the settings and the different humidifier types.
Why test carbon monoxide alarms in the fall?
Fall is when the furnace starts running every day and the windows stay shut until spring, which makes it the right week to test alarms. Press the test button on every CO and smoke alarm, replace the batteries, and check the replace-by date printed on each one. Minnesota law requires a working CO alarm within 10 feet of every sleeping room, and our CO alarm placement guide shows where they belong. If a CO alarm ever sounds, get everyone outside and call 911.
What does a furnace tune-up check that you can't?
Your checks keep the furnace breathing and draining. A tune-up measures the parts you can't judge by looking: the igniter and flame sensor, gas pressure, combustion and carbon monoxide at the furnace, the heat exchanger, blower amp draw and the safety switches. September or early October is the time. The first cold week is when a furnace that sat since spring gets its first hard test, and a weak igniter is far easier to deal with on a mild afternoon than on a 10-degree night.
With us, a furnace tune-up is $129, or $219 for the furnace and air conditioner together, at the same price in every town we serve. If you'd rather not remember every fall, ask us about a maintenance plan. Our maintenance service page has the details.
What if the furnace doesn't start in the fall?
Reset once: turn the furnace switch off for about 30 seconds, then back on, and raise the thermostat. If it still won't light, our no-heat checklist goes through the rest in order. Then call 612-429-3465. We're open 7 days, 7am–9pm, and no-heat calls go to the front of the schedule. A furnace repair visit is a $79 trip charge, waived when you approve the repair.
Questions
How long does the burning smell last when you first turn on the furnace?
It usually fades within the first few heating cycles. It should smell dusty or toasty and get weaker each cycle. If it's still strong after a day of normal running, or it smells sharp, like hot plastic, wiring or exhaust, turn the furnace off at the switch and have it checked. A new filter and a quick vacuum around the furnace before the first start keep the smell down.
Should I use Heat or Auto on my thermostat in the fall?
Either works. Heat is the simplest choice once the nights turn cold, because the air conditioner can't kick on during a warm afternoon. Auto switches between heating and cooling on its own, which is handy in September if the heat and cool setpoints sit a few degrees apart. On a heat pump, don't use Emergency Heat for everyday heating.
Do I need to turn the gas back on before starting my furnace?
Normally no. The gas to a furnace stays on all year, and the furnace simply doesn't get a call for heat in summer. If the handle on the furnace's gas line sits crosswise to the pipe, someone shut it off. Don't turn it back on yourself. Have a technician open it, light the furnace and check it, since there may have been a reason it was shut.
When should I get a furnace tune-up in Minnesota?
September or early October, before the first cold week. A tune-up measures what a homeowner can't: the igniter and flame sensor, gas pressure, combustion and carbon monoxide at the furnace, the heat exchanger and the safety switches. With us, a furnace tune-up is $129, or $219 for the furnace and air conditioner together, at the same price in every town we serve.