Every October, the first overnight low in the 30s hits the Fox Valley and thousands of furnaces that have sat idle since April get asked to light on the same morning. The ones that don’t become that week’s no-heat calls. Ten minutes with this checklist, done on a mild afternoon before you actually need heat, catches most of the reasons a furnace fails to start and gives you time to fix them on your schedule. It is not a substitute for a furnace tune-up, but it’s the homeowner half of getting ready for winter.
Why the First Cold Snap Turns Into a Rush
A furnace that runs fine in March and then sits for six months doesn’t age gracefully. The igniter is a brittle ceramic element that survives its last cycle in spring and cracks on its first in fall. The flame sensor picks up oxide while it sits. The filter that was “fine” in April has been catching dust from a summer of AC use. Condensate traps dry out, spiders build in PVC terminations, and thermostat batteries go flat. None of it shows until the first call for heat.
In northern Illinois that call usually comes in October, sometimes late September, and it arrives for everyone at once. The result is a week or two where furnace repair calls stack up and the furnaces that fail at 6 a.m. on a Saturday become emergency furnace repair calls, which are priced differently than a weekday visit and quoted on the phone before dispatch. The point of a pre-season check is to be the house that isn’t calling that week.
The 10-Minute Furnace Checklist
Do this on a day when you don’t need the heat yet. You’ll want a flashlight, a new filter of the right size, and fresh batteries for the thermostat if it takes them.
- 1. Replace the filter. Pull the old one and hold it to a light. If you can’t see through it, it goes. Note the arrow on the frame; it points in the direction of airflow, toward the furnace. A clogged filter is the single most common reason a furnace overheats and shuts down on its high-limit switch.
- 2. Set the thermostat to Heat and check the batteries. Switch the mode from Cool or Off to Heat, set the fan to Auto, and raise the setpoint a few degrees above room temperature. If the display is dim or blank, replace the batteries before you decide the furnace is dead.
- 3. Open every supply register and clear every return. Walk the house. Registers closed for summer, rugs over floor vents and furniture pushed against return grilles all cut airflow, and a furnace short on airflow cycles on its limit switch and blows lukewarm air.
- 4. Check the PVC intake and exhaust outside. On a 90%-plus furnace, find the two white pipes coming out through the rim joist. Clear leaves, nests, mulch and anything piled against them. Make sure the screens aren’t clogged and nothing has been stacked in front of them over the summer.
- 5. Look at the furnace itself. The power switch on or near the cabinet is on, the breaker isn’t tripped, the blower door is seated flat so the safety switch is pressed, and the area around the furnace is clear of paint cans, boxes and anything else stored against it since spring.
- 6. Check the condensate line. On a condensing furnace, follow the clear or white tubing from the furnace to the floor drain or condensate pump. It should run downhill without kinks or sags holding standing water. If there’s a condensate pump, pour a cup of water into it and confirm it kicks on and empties.
- 7. Test the CO detector. Press the test button on every carbon monoxide alarm in the house, replace batteries, and check the date on the back; most units are rated for 7 to 10 years and are quietly past it in a lot of homes. There should be one on every level and near the bedrooms.
- 8. Run it, and listen. With the thermostat calling, stand by the furnace for one full cycle. The next section covers what you should hear and what you shouldn’t.
If you have a whole-home humidifier, this is also the time to open its damper for the season and make sure the water supply is on; the water panel itself gets replaced once a season, and we do that at the fall visit. Details at humidifier installation.
What a Healthy Start Sounds Like
A furnace starting from a cold call follows the same sequence every time, and you can hear each step. First a small motor spins up: that’s the inducer, clearing the heat exchanger and proving draft. A few seconds later you may hear a click as the pressure switch closes. Then the igniter glows, the gas valve opens with a soft click, and the burners light with a low whoosh. Thirty to sixty seconds after that, the main blower comes on and warm air reaches the registers. On a two-stage or modulating furnace the first minute may sound quieter than you remember; that’s normal.
Here is what to note and mention when you call, rather than what to fix yourself:
- Burners light and go out within seconds, then retry. Almost always a dirty flame sensor. A cheap, quick repair, but not a DIY one on a gas appliance.
- Inducer runs, nothing lights, and the cycle repeats. Cracked igniter or a pressure switch that won’t close, often because of a blocked vent or a flooded condensate trap.
- A bang or thump a moment after the burners light. Delayed ignition from dirty burners. Worth a call before it cracks something.
- Squeal or grinding once the blower starts. A blower motor or bearing on its way out.
- Heat for a few minutes, then cool air, then heat again. The high-limit switch tripping on low airflow. If you just installed a clean filter and opened the registers, the blower or limit switch needs a look.
A faint dusty smell on the first run is normal and clears in a few minutes. A smell that lingers, a sharp electrical smell, or a CO alarm going off is not. Smell gas? Leave the house right away, call 911 from outside, and call your gas utility’s emergency line from outside before calling us.
What This Check Can’t Catch
The 10-minute check finds the obvious. It does not tell you whether the heat exchanger is cracked, whether the igniter is at 70 percent of its rated resistance and about to fail, whether the gas pressure is set to the nameplate, or whether the combustion numbers say the furnace is burning cleanly. Those need a combustion analyzer, a meter and a technician, which is what a fall furnace maintenance visit is for.
The two go together. Do the homeowner check in early October so you know the furnace runs, and have the tune-up done in September or early October so the parts that are about to fail get replaced on a mild day rather than a cold night. If the furnace is old enough that a tune-up keeps turning up parts, our furnace repair and replacement hub walks through the repair-or-replace decision, and a replacement quote is free and in writing.
If Something Fails Anyway
Some furnaces will fail no matter how carefully you check. When yours does, note the flash code on the control board (visible through the sight glass on the lower door, with the key printed inside the door), turn the thermostat off for a minute and back on to reset a lockout once, and if it still won’t run, call. The pricing is the same whether it’s October or January: a flat diagnostic fee to find the problem, then a flat-rate repair price you approve before any work starts. Common furnace repairs in our area typically run $300 to $1,800. After-hours and emergency calls are priced differently than business-hours visits, and we quote that on the phone before anyone is dispatched to your home.
If you’re going to be away for a stretch when the cold arrives, a smart thermostat that alerts you when the house temperature drops is cheap insurance against a furnace that quits while nobody’s home; a house that sits below freezing for a day risks frozen pipes on top of the repair.
Skip the Rush Entirely With the Maintenance Plan
The $240 semi-annual maintenance plan puts the fall furnace tune-up and the spring AC or heat pump tune-up on our calendar before the season starts, each up to a $130 value, plus one free business-hours service call, 5% off services and priority scheduling.
Priority scheduling is the part that matters in October: when the first cold snap fills the phones, plan members are scheduled ahead of the queue. Read more at furnace maintenance.
First Cold Snap Furnace FAQs
When should I test my furnace before winter in Elgin?
Late September or the first week of October, on a mild day when you don’t need the heat yet. That leaves time to schedule a repair before the first cold snap, which usually arrives in October and fills the calendar with no-heat calls.
What should I check on my furnace before turning it on for the first time?
Replace the filter, set the thermostat to Heat with fresh batteries, open every register and clear the returns, clear the PVC intake and exhaust outside, confirm the power switch and blower door, check the condensate line, test the CO detectors, and then run one full cycle and listen.
Is it normal for a furnace to smell when it first runs in fall?
A faint dusty smell for the first few minutes is normal; it’s dust burning off the heat exchanger. A smell that lingers, a sharp electrical smell, or any gas smell is not. If you smell gas, leave the house right away, call 911 from outside, and call your gas utility’s emergency line from outside before calling us.
My furnace won’t start after sitting all summer. What’s the most likely cause?
A cracked hot surface igniter or a coated flame sensor, followed by a pressure switch problem from a blocked vent or flooded condensate trap. Check the thermostat batteries and filter first, note the flash code on the control board, then call. These repairs typically run $300 to $1,800 in our area.
Does a 10-minute check replace a furnace tune-up?
No. The homeowner check catches filters, settings, blocked vents and dead batteries. A tune-up tests the igniter and flame sensor, inspects the heat exchanger, runs combustion analysis and sets gas pressure, which is where the parts that fail on the first cold night get caught.
Found Something on the Checklist? Get It Fixed Before It’s Cold
Tell us what you heard or what the status light is flashing, and we’ll schedule a repair on a day that suits you, before the rush. Flat diagnostic fee, flat-rate repair price approved before work starts.
Want the full tune-up too? See the $240 maintenance plan or request a free quote.
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