The first real heat demand in Skokie usually lands somewhere in mid-October, when an overnight low drops into the 30s and the furnace fires for the first time since March. That's when the phone starts ringing. Not because furnaces break in October, but because they broke sometime last spring and nobody noticed — a cracked igniter, a flame sensor coated in carbon, a pressure switch hose full of condensate. The unit sat idle for seven months and the failure sat with it.
Skipping a fall check doesn't usually cost you a furnace. It costs you a night. You lose heat on the coldest evening of the month, you're competing with every other no-heat call in Cook County, and a $180 flame sensor cleaning turns into a $400 after-hours diagnostic because that's when it happened. That's the whole argument for a furnace tune-up in Skokie before November: not drama, just timing.
Fire it up in September, not December
Run the furnace for 20 minutes on a mild day while it's still warm enough that a failure is an inconvenience instead of an emergency. Set the thermostat 5 degrees above room temperature and stand near the unit.
You're listening and looking for four things. Does it light within about 30 to 45 seconds of the call for heat, or does it try, fail, and retry? Does the blower come on and stay on, or does it cut out after a few minutes and restart — that's short cycling, and it points to a limit switch tripping on high temperature, usually because of restricted airflow. Do you smell burning dust for the first 10 minutes? Normal. Do you smell anything sharp or acrid, or anything like rotten eggs? Shut it down and call the gas utility first.
This one's DIY. Everybody should do it, and almost nobody does.
The checklist that actually matters
- Change the filter and check the thickness. A one-inch filter needs changing every one to three months during the heating season. If you've got a four- or five-inch media cabinet, once or twice a year is fine. Don't buy the highest MERV rating on the shelf for a one-inch slot — a MERV 13 pleated filter in a thin frame chokes the blower, raises static pressure, and trips the high-limit switch. MERV 8 to 11 is the sweet spot for most residential systems. DIY.
- Look at the burner flames. Pop the front panel while the burners are lit. You want steady blue flames, uniform across all burners. Yellow, lazy, or flickering flames mean incomplete combustion — dirty burners, a cracked heat exchanger, or inadequate combustion air. That's a stop-and-call situation, not a wait-and-see one. Look yourself, but a yellow flame is a pro call.
- Test the CO detectors. Press the button on every one. Detectors have a service life of five to ten years depending on the model, and the date is printed on the back. If yours is older than the furnace, replace it. Any gas-fired appliance deserves a working detector on every floor and outside every sleeping area. DIY.
- Clear the intake and exhaust terminations outside. If you have a high-efficiency condensing furnace, those two white PVC pipes coming out the side of the house are its lungs. Skokie's older bungalows and Cape Cods often have them terminating a foot or two above grade, which is fine until we get a 14-inch snowfall and a plow berm. A blocked intake shuts the furnace down on a pressure switch fault. Keep 12 to 18 inches clear all winter and check after every big storm. DIY, and it's the most seasonal item on this list.
- Check the condensate drain and trap. High-efficiency furnaces produce a few gallons of acidic water per day when running hard. That water goes through a trap and a drain line, often into a condensate pump in a Skokie basement. Algae and sediment clog the trap, water backs up into the inducer, and the furnace locks out. A pro clears this properly; if you're comfortable, you can at least confirm the pump runs and the discharge line isn't kinked. Mostly pro.
- Have the flame sensor and igniter checked. The flame sensor is a thin metal rod that proves the burner lit. It builds up a film of oxidation over a season and stops conducting, so the furnace lights, runs three seconds, and shuts off — over and over. Cleaning it takes two minutes with an abrasive pad and it's the single most common no-heat call there is. Hot-surface igniters are brittle silicon carbide and typically last four to seven years. Pro, mainly because it's a good excuse to have combustion checked at the same time.
- Get static pressure measured. This is the number nobody looks at and everybody should. Most residential ductwork is designed for about 0.5 inches of water column total external static pressure. Plenty of Skokie homes measure 0.9 or higher because a 1970s duct system got asked to carry a modern high-static blower. High static pressure shortens blower life, kills efficiency, and makes the second floor cold no matter what thermostat you install. Pro. It takes a manometer and two drilled test ports.
What Skokie's housing stock does to furnaces
The brick bungalows and post-war ranches around here mostly got their ductwork designed for gravity or low-static systems, then retrofitted. Return air is almost always the shortfall — one big central return in a hallway, nothing on the second floor, bedroom doors closed all night. Add duct runs through an unconditioned attic or a knee wall and you can lose 20 to 30 percent of your conditioned air before it reaches a register.
That's why a cold back bedroom usually isn't a furnace problem. Be skeptical of anybody whose first move is a replacement quote. Swapping a working 80,000 BTU furnace for a new 80,000 BTU furnace changes nothing if the air can't get where it's going. And if somebody suggests going bigger to fix it, that's backwards — oversized equipment short cycles, never reaches steady state, and heats unevenly. A load calculation beats a rule of thumb. Every time.
The repair-versus-replace math, briefly
A reasonable rule of thumb: multiply the repair cost by the age of the furnace. If that number is more than about $5,000, replacement deserves a serious look. A $600 inducer motor on a nine-year-old furnace is worth fixing. The same repair on a 22-year-old unit with a suspect heat exchanger is not.
Typical national ranges for a gas furnace replacement run roughly $4,500 to $9,000 installed, with high-efficiency modulating equipment and any duct or venting modifications pushing toward the top. Local quotes vary with sizing, efficiency tier, and how much of the existing duct system has to be reworked. If you're weighing efficiency upgrades, check current federal and state incentives before you sign anything.
If you only do one thing
Change the filter and run the furnace for 20 minutes this week. That single step catches most of what turns into a January no-heat call, and it costs you $15 and a half hour. If it lights clean, runs steady, and the flames are blue, you're in decent shape. If it hesitates, cycles, or smells wrong, you've found the problem in September instead of at 2 a.m. in a cold house — which is the entire point.