Every October, the same call comes in.
Someone flipped the thermostat from cool to heat for the first time since April. Nothing happened. Or something worse — a burning smell, a click-click-click that won't stop, the blower running but blowing cold air. Now it's 38 degrees at night, the kids are in hoodies at the breakfast table, and we're driving over with a flashlight and a multimeter.
We've been doing this in Lehi since 1996. We know the rhythm. And we know that almost none of those calls had to happen.
This is the checklist we actually walk through on a fall tune-up. Not the marketing version with twelve bullet points. The real one — the things that, if you do them in September, will keep your furnace from becoming a crisis in January.
Swap the filter. For real, not "eventually."
The filter is the single thing everyone knows about and almost nobody does on time. Here's what we see when we pull the front panel in October: a filter that should have been changed in June, crusted with dust and pet hair, pulled into a concave dish from being sucked on all summer.
A dirty filter is the root cause of more furnace problems than any other single thing we walk into. It chokes the blower. It stresses the motor. It dumps debris onto the heat exchanger. If the blower can't move enough air through the thing, the furnace overheats and the high-limit switch shuts it down as a safety — and that's the call we get at 6am when the house is 52 degrees.
What to actually do: Pull the filter. Look at it. If you can't see light through it when you hold it up, it's done. Replace it with a MERV 8–11 pleated filter from any hardware store, oriented so the arrow on the frame points toward the furnace. Write the date on the new filter with a Sharpie so you know when you changed it next time.
If you have pets, kids, or you're running the system hard because of Utah's summer inversions, check it monthly. Otherwise every 60–90 days is fine.
Run the heat for 15 minutes. In September.
This is the thing most people skip. They wait until the first cold snap, turn the heat on, and then find out something's wrong.
Pick a warm day in mid-September when you don't actually need heat. Set the thermostat to 75 and let the furnace run for 15 minutes. Stand near a vent and feel the air. It should come out warm, steady, and without any weird smells beyond the initial burn-off of summer dust (that burning-dust smell is normal the first time — it should go away in a few minutes).
What you're listening for:
- Clicking that won't stop. The furnace igniter clicks a few times, then the burners fire. If it clicks over and over and never lights, the igniter is dying or the flame sensor is dirty. Both are quick fixes, but only if you catch them before the first cold night.
- A thump or bang on startup. Delayed ignition. Gas is building up before it lights. Not a wait-and-see. Call someone.
- The blower runs but the air is cold. Gas valve, flame sensor, or pilot issue. Fixable.
- Weird burning smells that don't go away. Turn it off. Open windows. Call us. Could be a lot of things, none of them things you want to ignore.
If everything runs fine for 15 minutes and shuts off cleanly, you're in good shape. If anything felt off, you now have six weeks to deal with it before you actually need the furnace — instead of six hours.
Walk around the outside and look at the flue.
This sounds weird but it matters. Gas furnaces vent combustion gases out through a pipe that exits the side of your house or the roof. In Utah, those vents get blocked by:
- Bird nests. We pull these out constantly. Birds love warm flue pipes in spring, build nests, forget about them, and then the nest blocks the vent when you turn the heat on.
- Dryer lint, leaves, cobwebs. Anything that settled in over the summer.
- Snow, eventually. We're not there yet, but when the first real storm hits, make sure the vent isn't buried against the wall of the house.
A blocked flue is serious. Carbon monoxide can back up into the house. Modern furnaces have a pressure switch that shuts them down if the flue is blocked, which is good — but we'd rather you not find out that way.
Just walk around your house, find the white PVC pipe coming out of the wall (it's usually 2–3 inches wide, angled slightly downward), and look up it with a flashlight. If you see anything other than clean pipe, clear it out or call us.
Test your carbon monoxide detector.
Speaking of carbon monoxide: if you don't have a CO detector on every level of your house, stop reading this and go buy some. They're thirty bucks at any hardware store and they will literally save your family's life.
If you already have them, press the test button on each one. If the beep is weak or it doesn't respond at all, replace the batteries or replace the whole unit if it's more than seven years old.
Every fall we have at least one customer who calls with a "weird furnace smell" and we show up to a CO alarm they didn't know was beeping because the battery was dead. Don't be that customer.
Clear the airflow around registers.
Walk through every room and check that nothing is blocking the supply vents or the cold-air returns. Over the summer, things migrate: a couch gets pushed six inches closer to the wall, a rug ends up half-covering a floor register, a kid's beanbag ends up against the return grille in the hallway.
The furnace needs unrestricted airflow to work efficiently. A blocked return in particular is sneaky — you'll never notice it visually, but it starves the blower and causes the same overheating problems a dirty filter does.
Also: if you have vents in rooms you don't use, don't close them off to "save energy." That's an old wives' tale. Modern systems are balanced to move a specific volume of air. Closing registers doesn't reduce the load on the furnace — it just raises static pressure in the ducts and makes the blower work harder.
Know where the emergency shutoff is.
On the side of every gas furnace, there's a switch that looks like a light switch. It is not a light switch. It cuts power to the furnace.
Find it now, before you need it. Flip it off, flip it back on, make sure the furnace starts up normally. If your furnace ever starts making a horrifying noise, smelling like burning plastic, or doing anything else that freaks you out, that switch is the thing you want to know how to find in the dark at 2am.
Also know where your main gas shutoff is. It's usually near the gas meter on the outside of your house. A quarter-turn of that valve with a wrench shuts off all the gas to the house. You don't want to turn it unless you have to, but you want to know where it is.
If your furnace is over 15 years old, now is the time to plan.
We're not going to pressure you to replace a working furnace. If it's running, it's running.
But here's what we see: furnaces in Utah have a realistic lifespan of 15–20 years. Around year 15, we start seeing the signs — small parts starting to fail, efficiency dropping, repair costs creeping up. By year 20, most systems are running at 60% of their rated efficiency, and you're paying the difference on every utility bill.
If yours is in that zone, the time to plan for replacement is in September or October, when technicians are booked but not slammed, prices haven't spiked, and you can actually shop around. The worst possible time to replace a furnace is the middle of January, when it just failed, it's 12 degrees outside, and you'll take whatever's available.
Get a few estimates. Ask what brands the technicians actually install in their own homes. Ask about rebates — Utah has decent ones for high-efficiency systems, and most installers know which ones you qualify for.
The short version
If you do nothing else this fall, do these four things:
- Change the filter.
- Run the heat for 15 minutes in September, before you actually need it.
- Walk around outside and look at the flue vent.
- Test your CO detectors.
That's it. Twenty minutes of your time. It prevents probably 80% of the calls we get in October and November.
And if something feels off when you run it, call us. We'd rather come out on a warm day and catch a dying igniter before it leaves you in the cold than show up at midnight with a frozen house and a kid in a sleeping bag on the couch.
We've done both. The warm-day version is a lot nicer for everyone.