It's 20 degrees outside and your furnace is running — but the house isn't getting warm. Or maybe the furnace won't turn on at all. Either way, here's how to work through the most common causes in order, what you can check yourself, and what points to a bigger problem that needs a technician.
Start Here: A Few Checks Before Anything Else
Before digging into specific problems, run through these basics. They resolve more service calls than most people expect — and they take less than five minutes.
Quick checks first
- Thermostat is set to HEAT and the temperature is set above the current room temperature
- Thermostat fan setting is on AUTO — not ON (ON runs the blower constantly, even when there's no heat, which can feel like cold air)
- The furnace switch on the wall near the unit is in the ON position (it looks like a light switch)
- The circuit breaker for the furnace hasn't tripped — check your panel and reset it if needed
- The furnace filter isn't completely clogged — a blocked filter can cause the system to overheat and shut down as a safety measure
If the furnace starts after checking any of the above, great. If not, keep reading — the problems below are listed from most to least common.
Problem 1: Furnace Runs but Blows Cold Air
This is one of the most common complaints. The blower is running, air is coming out of your vents, but it's not warm. A few things can cause this:
Thermostat fan set to ON instead of AUTO
When the fan is set to ON, the blower runs continuously — including when the furnace isn't actively heating. You'll get room-temperature air from the vents in between heating cycles. Switch the fan to AUTO and see if the air warms up.
Pilot light or ignition failure
Older furnaces use a standing pilot light; newer ones use electronic ignition. If the burner isn't lighting, you'll get air movement but no heat. On a gas furnace, you can sometimes see the burner flames through a small window on the unit. No flames when the system is calling for heat means an ignition problem.
Overheated system (dirty filter)
A clogged air filter restricts airflow, which causes the furnace heat exchanger to overheat. Most furnaces have a high-limit switch that shuts off the burner when this happens — but leaves the blower running to cool the system down. The result is cold air from the vents. Replace the filter and give the unit 30 minutes to cool before testing again.
Gas supply issue
Check that the gas shutoff valve on the supply line to the furnace is fully open (handle parallel to the pipe). If your other gas appliances aren't working either, the issue may be with your gas service.
Problem 2: Furnace Won't Turn On at All
If the furnace is completely unresponsive — no blower, no noise, no ignition attempt — work through these in order:
Power issues
Check the breaker, the furnace switch near the unit, and — if your furnace has a door panel — make sure the door is fully closed. Many furnaces have a safety switch that cuts power when the access panel is open or not seated properly.
Thermostat not communicating
Try replacing the thermostat batteries if it's battery-powered. If you have a smart thermostat, check that it's connected to Wi-Fi and hasn't lost its settings after a power outage. You can also test the thermostat by manually raising the set temperature significantly — if the furnace doesn't respond at all, the issue may be the thermostat itself or the wiring.
Condensate drain clog (high-efficiency furnaces)
High-efficiency (90%+ AFUE) furnaces produce condensate — water as a byproduct of combustion. If the condensate drain line clogs, many units shut down completely as a safety measure. Check the drain line and float switch near the furnace for standing water or blockage.
Problem 3: Furnace Cycles On and Off Too Often
Short-cycling — when the furnace turns on, runs briefly, shuts off, and repeats — usually points to one of three things:
- Dirty filter: Restricts airflow, triggers the high-limit safety switch, shuts the burner off early.
- Overheated heat exchanger: Often caused by a dirty filter, closed vents, or blocked return air — same result as above.
- Flame sensor issue: The flame sensor tells the control board that the burner has lit. If it's coated in residue, the sensor can't confirm ignition and shuts the burner off after a few seconds. This is a technician fix — the sensor needs to be cleaned or replaced.
Problem 4: Furnace Makes Unusual Noises
Not every noise means an expensive repair — but some do. Here's a quick guide:
Signs That Point to a Bigger Problem
Some symptoms go beyond simple troubleshooting and warrant a call regardless of what you find on your own:
A note on carbon monoxide
A cracked heat exchanger is one of the more serious furnace failures — it allows combustion gases, including carbon monoxide, to mix with your heated air. You can't see a cracked heat exchanger without opening the furnace. If you don't have a CO detector near your furnace and sleeping areas, install one. If a technician diagnoses a cracked heat exchanger, replacing the furnace is typically the right call — that repair often costs more than a new unit.
When to Call vs. When to Wait
Here's a practical breakdown for St. Louis homeowners:
- • Replace a clogged filter and reset the system
- • Change thermostat batteries or correct thermostat settings
- • Reset a tripped breaker
- • Clear a condensate drain line (if you're comfortable doing it)
- • Ignition failures — pilot light won't stay lit, electronic igniter not firing
- • Flame sensor cleaning or replacement
- • Blower motor or belt issues
- • Thermostat wiring problems
- • Furnace error codes you can't resolve
- • Any smell of gas — evacuate first, then call
- • Yellow or orange burner flame
- • CO detector alarm
- • Cracked heat exchanger diagnosis
One More Thing: Preventing Most of These Problems
The majority of furnace problems we see in St. Louis — short-cycling, overheating, ignition failures — trace back to deferred maintenance. A furnace tune-up once a year (typically in fall before heating season) catches issues before they become failures. The tech cleans the flame sensor, checks the heat exchanger, tests safety controls, and makes sure the unit is running as efficiently as it should be.
It's not a sales pitch — it's just cheaper to catch a dirty flame sensor in October than to pay for an emergency call in January when it's 10 degrees out and the furnace won't light.
Having furnace problems in St. Louis?
We've been diagnosing and repairing furnaces in St. Louis homes since 2010. If you've run through the basics and your system still isn't heating, give us a call. We'll tell you exactly what's wrong, what it costs to fix, and whether a repair makes more sense than a replacement — no pressure, no upselling.