Why Won't My Furnace Turn On?
When a furnace won't start, the cause is almost always one of a handful of common issues. Most are straightforward to diagnose and many are quick to fix. Here's what to look at — in order of likelihood.
1. Thermostat Settings
Before anything else, confirm the thermostat is set to Heat, not Cool or Fan Only, and that the set temperature is above the current room temperature. It sounds obvious, but it's the first thing any technician checks. Also verify the thermostat has power — replace the batteries if it's been over a year.
2. Tripped Circuit Breaker or Power Switch
Furnaces have two power sources — the breaker in your electrical panel and a dedicated on/off switch on or near the unit that looks like a light switch. Check both. A tripped breaker will be halfway between On and Off — reset it fully to Off, then back to On. If it trips again immediately, there's an electrical issue that needs a technician.
3. Clogged Air Filter
A severely restricted air filter cuts airflow to the point where the furnace can't operate safely. Most modern furnaces will shut down before they overheat. Check your filter — if it's gray and dense with dust, replace it. Filters should be changed every 1 to 3 months depending on your home.
4. Ignition or Pilot Light Issues
Older furnaces use a standing pilot light. If it's out, follow the instructions on the furnace door to relight it. Modern furnaces use electronic ignition — you may hear it clicking repeatedly when the thermostat calls for heat. If it clicks but the burner never lights, the igniter may have failed and needs replacement.
5. Gas Supply Interrupted
Confirm your gas supply is on. Check that the gas shutoff valve near the furnace is in the open position (handle parallel to the pipe). If other gas appliances in your home — water heater, stove — are also not working, contact your gas utility. If only the furnace is affected, the issue is in the furnace itself.
Why Is My Furnace Blowing Cold Air?
A furnace that runs but delivers cold or lukewarm air is a different problem from one that won't start. The most common causes are below.
Dirty or Faulty Flame Sensor
The flame sensor is a small rod that confirms the burner is actually lit before allowing gas to continue flowing. When it gets coated with residue — which happens gradually with normal use — it can no longer detect the flame and shuts the burner off as a safety measure. The furnace will start, run briefly, then cut off. The blower continues running, pushing unheated air through the vents. Flame sensor cleaning is one of the most common furnace repairs and typically takes under an hour.
Overheating and Limit Switch Activation
When a furnace gets too hot — usually due to restricted airflow from a clogged filter or blocked vents — the high-limit switch shuts the burner off to prevent damage. The blower keeps running to cool the heat exchanger, which is why you feel cold air. Replacing the filter and clearing any blocked registers often resolves this. If the limit switch itself has failed, it will need replacement.
Cracked Heat Exchanger
The heat exchanger is the component that transfers heat from the combustion gases to the air in your home — without the two mixing. A crack in the heat exchanger allows combustion gases, potentially including carbon monoxide, into your breathing air. This is a serious safety issue. If a technician identifies a cracked heat exchanger, the furnace should not be run until it is replaced. In most cases, a cracked heat exchanger means the furnace needs to be replaced.
What to Check Before You Call
- Thermostat set to Heat, temperature above current room temp
- Furnace power switch in the On position
- Circuit breaker not tripped
- Air filter — replace if visibly clogged
- All supply and return vents open and unblocked
- Gas supply on (other gas appliances working)
- Furnace door fully closed and latched (safety switch)
When to Call a Technician
Call Licking County HVAC if the furnace still won't operate after working through the checklist above, if you smell gas or burning near the unit, if your carbon monoxide detector activates, if the furnace is short cycling repeatedly, or if it's showing an error code on the control board. We offer same-day service throughout Newark, Heath, Granville, Pataskala, and all of Licking County — and 24/7 emergency service when heat is out entirely.