Furnace Not Heating? What to Check Before Calling HVAC
1. Quick Checks (Free, 10 Minutes)
Do all of these before calling anyone:
- Thermostat: Set to HEAT, temp 5–10 degrees above room temp. Replace batteries even if the screen is on (weak batteries can stop the heat signal).
- Breaker: Check for tripped breaker labeled "furnace" or "air handler." Flip fully OFF then ON.
- Furnace power switch: Looks like a light switch on or near the furnace. These get accidentally bumped off. Make sure it's ON.
- Furnace door panel: Remove and re-seat firmly. There's a safety switch that kills power if the panel is loose.
- Air filter: A completely clogged filter can prevent the furnace from starting. Pull it out — if it's gray and packed with dust, replace it.
2. Furnace Starts Then Shuts Off? Clean the Flame Sensor
If the furnace ignites but shuts down after 5–10 seconds, the flame sensor is dirty. This is the #1 HVAC service call and it's a free DIY fix.
- Turn off power to the furnace.
- Find the flame sensor — a thin metal rod near the burners.
- Pull it out (usually one screw).
- Lightly sand the metal rod with fine sandpaper or steel wool.
- Reinstall, restore power, test.
Furnace doing something weird?
Describe what happens (or doesn't happen) when you turn the heat on. homemd.ai will narrow it down to the most likely cause and tell you if it's a DIY fix or needs a tech.
Get Your Free Diagnosis3. Parts You Might Need
Filtrete 1500 MPR Air Filter — ~$10 (match your size) — View at Home Depot
Honeywell Home T1 Thermostat — ~$25 (if thermostat is dead) — View at Home Depot
4. Cost: Repair vs Replace
| Issue | DIY | HVAC Tech |
|---|---|---|
| Thermostat / filter / switch | $0–$25 | — |
| Flame sensor cleaning | $0 | $100–$180 |
| Igniter replacement | — | $150–$350 |
| Blower motor | — | $500–$1,200 |
| Full furnace replacement | — | $4,000–$9,000 |
Rule of thumb: If the furnace is 15+ years old and the repair is over $1,000, replace it.
5. When to Call HVAC
- You smell gas — leave immediately and call your gas company.
- Furnace won't start after all quick checks above.
- Furnace starts but blows cold air continuously.
- You see error codes blinking on the furnace control board.
- Strange smells (burning, electrical) when it runs.
FAQ
Why is my furnace running but not blowing warm air?
Most common: clogged filter restricting airflow, or the furnace is in "fan only" mode on the thermostat. Check both first.
How long does a furnace last?
15–25 years for a gas furnace. If yours is 15+ and needs a major repair, replacement is usually the better investment.
Frequently Asked Questions
Why is my furnace running but not heating?
Check the thermostat setting and batteries first. Then check the air filter (a clogged filter can cause the furnace to overheat and shut down). If those are fine, the ignitor, flame sensor, or gas valve may have failed.
How much does furnace repair cost?
Thermostat or filter fix: $0 to $30 (DIY). Ignitor replacement: $150 to $300. Flame sensor: $100 to $250. Blower motor: $400 to $700. Heat exchanger: $1,500 to $3,000 (often worth replacing the whole unit).
Why does my furnace start then shut off after a few seconds?
A dirty flame sensor is the most common cause. The sensor can't detect the flame, so the furnace shuts off as a safety measure. Clean it with fine sandpaper, free fix.
How often should I replace my furnace filter?
Every 1 to 3 months during heating season. A clogged filter restricts airflow and can prevent the furnace from starting. Check it monthly and replace when it looks gray.
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