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Learn · 6 min read

My Furnace Isn't Heating — Homeowner Troubleshooting Before You Call

First cold snap in November, phone rings off the hook. Half of those calls could have been solved in five minutes by the homeowner. Here's the safe homeowner checklist — and the hard line where you absolutely stop and call a licensed HVAC tech.

By Eric Hixson · Owner & Master HVAC Technician NC Licensed Mechanical Contractor — License # L.34508 Updated

What can I safely check myself?

  1. Thermostat batteries and settings. Fresh batteries; set to Heat, not Off or Cool; call temp 3–5°F above room temp.
  2. Filter. Choking a furnace on a filthy filter is the fastest way to trip the high-limit safety.
  3. Breaker. Furnace breaker (usually a 15A) at the panel — reset it once. If it trips a second time, stop and call.
  4. Gas supply. Is the gas valve at the furnace open? Is your gas meter working? Does the stove light?
  5. Vent. Direct-vent furnaces intake and exhaust through PVC. Check for snow, ice, bird nests, insulation debris.

When do you STOP and call?

Immediately, no exceptions:

  • You smell rotten eggs (gas leak). Leave the house, then call the gas utility from outside.
  • The furnace fires but soot appears at the vent.
  • The blower runs continuously but no heat is produced (heat exchanger crack risk).
  • The furnace fires for a few seconds then shuts off — three times or more. This is the "lockout after failed ignition" pattern, and continuing to reset it can flood the combustion chamber with unburned gas.

How does DIY furnace troubleshooting fail?

Homeowners commonly bypass a safety limit switch to "get the heat working." Please don't. Those switches are the difference between an inconvenient shutdown and a house fire. If a limit is tripping, the underlying cause (airflow, gas pressure, cracked heat exchanger) needs to be diagnosed.

Frequently asked questions

Why does my furnace turn on then off after a few seconds?

That's a short-cycle / ignition-lockout pattern — usually a dirty flame sensor, an issue with the pressure switch, or a gas supply problem. The board is intentionally locking out to protect the equipment. Stop resetting it and call.

Is it safe to run a furnace with a cracked heat exchanger?

No. A cracked heat exchanger can leak carbon monoxide into your supply air. If a tech red-tags your furnace for this, don't relight it.

Action

Ready for real answers about your system? Eric Hixson and the HVAC team can be in your driveway the same day.

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