No heat — what to check before calling for repair
First the safety check below — if you smell gas or burning, stop and follow the emergency steps, don't troubleshoot. Otherwise, work top-down: thermostat set to heat and above room temp, the furnace switch and breaker on, the filter not clogged, and the door panel fully seated (many furnaces won't run with it off). On gas units, check whether the pilot or ignitor is working. If it still won't fire or keeps short-cycling, it's a technician call.
First, a safety check — is any of this true right now?
No hazard flagged — the diagnostic steps below are appropriate. If any of the above changes, stop and follow the safety guidance.
The five safe checks, in order
1) Thermostat: set to Heat, target a few degrees above the room, fresh batteries. 2) Power: the furnace switch (looks like a light switch nearby) on, and the breaker not tripped. 3) Filter: a clogged filter can shut a furnace down on a safety limit — replace it if it's dirty. 4) Door panel: many furnaces have a switch that stops them running with the panel off; seat it firmly. 5) Gas supply: the valve to the furnace open.
A furnace that starts then shuts off after a minute (short-cycling) is often an overheating limit from a dirty filter or a blocked vent — but repeated short-cycling needs a technician.
When it's beyond the checklist
If the ignitor doesn't glow, the pilot won't stay lit, or it trips its safety repeatedly, the cause is usually the ignitor, flame sensor, thermocouple, or control board — a technician's job. A diagnostic visit is an estimated $70–$250 (industry estimate). Never bypass a safety switch to force it to run.
When to call a professional
- • Any gas or burning smell — stop and follow the safety steps above.
- • The ignitor/pilot won't work, or it short-cycles repeatedly after you've changed the filter.
- • You'd have to bypass a safety switch to make it run — don't.
Common questions
- Why does my furnace blow cold air?
- Often the thermostat fan is set to ON (so it blows even when not heating) rather than AUTO, or the burner isn't igniting. Set the fan to AUTO first; if air is still cold, the burner/ignitor needs a look.
- Can a dirty filter really stop the furnace?
- Yes — restricted airflow can trip a high-limit safety switch that shuts the burner down to prevent overheating. It's the cheapest thing to rule out first.
Sources
- Inspect central AC/furnace filters monthly; replace 1-inch filters every 1–3 months and thicker media filters every 6–12 months. Tier 1 (official)
ENERGY STAR (EPA/DOE) · verified 2026-07-26 - HVAC service call / diagnostic: $70–$250 (US, industry estimate). Tier 3 (industry estimate)estimate · has commercial bias
Angi · verified 2026-07-26