You have no heat. Before anything else, one question: do you smell gas?
If you smell a rotten egg or sulfur odor anywhere in the house — stop reading this. Get everyone out of the house immediately, leave the door open, don't touch light switches or your phone until you're outside, and call your gas utility's emergency line or 911 from a neighbor's house. Gas leaks are not troubleshooting situations.
If there's no gas smell, keep reading. There's a real chance you can get your heat back in the next 20 minutes without a service call.
Step 1: The Thermostat (Always Start Here)
Most heating calls that techs go on in winter — more than you'd expect — turn out to be thermostat issues. Check these before anything else:
- Is it set to HEAT, not COOL or FAN ONLY?
- Is the setpoint at least 3 to 5 degrees above the current room temperature? If the house is at 68°F and the thermostat is set to 68°F, there's no call for heat.
- Is the fan set to AUTO rather than ON? The ON setting runs the fan constantly — including when nothing is heating — and that air blowing across your face feels cold.
- Is the display blank? Replace the batteries. A dead battery kills the system completely.
If you have a Nest, Ecobee, or Honeywell Home smart thermostat, check the app. Schedules get accidentally modified. Systems get manually switched to OFF and left that way.
Step 2: The Circuit Breaker and Power Switch
Gas furnaces still run on electricity. The blower motor, control board, and ignition system all need power. Heat pumps are fully electric. Either system can go dark if a breaker trips.
Find your electrical panel and look for the breaker labeled "furnace," "air handler," "heat pump," or "FAU." If it's tripped, flip it fully to OFF first, then back to ON.
Also check for the on/off switch on or near the unit itself — it looks exactly like a light switch. These get flipped accidentally more often than they should.
One warning: if the breaker trips again within an hour, don't reset it a third time. A breaker that keeps tripping is telling you there's an electrical fault. Continuing to reset it risks damaging the equipment.
Step 3: The Air Filter
A clogged filter can shut down a furnace or heat pump entirely. When airflow drops too low, the system overheats, triggers a high-limit safety switch, and shuts off. You may hear it start and stop repeatedly — that's the limit switch cycling.
Pull the filter out and hold it up to a light. If you can't see light through it, replace it now. A new filter costs $5 to $25 and takes two minutes. This is the most common fixable cause of a system that won't heat.
Step 4: Check the Emergency Shutoff
There's often a red emergency shutoff switch near the furnace or air handler, usually at the top of the basement stairs or just outside the mechanical room. It looks like a light switch with a red cover plate. Make sure it's in the ON position.
While you're at it, look at the furnace door. Modern units have a safety interlock built into the front access panel — if the door isn't fully latched, the furnace won't run. Remove it, reseat it firmly, and try the thermostat again.
Now: Two Different Systems, Two Different Problems
From here, what you check depends on whether you have a gas furnace or a heat pump. These systems fail differently.
Gas Furnace: What to Check
Pilot Light or Ignitor
Older furnaces (pre-2000) use a standing pilot — a small continuous flame that lights the burners. No pilot, no heat. Look for a small viewport or window on the furnace body. If there's no flame visible, the pilot is out. The relighting procedure is printed on the furnace door: turn the valve knob to PILOT, press and hold, hold a flame to the pilot, and keep holding the knob for 30 to 60 seconds after it catches.
If the pilot lights but goes out immediately when you release the knob, the thermocouple is faulty. A tech can replace it for $100 to $200.
Newer furnaces (post-2000) use a hot surface ignitor — a small element that glows orange-hot to ignite the gas. You can sometimes see it glow through the observation window during a startup attempt. If it's not glowing, it's likely failed. Don't touch it; even skin oils can shorten the life of a new one. This is a $150 to $350 repair.
Check the Gas Supply
Is the furnace attempting to start — ignitor glowing — but the burners won't light? Check whether other gas appliances are working. Fire up a burner on the stove. If the stove works, gas is getting to the house; the problem is local to the furnace.
Look at the manual shutoff valve on the gas line going directly to the furnace. The handle should be parallel to the pipe (open). If it's perpendicular, it's closed. This gets turned off during plumbing work and not always turned back on.
Read the Error Codes
Most furnaces built after the mid-1990s have a diagnostic LED on the control board that blinks a pattern when something is wrong. The decode chart is on the inside of the access panel door. Common patterns:
- 3 blinks — Limit switch tripped (often a clogged filter — did you replace yours in Step 3?)
- 2 blinks — Pressure switch fault (blocked flue, failed inducer motor, kinked condensate drain)
- 1 blink — Ignition failure or flame sensor issue
- 5 blinks — Flame sensed without a call for heat. This means gas is flowing without ignition. Turn the system off and call a technician immediately.
For a deeper walkthrough of furnace error codes and ignitor diagnosis, see the furnace not working troubleshooting guide.
Heat Pump: What to Check
Heat pumps confuse a lot of homeowners because they fail differently than furnaces — and because some things that look like failures are actually normal.
Is the Outdoor Unit Running?
Go outside and look at the heat pump's outdoor unit. The fan on top should be spinning when the system is running in heat mode. If the unit is completely off and silent, the issue may be the circuit breaker, thermostat, or a refrigerant-related lockout.
The Defrost Cycle — This Is Normal
If the outdoor unit is coated in frost or light ice, and the system seems to be blowing cooler air for a few minutes, your heat pump may be in a defrost cycle. This is normal operation.
When temperatures drop near freezing, moisture in the air condenses and freezes on the outdoor coil. The system periodically reverses to melt that frost — essentially running briefly in cooling mode — which means the air coming out of your vents will feel cool or room temperature for 5 to 15 minutes. Many systems automatically activate auxiliary heat strips during defrost to offset this.
If the outdoor unit is encased in a solid block of ice — not a light frost coating but actual ice buildup several inches thick around the entire unit — that's abnormal. The defrost control may have failed, or airflow around the unit may be too restricted. Call a technician.
Check the Thermostat Mode
Heat pumps require the thermostat to be set to HEAT mode, and the system type should be correctly configured. If a thermostat was recently replaced or a battery swap reset the programming, the system type setting can revert. A heat pump configured as a "conventional" system may try to run in the wrong mode entirely.
Also look for a EMERGENCY HEAT or AUX HEAT indicator on the thermostat. Emergency heat mode bypasses the heat pump entirely and runs only the backup electric heat strips — it's for when the outdoor unit has failed. It's expensive to run but it will heat the house while you wait for a tech.
Thermostat Not in the Right Mode
One scenario that catches people: a heat pump thermostat left in COOL mode, or with the system switch set to OFF. With a heat pump, cooling and heating use the same outdoor equipment, so the system can appear "on" while actively working against you. Verify the mode is HEAT before anything else.
Refrigerant Issues and Low-Charge Symptoms
A heat pump low on refrigerant will struggle to produce heat, especially when outdoor temps drop below 40°F. Signs: the system runs continuously, the indoor air is only slightly warmer than the room, and the outdoor unit may ice up even in mild weather. This requires a technician with recovery equipment and an EPA 608 license to diagnose and repair.
When to Call Emergency Service vs. Wait Until Morning
Not everything requires an after-hours call. Emergency HVAC service runs 1.5 to 2 times the standard rate — diagnostic fees alone run $150 to $300 after hours and on weekends, compared to $75 to $150 during business hours.
Call emergency service now if:
- You smelled gas and have returned to the house (don't — but if a tech is needed after the gas utility clears the space, that's an emergency dispatch)
- The furnace error codes indicate a flame-without-ignition fault (gas leaking past the valve)
- You have infants, elderly family members, or anyone medically vulnerable in the house and indoor temps are dropping toward 50°F
- Indoor temperature is below 55°F with no ability to keep it above that through other means
- The system is making banging, grinding, or screeching noises that suggest imminent mechanical failure
Wait for regular hours if:
- The house is cold but stable above 60°F with space heaters
- No one in the home has a medical condition aggravated by cold
- The outside temperature is mild enough that the house won't drop dangerously overnight
- The issue appears to be a part failure (bad ignitor, failed capacitor) with no safety risk
For a realistic picture of what repairs cost, the HVAC repair cost guide breaks down common failure scenarios.
While You Wait: Staying Warm Safely
If you're waiting for a tech — whether tonight or tomorrow morning — these steps will help keep the house livable and your pipes safe.
Space heaters: use them, but carefully. Plug directly into a wall outlet — never an extension cord or power strip, which can overheat. Keep the heater at least three feet from anything that can burn. Don't run it unattended or while sleeping. Focus one heater per occupied room rather than trying to heat the whole house; they're not efficient enough for that, and running multiple units on a single circuit can trip a breaker.
Manage the house. Close off rooms you're not using — a smaller space is easier to keep warm. Open curtains on south-facing windows during daylight to let in solar gain; close them all at night. Hang a blanket over drafty exterior doors.
Protect your pipes. If indoor temperatures drop below 55°F, the pipes in exterior walls, unheated crawl spaces, and attics are at risk of freezing. Open cabinet doors under sinks on exterior walls to let warmer air reach the pipes. A trickle of water running from a faucet on an exterior wall can prevent freezing. If you know a pipe is vulnerable, a small space heater pointed at the wall near it is a reasonable precaution.
Don't use the oven or gas range to heat the house. Carbon monoxide risk is real. Gas appliances aren't designed for space heating, and running them this way in an enclosed house can be dangerous.
When to Stop Troubleshooting
If you've been through this list and still have no heat, you've done what's reasonable. The problem is most likely a component failure — ignitor, control board, capacitor, reversing valve, or something else that requires test equipment and parts. That's what technicians are for.
When you call, tell them:
- Whether you have a gas furnace or heat pump
- What the system is doing when it tries to start (nothing at all, short cycling, error codes)
- What you've already checked and ruled out
- The system's approximate age and brand
That information gets you a faster diagnosis and a tech who arrives with the right parts. You can find licensed HVAC technicians in your area — describe the problem, and local techs will respond with availability.
For guidance on choosing a tech, the how to find a reliable HVAC technician guide covers what to look for and what questions to ask before you book.
Frequently Asked Questions
Why is there no heat in my house even though the furnace is running?
The most common reasons: the thermostat fan is set to ON instead of AUTO (pushing unheated air), the air filter is clogged and the system is short-cycling on the high-limit switch, or there's an ignition failure where the furnace starts but can't sustain a flame. Check the thermostat mode and filter before anything else.
What do I do if I have no heat and smell gas?
Leave the house immediately. Don't touch light switches, don't use your phone inside, and don't open windows. Call your gas utility's emergency line or 911 from outside or from a neighbor's home. Don't return until the utility has cleared the space.
Is it safe to sleep in a house with no heat?
At temperatures above 55°F, it's uncomfortable but not dangerous for healthy adults. Use space heaters safely in sleeping areas and keep the door closed to retain heat. If you have infants, elderly family members, or anyone medically vulnerable, consider staying somewhere warmer. Watch for pipe freezing if temps drop significantly.
Why is my heat pump blowing cold air?
Most likely: the system is in a defrost cycle (normal, lasts 5 to 15 minutes), the thermostat is set to COOL instead of HEAT, or the system is low on refrigerant. If the outdoor unit is completely encased in ice rather than a light frost coating, the defrost control may have failed.
How much does emergency heating repair cost?
After-hours diagnostic fees run $150 to $300, compared to $75 to $150 during business hours. Total repair costs for common emergency issues — failed ignitor, bad capacitor, dirty flame sensor — typically run $250 to $700 after hours, including the service fee. Major component failures cost more regardless of when you call.
What should I check before calling for no heat?
In order: thermostat settings and batteries, circuit breaker and power switch, air filter, emergency shutoff switch, and access panel door latch. For gas furnaces, also check the pilot light or ignitor and look for error codes on the control board. For heat pumps, confirm the thermostat is in HEAT mode and verify the outdoor unit isn't in a normal defrost cycle.
Can a clogged air filter cause no heat?
Yes. A severely clogged filter reduces airflow enough to trigger the high-limit safety switch, which shuts the furnace off to prevent overheating. The system may start and shut down repeatedly in short cycles. Replace the filter and the issue often resolves itself within one full heating cycle.
When is no heat a true emergency versus a next-day call?
Emergency: gas odor, flame-without-ignition error codes, temperatures dropping below 55°F with vulnerable occupants. Next-day: comfortable enough overnight with space heaters, outdoor temperatures mild, no safety-related symptoms. Emergency after-hours service costs substantially more — reserve it for situations that genuinely can't wait.