When a home heater stops warming, check thermostat mode, filter, power, gas supply, and safety sensors first.
Cold air blowing when you need warmth is stressful. The good news: many no-heat issues come down to a short list of settings, airflow blocks, and basic safety lockouts you can clear without tools. This guide walks you through fast checks, deeper steps, clear safety notes, and when it’s time to book service. Keep a flashlight handy, take your time, and only work on panels you can remove without defeating safety interlocks.
No Heat From The Furnace — Quick Checks
Start with the simple stuff. Small wins here save a service call and get heat back fast. Use the table below like a triage card, then move into the detailed sections that follow.
| Symptom | What To Check | DIY Step |
|---|---|---|
| Blower runs, air is cold | Thermostat mode, fan setting | Set to Heat, fan to Auto; bump setpoint 3–5°F above room |
| No airflow at all | Breaker, service switch, door switch | Reset tripped breaker, flip service switch On, reseat front panel |
| Short bursts of warm air, then off | Clogged filter, high-limit trips | Replace filter; check all supply/return grilles are open |
| Starts then shuts down in seconds | Flame sensor, ignition sequence | Power cycle; clean sensor only if accessible and safe |
| Outdoor vent icing or gurgle | Condensate drain, pressure switch | Clear drain trap; thaw outdoor terminations |
| Nothing happens after setpoint raised | Thermostat batteries/wiring | Swap batteries; reseat thermostat on base |
Thermostat And Power Basics
Make sure the control is actually calling for heat. Set the mode to Heat, fan to Auto, and raise the setpoint by at least 3–5°F above room temperature. If the screen is blank or fading, replace the batteries. Many wall controls cut heat calls when the batteries sag under load. If yours is hard-wired, pull it gently from the base and check that the control wires are seated and not corroded.
Next, confirm power. Find the dedicated breaker and reset it fully by switching Off, then On. Locate the nearby service switch—often a toggle on or near the unit—and set it On. If the front panel isn’t latched, the door switch will hold the unit off; reseat the panel until it clicks. Wait a full minute after restoring power so the control board can initialize.
Airflow And Filters
Restricted airflow can cause overheating and shutdowns that look like burner problems. Slide the filter out and check light transmission. If you can’t see light through it, replace it now. Size and orientation matter—match the arrow on the frame to the airflow direction. Keep all supply registers open and clear rugs or furniture off return grilles. Good airflow protects the heat exchanger and keeps safety limits calm.
Pros recommend regular filter changes to preserve airflow and prevent strain. Energy programs note that a dirty filter slows air and makes the system work harder; aim for monthly checks during heavy use and replace at least every three months—sooner with pets or remodeling dust. See the guidance on changing filters from ENERGY STAR for a simple schedule you can follow.
Gas Supply And Pilot/Ignition
Confirm that the manual gas shutoff near the appliance is open; the handle should be in line with the pipe. If you smell gas, stop and call your utility or a pro from outside the home. For older standing-pilot models, check the pilot status through the sight glass. If out, relight only if you can follow the printed instructions on the panel. For modern spark or hot-surface ignition, watch the sequence: inducer starts, pressure switch proves, igniter glows or sparks, gas valve opens, flame lights, then the blower ramps. If you see glow without flame, you may have a gas supply issue, a fouled burner, or a bad valve—time for service.
Safety Limits, Flame Sensor, And Lockouts
Safety devices stop the burner when conditions aren’t right. A high-limit switch opens when the heat exchanger runs too hot, often due to a clogged filter or closed vents. Reset by removing the cause and cycling power. If the unit lights and then drops out within a few seconds, the flame sensor may be dirty. This thin rod sits in the burner flame and tells the board that fire is present. Light oxidation can block the signal. If the sensor is easy to access, shut off power, remove the single screw, and wipe the rod with a clean, dry, non-abrasive pad. Reinstall and restart. If the problem returns, replace the sensor or call a tech.
Many units store a soft lockout after several failed lights. Kill power for 30–60 seconds to clear it, then watch the next attempt. If codes flash on the control board, capture the pattern (short/long blinks) and match it to the legend on the panel to narrow the fault before you call.
Vent, Combustion Air, And Condensate
Modern high-efficiency units create condensate that must drain freely. A slow drain floods the trap, trips the pressure switch, and stops the burner. Inspect the clear vinyl tubing and the trap near the cabinet. If water sits in the pan or trap, flush with warm water and a small drop of mild dish soap. Clear slime with a flexible brush or a short piece of weed-trimmer line. Re-prime the trap with clean water before restarting. Outside, check both PVC terminations. Clear snow, leaves, or insect nests and make sure the intake and exhaust aren’t blocked. If you see frost on the termination, thaw and recheck operation.
Electrical Checks You Can Do Safely
Look for a blown low-voltage fuse on the control board—a small automotive-style blade fuse. If it’s popped, a short in the thermostat wiring or accessory can be the cause. Replace with the same rating only once; if it blows again, you’ve confirmed a wiring fault that needs repair. Scan harness plugs for loose connectors and reseat them. Avoid opening sealed components or bypassing safety switches.
Deeper Symptoms And What They Mean
Use the table below to connect clues to likely parts. This helps you speak the same language as your tech and prioritize costs.
| Part | Typical Sign | What You Can Do |
|---|---|---|
| Hot-surface igniter | Never glows or cracks visible | Don’t touch the element; schedule replacement |
| Flame sensor | Lights then drops flame in 2–8s | Clean gently; replace if issue returns |
| Inducer motor | No pre-purge, pressure code shows | Check vent block; call for motor or switch testing |
| Pressure switch | Clicking without proof, long flash code | Clear drain/vent; avoid jumping the switch |
| Limit switch | Burner cuts out during long runs | Change filter, open vents; tech checks temps |
| Control board | Random resets, dead outputs | Confirm power and fuse; then service visit |
When Nothing Starts At All
If the thermostat is calling and you hear no inducer or blower, return to power sources. Check the outlet if the unit has a cord; plug in a lamp to confirm voltage. Some attic and crawlspace installs have a GFCI receptacle in line—reset it. If power is present and the unit stays dead, the internal transformer, board, or a safety chain may be open. At that point, stop and schedule a diagnostic.
Why Airflow Problems Cause Heat Loss
Heat must move off the exchanger. When the filter is packed or too restrictive for the rack, air stalls, temperature spikes, and the high-limit opens. That cuts the flame, cools the exchanger, and repeats. The cycle feels like short blasts of warmth followed by fan-only. This is the reason a simple filter swap often restores steady heat. For households with shedding pets or drywall dust, check more often than a seasonal schedule. The ENERGY STAR page above outlines a practical cadence you can adopt today.
Read Error Codes Like A Pro
Most control boards flash long and short blinks to point you to a system. Keep the front panel off only as long as needed to note the pattern, then close it so the door switch re-engages. Codes pointing to pressure or limit usually trace back to airflow, venting, or condensate. Codes tied to ignition sequence steer you toward the igniter, flame sensor, or gas flow. Bring the code list to your call; it speeds parts decisions.
Safety Notes You Shouldn’t Skip
Fuel-burning appliances can create carbon monoxide during fault conditions. Install CO alarms with battery backup on each level and near sleeping areas, and test them regularly. Guidance from the U.S. safety authority confirms that alarms save lives and that annual inspections of fuel-burning equipment are wise. See the CPSC CO information center for placement and maintenance advice. If your alarm sounds, go outside and call for help. Don’t re-enter until responders clear the home.
Step-By-Step: Clear A Drain Block
Many no-heat calls in condensing units trace back to a slow drain. Here’s a simple clearing routine you can do without disassembling sealed parts:
- Shut off power at the switch and breaker.
- Remove the service panel and locate the trap and clear tubing.
- Detach the tubing at the cleanout point and drain water into a small pan.
- Flush with warm water. Add a small drop of dish soap to cut biofilm, then rinse.
- Re-prime the trap with clean water so the pressure switch can prove.
- Reconnect tubing, secure clamps, and restore power.
If the pan refills within minutes, the downstream line or outdoor termination may still be restricted. Clear ice, leaves, or debris. If backups return, ask a tech to inspect slope, trap design, and vent lengths.
Step-By-Step: Clean A Flame Sensor
Only proceed if the sensor is plainly reachable near the burners and removal doesn’t require disassembling sealed chambers.
- Cut power. Give the unit a full minute to discharge.
- Remove the single retaining screw and gently pull the rod free.
- Wipe the rod with a dry, clean, non-abrasive pad. Avoid sandpaper that removes protective coating.
- Reinstall, snug the screw, and restore power.
- Call for heat and watch the light-off. If it still drops out, the part may be weak or the ground path poor—book service.
When To Call A Technician
Call in a pro if you smell gas, see water near the cabinet, hear grinding from the blower or inducer, or the breaker trips twice. Book service when the unit shuts down with repeated ignition failures after you’ve checked airflow and settings. Ask for a full combustion check, temperature rise measurement, static pressure reading, and a drain/vent inspection. Those four tests reveal most hidden issues and protect the exchanger.
Prevent Issues Before The First Cold Snap
A short preseason checklist pays back all winter:
- Stock the correct filter size and set a calendar reminder.
- Vacuum return grilles and pull furniture away from supplies.
- Confirm the breaker label matches the unit and that the service switch is accessible.
- Clear leaves from outdoor terminations and keep snow below the intake height.
- Test CO alarms and replace batteries as needed.
- Schedule a tune-up if you skipped last season. Ask for the readings listed above, plus a heat exchanger inspection.
Helpful Benchmarks After You Fix It
Once heat is back, note a few baseline numbers on a sticky near the unit:
- Typical temperature rise between return and supply during a 10-minute run.
- Static pressure reading if your tech provides it.
- Filter change dates and the brand/model that fits your rack best.
These quick notes make future troubleshooting smoother and help you notice changes early.
Cost Awareness And Smart Next Steps
Simple fixes cost little: filters, a sensor, or a drain cleaning. Parts like igniters or pressure switches are usually mid-range. Boards and motors sit higher on the scale. If your unit is nearing the end of its expected life and repair quotes are stacking up, ask for a side-by-side estimate that includes efficiency and warranty terms. Keep receipts and model/serial info in a plastic sleeve inside the cabinet so any tech can see the service trail.
Bottom Line
Most heat outages start with settings, airflow, or a basic lockout. Work through the quick checks, clear drains, and keep a fresh filter in place. Add CO protection, keep vents clear, and book a tune-up if the same fault returns. You’ll spend less, stay safer, and keep the house warm when it counts.