April 2026

Home HVAC Problems Furnace Not Blowing Hot Air? Guide Mount Pleasant Heating & Air Conditioning · Heating Troubleshooting Furnace Not Blowing Hot Air? Guide A furnace that runs but blows cold air, or a heater that will not produce heat at all — it is one of the most frustrating problems on a cold morning. Work through the fast checks below, then call a technician for anything involving gas, ignition, or electrical parts. Call (843) 714-0863Request Service 24/7 emergency serviceLicensed, bonded & insuredBackground-checked techs 24/7 Emergency ServiceCooling and heating emergencies, day or night Quick Answer If your furnace is not blowing hot air, start with the thermostat (make sure it is set to HEAT and above room temperature), replace a dirty filter, and confirm the furnace has power and fuel (gas valve on, breaker on). If it still blows cold, the cause is usually the igniter, flame sensor, thermocouple, gas supply, or a limit switch — those are technician repairs. Common Causes What Usually Causes This Problem Most of these problems come from a handful of causes. Each one links to the service that fixes it. Thermostat on the Wrong Setting A thermostat set to FAN or COOL, or set below room temperature, means the furnace runs the blower without calling for heat. Dirty Air Filter A clogged filter can starve the furnace of airflow, trigger a high-limit shutdown, and stop the burner. Igniter or Flame Sensor Failure The igniter lights the burners; the flame sensor confirms the flame. If either fails, the furnace runs but never heats. Gas Supply Off If the gas valve is closed or the pilot is out, the burners cannot fire. Only check the valve handle position — leave the rest to a pro. Tripped Limit Switch The limit switch shuts the burner off if the furnace overheats — often from restricted airflow or a dirty filter. Blower Motor or Belt Problem If the blower runs weak or not at all, warm air stays trapped in the furnace instead of reaching your rooms. Start-Up Sequence How a Gas Furnace Starts, and Where It Stops If the blower runs but the air stays cold, the start-up sequence usually stopped somewhere. Stand near the unit and listen. Inducer motor and pressure switch. You should first hear a small motor whirring. If it fails, or a clogged vent pipe or leaking tubing keeps the pressure switch open, the furnace stops before ignition. Igniter. Most modern furnaces use a hot surface igniter that glows like a light bulb filament. If you never see a bright orange glow through the sight glass before the fan starts, the igniter has likely failed. Do not touch it. Flame sensor. If the furnace starts for about three seconds and then shuts off, a dirty flame sensor is the classic cause. Grime on the rod keeps it from sensing the flame, so it shuts off the gas. Try These First What You Can Check Yourself Safe, free checks you can do in a few minutes before deciding whether to call. 1 Check the Thermostat Confirm the mode is HEAT and the setpoint is above room temperature. If it is a programmable thermostat, check the schedule. 2 Replace the Air Filter The most common fix. A clean filter restores airflow and clears many high-limit shutdowns. 3 Check Power and Fuel Confirm the furnace breaker is on and, for gas furnaces, that the gas valve handle is parallel to the pipe (open). 4 Look for a Reset Button Many furnaces have a reset button near the burner. If it is tripped, press it once — if it trips again, call a technician. 5 Listen for the Burner When the thermostat calls for heat, you should hear the burner fire within a minute or two. No flame means ignition trouble. 6 Check the Exhaust Snow, ice, or debris blocking the exhaust vent can shut the furnace down for safety. Keep vents clear. Electric Furnaces If You Have an Electric Furnace An electric furnace has no burner, igniter or gas valve, so the gas-furnace causes above do not apply. The thermostat, filter, vents and blower still do. Check the breaker. Turn the power off at the circuit breaker, wait a few minutes, then turn it back on. If there is a reset button, press it once after power is restored. Warm but not hot air. A dirty filter, a low thermostat setting or a problem with the heating elements can leave you with only warm air. Heating elements are electrical work for a technician. When to Stop Signs to Stop and Call a Technician Gas smell. Do not turn the furnace on or off or use light switches. Leave the house and call for help from a safe distance. Yellow or orange flame. A healthy flame is steady and blue. A flickering yellow or orange flame suggests incomplete burning, which can lead to carbon monoxide. Banging at startup. Often delayed ignition, where gas builds up before it catches. It can eventually crack the heat exchanger. Short cycling. A furnace that switches on and off quickly may have a problem with airflow, the thermostat or a safety sensor. Before you call, note any blinking lights on the control panel and what you saw and heard. It helps the technician. Get It Fixed Get It Fixed by a Pro If the checks above did not solve it, stop guessing. A licensed technician will diagnose the real cause, explain your options, and give you the price before any work begins. Heating & Furnace RepairCall (843) 714-0863 Related Problems More HVAC Troubleshooting Guides Diagnose other common Mount Pleasant HVAC problems with the same plain-language approach. Heat Pump Freezing Up The other common Lowcountry heating failure — ice on the outdoor unit. Read the guide Repair vs. Replacement When a furnace is old or failing, is repair or replacement the better call? Read the guide Thermostat Blank or Not Working A dead thermostat can stop the heat call before the furnace ever