Furnace Turns On However No Heat - Resolve It

Nothing's worse than waking up shivering under the covers, realizing your furnace isn't producing any heat. You hop out of bed to crank up the thermostat, only to hear the furnace running continuously without ever kicking into high gear to generate warmth. Dealing with a furnace that turns on but doesn't heat is frustrating, especially in frigid winter weather. But some basic troubleshooting and DIY repairs can get your furnace flaming hot again.

A variety of factors cause furnaces to activate without heating homes properly. From faulty thermostats to cracked heat exchangers, many issues prevent warm air from flowing. Understanding the root causes enables homeowners to correctly diagnose and fix furnace problems themselves without waiting days for a service appointment.

Common Causes of Furnace Running With No Heat

Several of the most frequent reasons a furnace turns on but doesn't heat or blows cold air include:

• Dirty air filter - A clogged filter blocks airflow, preventing heat from circulating.
• Faulty thermostat - An inaccurate thermostat fails to signal the furnace to produce heat.
• Cracked heat exchanger - Cracks in the exchanger create unsafe conditions and hinder heating.
• Bad ignitor or flame sensor - These prevent the furnace from lighting the burners.
• Faulty gas valve - Gas valves that stick open or closed affect heat generation.
• Blower motor or fan failure - No blower action means no hot air flow.

Issues like dirty filters or faulty thermostats are easy fixes, while cracked exchangers or blower motor failure require professional repairs. Understanding what each component does helps determine the cause.

How the Furnace Creates Heat

Furnaces generate heat through combustion, burning natural gas or propane to heat air. The basic sequence includes:

1. Thermostat signals furnace to activate
2. Draft inducer fan pulls in air for combustion
3. Gas ignites to fuel burners
4. Heat exchanger absorbs heat from the flame
5. Blower circulates warmed air throughout home

If any stage malfunctions, the result is a furnace running without producing sufficient heat. Examining each step isolates the culprit.

Diagnosing a Furnace That Turns On But Doesn't Heat

Furnace troubleshooting involves working through a diagnostic process. Steps include:

1. Inspecting filters for debris and replacing if necessary
2. Checking thermostat settings including temperature and fan settings
3. Listening to hear if draft inducer runs but no ignition occurs
4. Feeling for airflow to determine if blower is circulating air
5. Examining heat exchanger for cracks or corrosion

Issues like dirty filters, faulty thermostats, or ignition failures are simple fixes. However, no airflow indicates blower problems, while heat exchanger cracks require professional repairs.

Troubleshooting Furnace That Turns On But Blows Cold Air

A furnace that turns on but only blows cold air never fully fires up. Several key troubleshooting tips apply:

Check Thermostat Settings

Ensure the thermostat is set to "heat" mode. Check that the thermostat temperature setting exceeds the indoor temperature. Reset the thermostat if needed.

Replace Air Filter

Dirty filters commonly cause furnace problems. Shut off power before removing the filter. If dirty, replace with a new filter of the same size. Use proper filter ratings to allow airflow.

Inspect Heat Exchanger

Cracks or holes in the exchanger create safety risks and heating issues. Visually inspect for corrosion or cracks. Small holes may be hard to find without a professional inspection.

Diagnose Ignitor and Flame Sensor

The ignitor sparks to light the gas burners. The flame sensor confirms ignition occurred. If the burners don't ignite, the issue may lie with the ignitor or flame sensor.

Test Gas Valve

The gas valve regulates fuel flow to the burners. Malfunctions like sticking open or closed affect heating. Listen for a "click" indicating proper opening and closing.

Check Blower Motor and Fan

No airflow means the blower isn't circulating heated air. Feel near vents for airflow when the furnace should be running. Replace motors or fan blades if not operational.

DIY Repairs When Furnace Activates But Doesn't Heat

Many furnace problems resulting from running without heating have DIY fix options. Some repairs include:

Replace Old Thermostat

Thermostats eventually fail and require replacing. Turn off power, disconnect wires, remove old unit, install and wire new thermostat. Program temperature and fan settings.

Swap Clogged Air Filter

Routinely replacing filters prevents debris buildup. Shut off power, remove old filter, inspect sizing, insert new filter in correct direction, restore power.

Patch Cracks in Heat Exchanger

Small cracks and holes may be patchable using high-temperature epoxy adhesive. Larger cracks require professional replacement for safety.

Replace Bad Ignitor or Flame Sensor

Use a multimeter to test ignitors and sensors. Replace defective ones with identical models. Reconnect wires securely.

Adjust or Replace Faulty Gas Valve

Clean debris from gas valves sticking open. Replace valves sticking closed. Install same make and model for proper sizing and performance.

Oil Blower Motor or Replace Fan

Motor oil lubricates and extends motor lifespan. Replace seized motors. Unscrew fan cage, detach fan and replace with identical size.

Warning Signs of Furnace Malfunctions

Certain signs indicate developing furnace issues:

• Unusual noises like squealing or grinding
• Burning smells from overheating components
• Pilot light frequently goes out
• Blower runs nonstop regardless of thermostat
• Some rooms aren't heating evenly or sufficiently
• Excess dust/debris around vents and furnace

Addressing these early warning signs prevents bigger problems down the road. Don't ignore furnace symptoms that reduce efficiency and comfort.

Benefits of Addressing Furnace Issues

It's tempting to delay repairs, but fixing problems promptly prevents headaches. Benefits include:

• Preventing dangerous CO leaks from cracked heat exchangers
• Lowering energy usage and utility costs
• Maintaining comfortable indoor temperatures
• Extending furnace lifespan with preventative maintenance
• Removing dust and allergens for better indoor air quality

Technician costs for emergency repairs also increase compared to routine maintenance. Staying on top of furnace issues improves safety, comfort and savings.

Learning about furnace components and operations assists troubleshooting. Helpful concepts include:

• Pilot vs. electronic ignition systems
• Sequence of operation during heating cycles
• Blower motor types and lubrication needs
• Replacing air filters based on ratings
• Automatic safety features like overheat sensors
• Understanding AFUE efficiency ratings

Arming yourself with furnace knowledge prepares you to diagnose issues quickly. Technical understanding combined with a methodical approach keeps your furnace flaming away.