Furnace Turned On With No Heat Output - What To Check

As winter sets in and temperatures start dropping, having a working furnace is essential. But when you flip that switch and hear the furnace running yet feel no warmth or heat coming from the vents, it can be intensely frustrating. This problem--a furnace that turns on but does not generate any heat--can have many causes depending on the type of system.

Inspecting key components methodically and troubleshooting common issues can help identify why your furnace activates without producing heat. In many cases, some simple DIY repairs may have it blowing hot air once again. Understanding the possible reasons and solutions will equip you to restore furnace heat when it mysteriously vanishes.

Check Thermostat Settings and Wiring Connections

The first place to check is your thermostat. Verify that the temperature setting is actually calling for heat--it should be set a few degrees above the current room temperature. Ensure the system switch is set to "Heat" rather than "Cool" or "Auto." If adjusting these does not trigger furnace heat, there may be a wiring problem.

Carefully inspect where all wires connect to the back of the thermostat and at the control panel on the furnace itself. Tug gently on wires to confirm snug and secure fit. Loose wiring is a simple explanation for activated furnace yet no heat production. Consult your model's manual to double check all wire connections are matched correctly.

Assess Thermostat Power

Another thermostat-related issue may simply be low or dead batteries if you have a standard unit with batteries. Replace old batteries and verify that the thermostat display illuminates normally. For thermostats powered by the heating system itself, test the voltage at the unit to rule out voltage supply issues triggering the no heat situation.

Replace Faulty Thermostat Altogether

If adjusting settings and connections does not resolve the lack of heat, then replacing the entire thermostat may be necessary. Whether issues like failed internal components or storm damage, malfunctioning thermostats certainly can cause furnace activation without heat. Install a new thermostat properly following our step-by-step process to restore normal operation.

Examining the Flame Sensor

Furnaces rely on a small device called the flame sensor to detect if a flame is actually present after the burners ignite. If this sensor is dirty, damaged, or improperly aligned, it will fail to confirm flame and shut off the gas valve--leading to an running furnace without burning gas or generating head. Carefully remove the flame sensor and clean gently with emery cloth if sooty. Ensure correct horizontal alignment with tips of burners before reinstalling.

Checking Sensor Condition

Also examine the sensor for any cracks, corrosion, or compromises to insulation which could all hinder flame detection. If cleaning does not restore proper performance, flame sensors are an inexpensive component easily replaced by homeowners in less than 15 minutes following disconnection and removal of old sensor.

Resetting Flame Sensor Gap

Precision alignment matters for the sensor triggering appropriately. Though specs vary slightly across furnace makes and models, general rule of thumb is about 1/4" horizontal distance from the tips of igniting flames. Refer to your owner's manual for the ideal gap required between burners and sensor on your unit, and adjust as needed with screw fasteners.

Checking Condition of Air Filter

Don't overlook something as simple as a dirty air filter choking airflow as the culprit for a running furnace and no heat. Air filters should be checked monthly and replaced every 1 to 3 months depending on use. Overlook this routine maintenance and excessive debris, dust and pet hair will impede critical airflow.

Removing Existing Filter

Tracking down the air filter location can be tricky depending on system type and installation configuration. Many modern filters slide out easily from return air grilles. On older units, accessing filters may involve unfastening cover panels either at the blower unit itself or removal from ductwork joints. Refer to the owner's manual if unsure. Once located, pull out filter and inspect condition.

Determining if New Filter Needed

Evaluate if accumulated dirt has significantly reduced airflow through the filter. Although monthly checks are wise, many experts suggest replacing filters every 60 to 90 days regardless given how quickly they collect debris. Confirm your filter's MERV rating matches recommendation for system--using overly dense filters also hinders airflow. Keep some replacement filters on hand for quick swap outs.

Inspecting Blower Assembly and Fan

Furnace blower units move heated air through ductwork to warm your home. If the blower malfunctions with a defective fan or motor problems, this prevents heated airflow even with properly burning furnace. Locate your unit's blower assembly and visually inspect when facing no heat issues.

Testing Fan Operation

First step is verifying fan activation since seizing bearing or stuck pulleys could halt rotation. The fan should turn freely without odd noises when spun by hand (with power off of course). Use a multimeter to check for current if fan does not run with furnace on. Replace fan if necessary matching CFM and other specs.

Checking Fan Belts

On belt drive systems examine for broken, loose or severely worn belts inhibiting proper fan speeds. Replace belts every 2-3 years as preventive maintenance. Adjust tension appropriately taking care not to overtighten. Take photos before disassembly making reinstallation straightforward if adapting custom alignments.

Inspecting Burners and Ignitor

Problems with furnace burners or ignitors also cause an operational furnace without generating any heat. Locate these components by removing outer burner compartment cover. Give the ignitor, burners, and surrounding area a close inspection searching for issues.

Examining Ignitor

The ignitor is responsible for lighting the furnace gas to produce flame when activated and should glow reddish. Cracks in the ceramic element prevent proper ignition. Ensure electrodes are clean without dust or debris buildup which can obstruct sparking. Replace entire hot surface ignitor assembly if defective or damaged.

Checking Burners

Thoroughly clean burners with compressed air or vacuum removing signs of rust or corrosion which lowers efficiency. Ensure burners ignite easily when tested and check for obstructions from insect nests, soot etc. blocking flow. If adjusting burners does not resolve ignite problems, deeper gas valve examinations by an HVAC technician may be needed.

Inspecting Exhaust Venting

Safely exhausting combustion byproducts relies on properly functioning furnace venting whether using metal pipes or a masonry chimney. Inspect entire exhaust pathway seeking cracks, leaks, separations or obstructions which would cause improper ventilation.

Checking for Blockages

Look for nesting animals or hives blocking vents. Debris like leaves/plants can accumulate restricting flow. Confirm ventilation terminals are clear outdoor. Detach pipes for internal inspection too. Repair leaks with proper sealing methods and materials.

Examining Vent Pipe Condition

Evaluate vent pipe orientation and joints with a level confirming slope upwards allowing drainage. Horizontal runs should slope gradually avoiding sags holding condensation. Check for signs of water damage, rust, or holes indicating compromised integrity. Consult codes before attempting vent pipe modifications.

Consider Testing Reset Button

Many furnace models have auto reset buttons to disable heating when internal temperature or pressure safety switches trip detecting issues like blocked vents. Other programmable circuit boards may also use internal sensors and logic disconnecting components upon sensing problems.

Locating Reset Button

Reset buttons or switches are typically found near the burners or blower unit but location varies across furnace types. They usually are colored red and may click in and out when pressed. Consult the owner's manual for your system if unsure regarding the functionality or location of a reset control.