Furnace Turns On But Doesn’t Heat? Check This First

Finding ourselves suddenly without heat on a cold winter day can be frustrating and concerning. The good news is that there are a few simple things we can check ourselves before calling an HVAC professional for furnace repair.

When a gas furnace is running but not producing any warm air, the issue is likely an easy fix. By methodically examining some key components and sequences, we can get our furnace heating again. The goal is to troubleshoot the problem, restore heat, and avoid an expensive service call unless absolutely necessary.

Check The Thermostat First

The first and easiest item to inspect is the thermostat. This control device dictates when the furnace should kick on based on the home’s temperature. If it’s not set properly, the furnace won’t know to start heating.

gas furnace turns on but no heat

Go through these thermostat checks step-by-step:

  • Ensure the thermostat is set to “heat” mode, not “cool” or “auto”. Selecting the heat setting tells the furnace to start producing warm air.
  • Check that the fan is set to “auto” not “on”. Auto mode will only run the fan when heat is being produced.
  • Confirm that the temperature is set higher than the current room temperature. The furnace won’t ignite unless heating is required.
  • Replace old batteries if you have a battery operated programmable thermostat. Low power can cause thermostat malfunctions.
  • If issues persist, try resetting the thermostat to factory default settings in case of any programming errors.

Take the time to carefully inspect the thermostat settings before moving on to other troubleshooting. A simple thermostat misconfiguration is one of the most common reasons a furnace fails to heat properly.

Examine The Air Filter

A dirty, clogged air filter is another top culprit for reduced furnace heating. The filter removes dust and allergens from the air, but over time it can become obstructed.

On most furnace models, the filter is located in the return duct just before the blower motor. Refer to the installation manual to find its specific spot. Signs it needs replacement include:

  • Discoloration or debris buildup visible through the filter material.
  • The plastic filter frame does not fit flush against the housing.
  • Reduced airflow coming from the vents indicating blockage.
  • The current filter has been in place for over 60 days already.

When checking the filter, turn off the furnace at the circuit breaker for safety. Then slide out the old filter, inspecting it for dirt accumulation. If it looks overly dirty, replace it with a new filter of the same size and type. Use caution handling old filters to avoid freeing trapped particulates. Wash hands after replacement.

As a maintenance practice, mark the install date on new filters with a marker. That makes it easy to check elapsed time and schedule a swap every 1-3 months depending on filter type. Proper filtration is required for adequate airflow and heating performance.

Is There Adequate Airflow?

Airflow is how the hot air generated by the furnace circulates through the home via ducts and vents. Any restrictions or obstructions along that path will prevent proper heat distribution and make it seem like the furnace isn’t working.

Inspect the following areas for anything blocking free airflow:

  • Vents and registers in individual rooms. Close proximity to furniture or belongings can restrict output.
  • Main plenum ducts that branch off to vents. Crushed or disconnected ducts reduce performance.
  • Return air intake ducts going back to the furnace. Blockage here hurts efficiency.
  • The furnace blower motor itself. Slow RPM or jams can reduce speed.

Use a bright flashlight to peer down duct openings and look for obstructions. Also check for lint buildup around vents and the blower motor housing which can slowly choke off airflow. Use a vacuum with a soft brush attachment to gently clean away any accumulated lint or debris.

Try holding a tissue up to vents to feel output intensity and find weaker spots. Any vent lacking forceful airflow could indicate a blockage needing cleared. Proper airflow is required to circulate heat effectively to warm the home.

Analyze The Ignition Sequence

The ignition sequence on a gas furnace refers to the specific steps which must happen to safely light the burner and start generating heat. Issues here can prevent successful ignition and heating.

Stand near the furnace and listen closely to noises it makes when starting up. This provides clues to where the ignition sequence fails:

  • Listen for the hum of the inducer fan pulling in air for combustion.
  • The igniter should click signaling it is sparking to light the burner.
  • Finally, the gas valve should open releasing fuel to the ignited burner.

If the igniter clicks repeatedly but the burner fails to ignite, the igniter itself could be defective and require replacement. No clicking at all may indicate a problem with the ignition control module.

If you smell gas but don’t hear ignition, the gas valve likely isn’t opening. This could require replacing the valve or fixing wiring issues preventing its activation. A fully operational ignition sequence is required for proper heating.

Flame Sensor

The furnace also relies on a flame sensor to confirm the burner has successfully ignited before continuing to operate. If this sensor is dirty or defective, it won’t detect flame and heating will cease.

Carefully inspect the flame sensor rod projecting into the burner flame. Clean any dirt or debris off the sensor with steel wool. Replace the sensor completely if cleaning doesn’t restore normal operation.

Could It Be A Gas Supply Problem?

Furnaces require a consistent supply of gas fuel to ignite their burner and generate heat. Disruptions to that fuel delivery will cause heating failures.

Sudden drops in gas pressure are rare but can happen from issues like:

  • Damage to the gas main line or service meter during underground construction.
  • Increased demand across the utility grid overwhelming supply.
  • A faulty regulator valve on the home gas line.

Check for small leaks in the interior gas pipes leading to the furnace. Apply leak detecting solution and watch for bubble formation. Tighten any loose fittings but call the gas company if leaks persist.

For suspected pressure problems, contact the gas utility company to inspect. They can confirm adequate fuel supply entering the home. Correct and consistent gas pressure is needed for proper furnace operation.

Is The Blower Working Correctly?

The blower motor and fan are responsible for pushing heated air through ducts to warm the home. Furnace heat production relies on them functioning properly.

Signs of potential blower motor problems include:

  • Loud grinding or humming noises coming from motor.
  • Fan visibly spinning slower than normal.
  • Intermittent changes in airflow from vents.
  • Burning smell indicating motor overheating.

Power down the furnace then try spinning the blower fan by hand to check for stiffness or grinding. If rotation is smooth, try adjusting blower components that can slip and cause speed changes.

Ultimately though, failed bearings or electrical issues in older units will require full blower motor replacement. Confirm correct RPM ratings plus dimensions to match furnace requirements.

Prevent Future Furnace Issues

Beyond addressing the immediate lack of heat, it helps to adopt preventative maintenance habits to avoid furnace problems down the road:

  • Replace air filters regularly – Clogged filters are a leading cause of furnace failure. Mark calendars for a new filter each month.
  • Check vents and ductwork yearly – Ensure airflow paths remain clear of blockages for maximum efficiency.
  • Annual professional tune-ups – HVAC technicians can spot small problems before they become big repairs.
  • Maintain comfortable indoor temps – Avoid frequent dramatic temperature swings that strain furnace components.

Avoid waiting until furnace issues leave us without heat. By staying on top of maintenance and making minor repairs promptly, we can keep our home’s heating running smoothly all winter long.

When To Call A Professional

While many furnace malfunctions have straightforward DIY fixes, more complex repairs are best left to qualified HVAC professionals. They have specialized training plus access to manufacturer information and parts.

Call a technician for diagnosis in these scenarios:

  • Electrical issues in the furnace components or wiring.
  • Cracked heat exchanger or gas leaks.
  • Problems with refrigerant pressure on more complex units.
  • Repeated failures and shutdowns where the cause is unclear.

When hiring an HVAC company:

  • Ask for referrals from recent satisfied customers.
  • Check for good standing with organizations like BBB.
  • Look for technicians certified on your specific furnace make and model.

While fixing fees vary, homeowners may pay an average of $280 for common furnace repairs. More significant issues like heat exchanger or blower motor replacement run $1000 or more in parts and labor. Compare multiple estimates before authorizing costly work.

Troubleshooting a gas furnace that turns on but doesn’t heat a home isn’t hard using a systematic approach. We covered several easy things homeowners can check themselves before calling in an HVAC service.

In most cases, the issue proves to be something simple like a dirty filter, blocked vent, or ignition failure. Don’t immediately assume the worst. Methodically inspect the thermostat, air filter, airflow paths, ignition sequence, gas supply, and blower fan when heat runs low.

With cold weather here, a warm home is a priority. In many instances, we can restore furnace heat ourselves without waiting days for a technician. If problems persist, qualified professionals can fully evaluate and make expert repairs so we can stay comfortable all winter long.

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