How to Repair Your Furnace: Complete DIY Troubleshooting Guide

Is your furnace acting up? Before calling for service, there are several things you can check and potentially fix yourself. This guide walks you through common furnace problems, DIY solutions, and when it's time to call a professional.

Before You Start: Safety First

Important safety warnings:

• If you smell gas: Leave your home immediately, don't flip any switches, and call your gas company from outside
• Carbon monoxide risk: Ensure you have working CO detectors on every floor
• Never bypass safety switches: They exist to protect you from fire and gas leaks
• Know your limits: If you're uncomfortable with any step, call a professional

Common Furnace Problems and Solutions

Problem 1: Furnace Won't Turn On

This is the most common furnace complaint. Here's what to check:

Check the thermostat:

• Ensure it's set to "Heat" mode
• Set the temperature at least 3-5 degrees above room temperature
• Replace the batteries if it's battery-powered
• Check that the display is working

Check the power supply:

• Verify the furnace switch (usually near the furnace) is ON
• Check your circuit breaker panel for a tripped breaker
• Reset the breaker by turning it fully OFF, then ON

Check the air filter:

• A severely clogged filter can cause the furnace to shut down
• Replace the filter if it's dirty (you should do this every 1-3 months)
• Use the correct size filter for your system

Check the condensate drain (high-efficiency furnaces):

• 90%+ AFUE furnaces produce condensate that drains through a tube
• If this tube is clogged, the furnace won't run
• Clear any blockages and ensure water can drain freely

Problem 2: Furnace Runs But No Heat

If your furnace turns on but doesn't produce warm air:

Check the pilot light (older furnaces):

• Look through the viewing window on the furnace
• If you don't see a flame, the pilot may be out
• Follow your furnace manual to safely relight the pilot
• If it won't stay lit, the thermocouple may need replacement

Check the ignitor (newer furnaces):

• Modern furnaces use electronic ignition
• A cracked or worn ignitor won't light the gas
• You can sometimes see the ignitor glow through the viewing window
• Ignitor replacement is a common repair that costs $150-$300

Check your gas supply:

• Verify the gas valve to the furnace is open
• Check if other gas appliances in your home are working
• If no gas appliances work, contact your gas company

Problem 3: Furnace Short Cycles (Turns On and Off Repeatedly)

Short cycling stresses your furnace and increases energy bills:

Dirty air filter (most common cause):

• A clogged filter restricts airflow
• This causes the heat exchanger to overheat
• The safety limit switch shuts down the furnace
• Replace the filter and see if the problem resolves

Thermostat issues:

• A thermostat in a poor location (near a heat source or drafty area) can cause short cycling
• Consider relocating the thermostat or shielding it from drafts

Oversized furnace:

• A furnace that's too large for your home heats quickly and shuts off
• This is an installation issue that requires professional evaluation
• The solution may involve adjusting blower speed or, in severe cases, replacing the furnace

Flame sensor issues:

• A dirty flame sensor can cause the furnace to shut down shortly after igniting
• This is a common issue that a technician can fix quickly

Problem 4: Strange Noises

Different sounds indicate different problems:

Loud bang or boom when starting:

• Often indicates delayed ignition
• Gas builds up before igniting, causing a small explosion
• This is dangerous and requires immediate professional attention

Squealing or screeching:

• Usually a worn blower belt or motor bearings
• Turn off the furnace and call for service
• Continuing to run can cause more damage

Rattling:

• Loose panels or ductwork
• Tighten visible screws on the furnace cabinet
• Check duct connections for loose joints

Humming:

• Normal operation hum is fine
• Loud humming may indicate a failing blower motor capacitor
• Have it checked during your next maintenance visit

Problem 5: Furnace Blows Cold Air

Check the thermostat fan setting:

• Set to "Auto" not "On"
• "On" runs the fan continuously, even when not heating
• "Auto" only runs the fan during heating cycles

Wait for warm-up:

• Modern furnaces have a delay before blowing warm air
• This prevents cold air blast at startup
• Wait 2-3 minutes after the furnace starts

Check for overheating:

• If the furnace overheats, it may blow cold air as a safety measure
• This is often caused by restricted airflow (dirty filter, blocked vents)
• Replace the filter and ensure all vents are open

DIY Furnace Maintenance Tasks

Regular maintenance prevents many common problems:

Monthly Tasks

• Check and replace air filter: The single most important maintenance task
• Check thermostat batteries: Replace annually or when low
• Ensure vents are open: Closed vents cause pressure imbalances

Seasonal Tasks (Before Heating Season)

• Clean around the furnace: Remove stored items and debris
• Test the thermostat: Raise the temperature and verify the furnace responds
• Check the flue: Ensure it's properly connected and free of obstructions
• Listen for unusual sounds: Address them before heavy use season

Annual Professional Maintenance

• Clean and inspect heat exchanger: Cracks can leak carbon monoxide
• Check gas pressure: Proper pressure ensures efficient operation
• Inspect electrical connections: Loose connections cause failures
• Clean and test safety controls: Critical for safe operation
• Lubricate moving parts: Extends equipment life
• Check ignitor and flame sensor: Clean or replace as needed

When to Call a Professional

Some furnace issues require expert diagnosis and repair:

Call immediately if:

• You smell gas or rotten eggs
• Your carbon monoxide detector alarms
• You see a yellow or flickering pilot light (should be steady blue)
• There's water pooling around the furnace
• You hear loud banging or screeching
• The furnace exterior is very hot to touch

Call for service if:

• Basic troubleshooting doesn't resolve the issue
• The furnace is short cycling repeatedly
• You notice a significant increase in energy bills
• Some rooms are much colder than others
• The furnace is more than 15 years old and having frequent problems

Common Furnace Repairs and Costs

Here's what typical repairs cost in the Bay Area:

Repair	Typical Cost
Thermostat replacement	$150-$400
Ignitor replacement	$150-$300
Flame sensor cleaning/replacement	$100-$200
Blower motor replacement	$400-$700
Draft inducer motor	$400-$700
Control board replacement	$500-$1,000
Heat exchanger replacement	$1,500-$3,000

Note: If your heat exchanger is cracked, it's often more cost-effective to replace the entire furnace, especially if it's older than 15 years.

Should You Repair or Replace?

Consider replacement if:

• Your furnace is over 15-20 years old
• Repair costs exceed 50% of a new furnace
• You've had multiple repairs in the past 2 years
• Your energy bills keep increasing
• The heat exchanger is cracked

Consider repair if:

• Your furnace is less than 10 years old
• The repair is straightforward (ignitor, sensor, thermostat)
• The system has been well-maintained
• Repair cost is reasonable compared to replacement

Get Professional Help

If you've tried these troubleshooting steps and your furnace still isn't working properly, it's time to call a professional. Our NATE-certified technicians can diagnose and repair any furnace problem quickly and safely.

Schedule a Furnace Repair or call us at (925) 578-3293 for same-day service in the Bay Area.

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Galaxy Heating & Air Conditioning | CSLB License #1076868 (C-20 HVAC, C-10 Electrical, B General Building)