Furnace Repair Troubleshooting: Top 10 Common Problems

By Denny's Heating
on
Technician inspecting home furnace for repair issues

When your furnace quits on a freezing night, panic sets in fast. At Denny’s Heating, we know how brutal cold snaps can turn minor HVAC hiccups into major emergencies. If your furnace acts up, you need answers—quickly and safely.

That’s why we’ve created this practical, easy-to-follow guide for the top 10 furnace problems. Learn what you can fix, what’s risky, and how our licensed techs tackle emergencies. Stay warm, stay safe, and don’t let furnace troubles catch you off guard.

Furnace Won’t Start ❄️

Nothing’s more stressful than a furnace that won’t turn on—especially with icy winds outside.

Likely Causes:

  • Power switch off or breaker tripped
  • Thermostat incorrectly set
  • Pilot light out (for older units)
  • Blown fuse
  • Clogged air filter causing safety shutdown

Simple Fixes:

  • Make sure the furnace switch is “On”
  • Check your home’s breaker panel
  • Set thermostat to “Heat” and raise temp above room temp
  • Replace dirty filters

Red Flags—Call for Emergency HVAC Help If:

  • You smell gas (Evacuate immediately. This is a fire/explosion risk.)
  • Breaker keeps tripping after reset
  • Furnace clicks but won’t start at all

Thermostat Issues 🕹️

Your thermostat is the brain of your heating system. If it’s not working right, your furnace can’t do its job.

Likely Causes:

  • Dead batteries
  • Incorrect settings
  • Dusty or misaligned components
  • Old or incompatible thermostat

Simple Fixes:

  • Replace batteries
  • Double-check the setting (set to “Heat”)
  • Gently clean inside with a soft brush

Red Flags—Call a Professional If:

  • The screen is blank after battery change
  • Thermostat wiring looks loose or burnt
  • System cycles on and off erratically

Strange Noises 🔊

Bangs, rattles, whines—furnace noises are like a car’s warning lights. Don’t ignore them.

Likely Causes:

  • Loose panels or screws
  • Worn blower belt
  • Bearings needing lubrication
  • Air in the fuel line (oil or gas systems)

Simple Fixes:

  • Tighten panels carefully
  • Replace (or tighten) blower belt if accessible

Red Flags:

  • Loud booms (could signal gas buildup)
  • Grinding or scraping (motor or blower failure)
  • Persistent metallic or burning odors

Short Cycling (Turning On and Off Frequently) 🔁

If your furnace keeps clicking on and off, it’s short cycling—wasting energy and wearing out parts.

Likely Causes:

  • Dirty air filter
  • Oversized furnace
  • Thermostat location issues (e.g., in direct sunlight or near heat sources)
  • Blocked vents

Simple Fixes:

  • Replace the filter
  • Ensure vents aren’t blocked by furniture or rugs

Red Flags:

  • Furnace shuts off within minutes of starting, repeatedly
  • Burning smell or overheating

Weak or No Airflow 🌬️

You crank the heat, but barely feel a breeze. This is often a simple fix—sometimes, it’s a warning.

Likely Causes:

  • Clogged filter
  • Obstructed ductwork
  • Blower motor failure

Simple Fixes:

  • Change the filter
  • Open all supply/return vents

Red Flags:

  • Ice forming on the furnace or ductwork
  • Blower runs but no air comes out

Burner or Pilot Light Problems 🔥

Without a properly working burner or pilot, your furnace can’t heat.

Likely Causes:

  • Dirty or clogged pilot
  • Thermocouple failure
  • Gas supply issue

Simple Fixes:

  • Gently clean the pilot assembly (if accessible)
  • Reset the pilot following your furnace’s manual

Red Flags:

  • Pilot won’t stay lit
  • Strong gas smell (leave the house, call emergency services)

Blower Won’t Shut Off 🛑

If your blower keeps running, you’ll waste energy and overheat your system.

Likely Causes:

  • Thermostat set to “Fan On” instead of “Auto”
  • Stuck relay or faulty limit switch

Simple Fixes:

  • Set “Fan” to “Auto”
  • Reset the system at the breaker

Red Flags:

  • Blower runs constantly regardless of settings
  • Heat never turns off

Furnace Is Blowing Cold Air 🧊

Nothing’s worse than cold air when you expect warmth.

Likely Causes:

  • Thermostat set to “On” not “Auto”
  • Dirty flame sensor
  • Overheated system (often from dirty filter)

Simple Fixes:

  • Set the fan to “Auto”
  • Change the filter

Red Flags:

  • Burner won’t ignite
  • Cold air persists after basic checks

Odd Smells 🚨

A furnace should never smell strange. Trust your nose.

Likely Causes:

  • Dust burning off (normal at season’s start)
  • Musty odors from mold in ducts
  • Electrical or burning smells signal trouble

Simple Fixes:

  • Wait a few hours for dust smells to fade
  • Replace the filter

Red Flags:

  • Rotten egg or sulfur smells (Gas leak! Leave immediately.)
  • Persistent burning or chemical odors

High Energy Bills with No Comfort Increase 💸

If your energy bill climbs but your home stays cold, something’s wrong.

Likely Causes:

  • Dirty filter
  • Poorly sealed ducts
  • Malfunctioning thermostat
  • Aging or oversized furnace

Simple Fixes:

  • Change the filter monthly
  • Seal visible duct leaks with mastic or foil tape

Red Flags:

  • Bills spike suddenly, no clear reason
  • Uneven heating room-to-room

Quick Safety Checklist

  • Always shut power at the breaker before opening your furnace panels
  • Never attempt repairs on gas lines or electrical components
  • If you smell gas, evacuate and call 911
  • Wear gloves and eye protection when handling filters or panels
  • Follow manufacturer’s instructions and local code requirements

For more, see guidelines from the National Fire Protection Association (NFPA) and Occupational Safety and Health Administration (OSHA).

DIY vs. Call a Pro

Some fixes are safe for handy homeowners—like changing filters, checking your thermostat, or resetting breakers. But if you’re ever unsure, or you see signs of gas, smoke, or electrical issues, don’t risk it.

Call an emergency HVAC technician from Denny’s Heating. We’re trained, licensed, and insured—your safety comes first.

What Pros Do On Arrival

When a Denny’s Heating pro arrives, here’s what you can expect:

  • Assess safety: Check for gas leaks, electrical hazards, and code compliance
  • System diagnosis: Use specialized tools to test circuits, sensors, and airflow
  • Targeted repairs: Clean, adjust, or replace faulty parts
  • Full safety check: Ensure your furnace is safe before leaving

We also keep you informed at every step—no mysteries, no guesswork.

Prevention & Maintenance

The best furnace repairs are the ones you never need. Regular maintenance can head off most problems before they start.

Prevention Tips:

  • Change filters every 1–3 months
  • Schedule annual furnace inspections (before winter hits)
  • Keep vents and registers clear of dust, toys, or furniture
  • Test your thermostat monthly

Ask about our HVAC Maintenance programs, IAQ Testing, and energy audits for a healthier, more efficient home.

Costs & Time Factors

Repair times and costs vary by issue:

  • Simple fixes (like filter changes): minutes and minimal cost
  • Electrical or control issues: 1–2 hours (plus parts)
  • Major repairs (motors, control boards): several hours; parts and labor add up

Emergency HVAC service costs more during off-hours, but for heat in a cold snap, it’s worth every penny.

When It’s an Emergency

If you face any of these, call for help—fast:

  • No heat on a freezing night (risk of frozen pipes)
  • Smell of gas or burning
  • Repeated breaker trips
  • Furnace won’t start at all, and you’ve checked basics

Denny’s Heating is here 24/7 for your peace of mind.

FAQ

How often should I replace my furnace filter?
Every 1–3 months, depending on pets, allergies, and filter type.

Is it safe to relight my own pilot light?
Only if your manual says it’s safe and you smell no gas. If unsure, call a pro.

How do I know if I need a new furnace?
If repairs are frequent, bills climb, or the unit is over 15–20 years old, ask about replacement options.

What’s included in a professional tune-up?
Inspection, cleaning, safety checks, airflow test, and a full report—following guidelines from ENERGY STAR and the Air Conditioning Contractors of America (ACCA).

Local/Seasonal Considerations

In cold climates, a working furnace isn’t just comfort—it’s survival. Schedule maintenance before the first frost. Keep an eye out for sudden temperature drops; that’s when older systems are most likely to fail.

If you’re searching for “emergency HVAC” or “Hood & Exhaust Installation near you,” remember: Denny’s Heating is always on call—rain, snow, or ice.

Stay warm this winter. If your furnace acts up, don’t wait.

Call Denny's Heating now — or reach us anytime at  888-840-5783 .

Heating issues? Denny’s experts are ready to help. | Call Now 888-840-5783