Furnace Troubleshooting Guide

Furnace Troubleshooting Guide

​When your furnace stops working in the middle of a chilly San Mateo evening, panic can set in fast. But before calling for a repair, there are a few things you can check yourself.
At Cloud Mechanical HVAC, we specialize in diagnosing and repairing heating systems across San Mateo and the Bay Area. In this guide, we’ll walk you through the most common furnace troubleshooting steps — from simple DIY fixes to signs you need professional help.

1. How a Furnace Works (Quick Overview)Understanding how your furnace operates can make troubleshooting much easier.
A standard gas furnace works like this:

1. Thermostat signals the furnace to turn on.
   
2. Ignition system lights the burners.
   
3. Burners heat the heat exchanger.
   
4. Blower fan pushes warm air through the ductwork.
   
5. Exhaust gases exit through the flue.
   

If any of these components fail, your furnace may stop heating properly.

2. Common Furnace Problems and How to Fix ThemHere are the most frequent heating issues we encounter — and what you can do before calling a pro.

Problem #1: Furnace Won’t Turn OnPossible Causes:

• Thermostat not set correctly
  
• Blown fuse or tripped circuit breaker
  
• Faulty power switch or control board
  

Quick Fixes:

• Make sure the thermostat is set to HEAT and above room temperature.
  
• Check your circuit breaker panel for a tripped switch.
  
• Confirm the furnace’s power switch (usually near the unit) is ON.
  

If your system still doesn’t start, it may be a control board or safety switch issue — call a technician.

Problem #2: Thermostat Isn’t RespondingSometimes the thermostat is the culprit.
What to check:

• Replace batteries (if battery-powered).
  
• Ensure wiring is intact.
  
• Restart smart thermostats by disconnecting and reconnecting Wi-Fi or power.
  

If you recently installed a new thermostat, it might not be compatible with your furnace.

Problem #3: Furnace Blowing Cold AirPossible Causes:

• Pilot light or ignition failure
  
• Dirty air filter
  
• Overheated system triggering safety shutoff
  

Quick Fixes:

• Check that your pilot light is on (for older furnaces). If it’s out, relight it following manufacturer instructions.
  
• Replace or clean the air filter — clogged filters can cause overheating.
  
• Turn furnace OFF for 15 minutes, then back ON to reset safety sensors.
  

If the problem continues, the flame sensor or igniter may need cleaning or replacement.

Problem #4: Furnace Turns On and Off Rapidly (Short Cycling)Causes:

• Clogged filter
  
• Oversized furnace
  
• Blocked vents or return grilles
  
• Faulty flame sensor
  

Short cycling wastes energy and causes extra wear. Start by changing the filter and ensuring all vents are open. If it continues, call for a diagnostic check.

Problem #5: Strange Noises (Banging, Rattling, Squealing)Each sound can point to a specific issue:

• Banging: Dirty burners or delayed ignition
  
• Rattling: Loose panels or duct vibration
  
• Squealing: Worn blower motor belt or bearings
  

Turn off the furnace if noises persist — mechanical issues can worsen quickly and become safety hazards.

Problem #6: Weak AirflowLow airflow often results from:

• Dirty air filters
  
• Blocked vents or ducts
  
• Blower motor issues
  

Replace filters regularly and inspect ducts for blockages. If airflow remains poor, the blower fan capacitor or motor might need professional servicing.

Problem #7: Furnace Smells Bad

• Burning Dust: Normal for the first use of the season.
  
• Musty Odor: Mold or mildew in ducts.
  
• Rotten Egg Smell: Gas leak! Turn off your furnace immediately, evacuate, and call PG&E or emergency services.
  

Never ignore strong or unusual odors from your heating system.

3. How to Reset Your FurnaceMany newer furnaces have a reset button (usually red or yellow) on the blower motor or control board.
To reset safely:

1. Turn the furnace power switch OFF.
   
2. Wait 30 seconds.
   
3. Press and hold the reset button for 5 seconds.
   
4. Turn power back ON.
   

If your furnace shuts off again, don’t keep pressing reset — repeated resets can damage the control board.

4. Maintenance Tips to Prevent Furnace ProblemsRegular maintenance is the best way to prevent breakdowns. At Cloud Mechanical HVAC, we recommend:
✅ Replace air filters every 1–3 months
✅ Schedule annual furnace tune-ups before winter
✅ Clean vents and return grilles
✅ Check thermostat settings seasonally
✅ Inspect exhaust venting for blockages
Proper maintenance improves efficiency, lifespan, and safety — and it’s often required to keep your manufacturer warranty valid.

5. When to Call a ProfessionalCall a licensed HVAC technician if:

• Your furnace keeps tripping breakers
  
• Pilot light won’t stay lit
  
• You smell gas or burning plastic
  
• You hear loud or persistent noises
  
• Your furnace is over 15 years old and inefficient
  

Our team at Cloud Mechanical HVAC has years of experience diagnosing complex issues for gas, electric, and high-efficiency furnaces in San Mateo. We’ll pinpoint the cause fast and get your heat back on safely.

6. Typical Furnace Repair Costs in San Mateo, CAType of RepairAverage CostFilter replacement$20–$50
Thermostat repair$100–$250
Igniter or flame sensor$150–$400
Blower motor replacement$500–$1,200
Full furnace replacement$4,000–$8,000We always provide transparent pricing and free estimates before starting any repair work.

7. Choose Cloud Mechanical HVAC for Furnace Repair in San MateoWhen you need fast, reliable furnace repair or troubleshooting, count on Cloud Mechanical HVAC — your local heating experts.
We provide:

• Same-day service calls
  
• Certified and insured technicians
  
• Honest, upfront pricing
  
• Maintenance plans to prevent future issues
  

We proudly serve San Mateo, Redwood City, Palo Alto, and the entire Bay Area.
📞 Call (650) [Your Number]
🌐 Visit www.cloudmechanicalhvac.com
We’ll restore your comfort — safely, efficiently, and affordably.

FAQs About Furnace TroubleshootingQ1: Why won’t my furnace turn on even though the thermostat is working?
It could be a tripped breaker, a safety switch, or a control board issue. Check for power at the furnace switch and breaker panel.
Q2: Why is my furnace blowing cold air?
Your pilot light or igniter might have failed, or your system is overheating and shutting off. Clean filters and reset the furnace before calling a pro.
Q3: What should I do if my furnace smells like gas?
Turn off the furnace immediately, evacuate your home, and call PG&E or 911. Never attempt to fix gas leaks yourself.
Q4: How often should I have my furnace serviced?
At least once per year, ideally before the heating season begins.
Q5: Is it better to repair or replace an old furnace?
If your furnace is over 15 years old and frequently breaks down, replacement is usually more cost-effective long-term.