When it’s cold outside, nothing’s more frustrating than turning on your furnace and feeling cold air coming through the vents. Whether you have a gas furnace, electric model, or a heat pump system with furnace backup, several common issues can cause this. Below, our certified HVAC technicians explain why this happens, how to safely diagnose the problem, and when to call in a pro—because sometimes it’s more than just a simple fix.
For furnace repair in Denver, call Squeak’s Plumbing Heating & Air today!
Common Reasons Your Furnace Is Blowing Cold Air
When your furnace fails to deliver warm air, it’s often due to one of these five issues. Let’s dive into each cause and how to address it.
1. Incorrect Thermostat Settings
Incorrect thermostat settings are a frequent culprit behind cold air from your furnace. A thermostat set to the wrong mode or temperature can prevent your system from heating properly.
What to Check:
Ensure the thermostat is set to “Heat” mode, not “Cool” or “Off.”
Verify the fan setting is on “Auto” (runs only when heating) rather than “On” (runs continuously, potentially circulating unheated air).
Follow energy-efficient temperature guidelines for optimal comfort:
Home: 68°F
Away: 60°F
Sleep: 62–65°F
Pro Tip: Smart thermostats, like those we install at Squeak’s, can optimize settings automatically and save energy. If your thermostat is outdated or unresponsive, contact our team for an upgrade or repair.
2. Clogged or Dirty Air Filter
A dirty air filter restricts airflow, forcing your furnace to overwork and potentially overheat. This can cause the system to shut off its heating function, resulting in cold air blowing through your vents.
What to Do:
Check your air filter monthly and replace it every 2–3 months, or as recommended by your furnace manufacturer.
Use high-quality filters (e.g., MERV 8–11) suitable for your system to balance air quality and airflow.
Why It Matters: A clean filter improves efficiency, extends your furnace’s lifespan, and enhances indoor air quality. Our technicians can recommend the best filter for your system during a maintenance visit.
3. Leaky Ductwork
Leaky ducts can cause significant heat loss before warm air reaches your living spaces. In Denver homes, ducts often run through uninsulated areas like attics, where small holes or gaps can let heat escape.
Signs of Leaky Ducts:
Uneven heating in different rooms.
Higher-than-expected energy bills.
Visible damage or disconnected ducts in accessible areas.
Solution: Schedule a duct inspection with Squeak’s Plumbing, Heating & Air. Our experts use advanced diagnostic tools to locate and seal leaks, ensuring efficient heat delivery.
4. Faulty Electronic Ignition or Pilot Light
Older furnaces rely on a pilot light, while newer models use electronic ignition systems. A malfunction in either can prevent your furnace from igniting and producing heat.
Common Issues:
A dirty or faulty pilot light (in older systems) may fail to stay lit.
Electronic ignition components, such as the ignitor or flame sensor, can wear out over time, especially in furnaces over 15 years old.
What to Do: Avoid troubleshooting ignition issues yourself, as they involve gas or electrical components. Our licensed technicians can safely inspect and repair or replace faulty ignition systems.
5. Technical or Electrical Issues
Modern furnaces with electronic control panels or circuit boards can experience technical glitches, similar to a computer malfunction. These issues may cause the furnace to blow cold air or fail to start.
Common Technical Problems:
Faulty wiring or loose connections.
A malfunctioning control board.
Tripped circuit breakers or blown fuses.
Solution: A professional reset or diagnostic test by our team can resolve most technical issues. We use state-of-the-art tools to pinpoint and fix electrical problems safely.
When It’s a DIY Fix vs When to Call a Pro for Furnace Repair
While some issues, like checking thermostat settings or replacing an air filter, are DIY-friendly, most furnace problems require professional expertise. Attempting to fix complex issues without proper training can lead to safety hazards or further damage. Here’s how to decide whether you can safely troubleshoot on your own or if you need a licensed technician.
DIY Possible
- Checking thermostat settings, replacing batteries.
- Replacing or cleaning air filter.
- Ensuring vents are open and filters are clean.
- Relighting pilot light (if familiar & safe) or replacing batteries in thermostat.
Furnace Repair Professional Needed
- Repairing gas valve, flame sensor, control board.
- Fixing leaks in gas line or dealing with combustion/gas safety.
- Diagnosing high limit switch, electrical faults, major ductwork issues.
- Ignition system repair, major component replacement, gas inspection.
Prevent Furnace Issues with Regular Maintenance
Preventive maintenance is the key to avoiding unexpected furnace breakdowns. Our comprehensive maintenance plans include:
Annual furnace tune-ups to optimize performance.
Air filter replacements and duct inspections.
Safety checks for gas and electrical components.
Fun Fact: Regular maintenance can reduce your energy bills by up to 15% and extend your furnace’s lifespan by years!
Our Customers Frequently Ask:
How long should I wait after turning on the furnace before it should feel warm?
Typically 5-15 minutes. That allows the burners to heat up and the warm air to move through ductwork. If it’s still cold after that, it indicates one of the issues above.
My thermostat is set correctly, and filter is clean, yet the furnace still blows cold air. What then?
If you’ve ruled out settings and airflow, likely culprits include ignition failure, flame sensor problems, a gas supply issue, or a control board failure. These generally require professional evaluation.
Does closing unused vents help reduce heating bills, or can that make cold air issues worse?
Closing too many vents can restrict airflow and cause the furnace to overheat, possibly triggering safety shutdowns or cold airflow. Balanced vent usage helps maintain proper airflow throughout.
Action Plan: What You Can Do Now
✅ Confirm thermostat is set to Heat and fan to Auto.
✅ Replace or clean the filter.
✅ Allow full warm-up time (5-15 minutes).
✅ Check for visible issues (burners, pilot light, ducts).
✅ If basic fixes don’t resolve, schedule professional inspection (especially if any gas, ignition, or electrical issues are suspected).
