The Frustrating Reality: Heat Is On, House Is Cold
It’s a chilly December evening in the Bay Area. Your thermostat shows the furnace is running. You can hear it. But your living room still feels like a refrigerator.
This is one of the most common HVAC complaints we receive-and it’s maddening because everything seems to be working.
The good news? Most causes are fixable. Let’s diagnose what’s happening.
Quick Check: Is Heat Actually Coming Out?
Before diving into causes, verify the basics:
- Put your hand near a supply vent. Is air coming out?
- Is the air warm, lukewarm, or cold?
- Are some rooms warm but others cold?
Your answers point to different problems:
| Symptom | Likely Cause |
|---|---|
| No air from vents | Blower motor issue |
| Cold air from vents | Ignition or gas problem |
| Lukewarm air | Heat exchanger or airflow issue |
| Some rooms warm, others cold | Ductwork or zoning problem |
The 10 Most Common Causes
1. Dirty Air Filter (Most Common)
The problem: A clogged filter restricts airflow. Your furnace produces heat, but the blower can’t push enough air through to distribute it.
Signs:
- Weak airflow from vents
- Furnace cycles on and off frequently
- Some rooms warmer than others
The fix: Check your filter right now. If you can’t see light through it, replace it. During winter, check monthly.
Bay Area note: Our dry climate creates dusty conditions. Homes near hills or construction zones need filter changes every 2-3 weeks.
2. Thermostat Issues
The problem: Your thermostat is misreading the temperature or not communicating properly with the furnace.
Signs:
- Display shows temperature that doesn’t match how the room feels
- Furnace short-cycles (runs briefly, stops, runs again)
- Temperature swings wildly
The fix:
- Check batteries (if battery-powered)
- Verify it’s set to HEAT, not COOL or AUTO
- Make sure temperature is set above current room temp
- Check if thermostat is in direct sunlight or near a heat source (causes false readings)
Older Bay Area homes: Many still have mercury thermostats from the 1970s-80s. These drift over time and cause 3-5 degree inaccuracies.
3. Closed or Blocked Vents
The problem: Supply vents are closed, or furniture/rugs are blocking return vents, disrupting airflow throughout the house.
Signs:
- Specific rooms are cold while others are fine
- Furnace runs longer than usual
- You recently rearranged furniture
The fix: Walk through every room. Open all supply vents. Clear furniture away from return air grilles. Yes, even in rooms you don’t use-closing vents causes pressure problems.
4. Leaky Ductwork
The problem: Your ducts have gaps, disconnections, or holes. Heat escapes into your attic, crawl space, or walls instead of your rooms.
Signs:
- Rooms farthest from furnace are coldest
- Excessive dust around vents
- Higher-than-expected heating bills
- You can hear whistling in the ducts
The fix: Duct sealing or repair by a professional. This is especially common in Bay Area homes with:
- Ductwork in unconditioned crawl spaces
- Flexible duct runs in attics
- Homes built before 1990 (duct tape degrades over time)
The impact: Leaky ducts can waste 20-40% of your heating energy. Sealing them often pays for itself in 2-3 years.
5. Undersized Furnace
The problem: Your furnace simply isn’t big enough to heat your home, especially on colder nights.
Signs:
- Furnace runs constantly but never reaches set temperature
- Problem is worse on coldest days
- Home was remodeled (added square footage, opened up walls)
- Previous owner installed a “builder-grade” system
The fix: A proper load calculation (Manual J) determines the right furnace size. Undersized systems need replacement-there’s no other fix.
Bay Area context: Many 1960s-70s homes have original or first-replacement furnaces sized for a different era (smaller homes, less insulation requirements).
6. Ignition Problems
The problem: The furnace isn’t igniting properly, so it’s blowing unheated air or cycling off before producing enough heat.
Signs:
- Cold air from vents
- Furnace clicks but doesn’t ignite
- Delayed ignition (boom or bang when starting)
- Burning smell when furnace finally lights
Common causes:
- Dirty or failed igniter
- Dirty flame sensor
- Gas valve issues
- Pilot light problems (older furnaces)
The fix: This requires professional diagnosis. Don’t attempt to fix gas-related issues yourself.
7. Blower Motor Problems
The problem: The blower motor isn’t running at full speed (or at all), so heat isn’t circulating even though the furnace is producing it.
Signs:
- Very weak airflow from vents
- Furnace runs, but you barely feel air
- Squealing or grinding noises
- Burning electrical smell
Common causes:
- Failed capacitor (most common)
- Worn bearings
- Motor failure
- Dirty blower wheel
The fix: Capacitor replacement is inexpensive ($150-$250). Full blower motor replacement costs $400-$800.
8. Heat Exchanger Issues
The problem: A cracked or failing heat exchanger reduces heating efficiency and poses safety risks.
Signs:
- Yellow or flickering burner flames (should be blue)
- Soot inside furnace
- Furnace shuts off frequently
- CO detector alerts
- Family members experiencing headaches or flu-like symptoms
The danger: A cracked heat exchanger can leak carbon monoxide into your home. This is a life-safety issue requiring immediate professional inspection.
The fix: Heat exchanger replacement ($1,500-$2,500) or full furnace replacement. If your furnace is over 15 years old, replacement usually makes more sense.
9. Gas Supply Issues
The problem: Your furnace isn’t receiving adequate gas flow.
Signs:
- Weak flames or delayed ignition
- Furnace struggles on coldest days (highest demand)
- Other gas appliances also underperforming
Common causes:
- Partially closed gas valve
- Gas line sizing issues (especially after remodels)
- Utility supply problems
The fix: Check that the gas valve to your furnace is fully open (handle parallel to pipe). If other appliances work fine, call for HVAC service. If nothing works, contact PG&E.
10. Poor Insulation or Air Leaks
The problem: Your furnace might be working perfectly, but your home is losing heat faster than the furnace can produce it.
Signs:
- Drafty windows and doors
- Cold walls or floors
- Furnace runs constantly on windy days
- Old single-pane windows
The fix: This isn’t an HVAC problem-it’s a building envelope problem. But it directly impacts your heating effectiveness. Consider:
- Weatherstripping doors and windows
- Sealing gaps around outlets and pipes
- Adding attic insulation
- Upgrading windows
Bay Area homes built before 1980 often have minimal insulation by today’s standards.
When to Call a Professional
DIY fixes:
- Replace air filter
- Open blocked vents
- Check thermostat settings
- Clear area around furnace
Call a professional for:
- Ignition or gas-related issues
- Blower motor problems
- Ductwork inspection and sealing
- Heat exchanger concerns
- Any time you smell gas
Emergency situations (call immediately):
- Gas smell (evacuate, call 911)
- CO detector alert (evacuate, call 911)
- Smoke from furnace
The Diagnostic Process
When you call Bay Area Climate Control, here’s what we’ll check:
- Thermostat operation - Communication with furnace
- Filter condition - Airflow restriction
- Ignition system - Igniter, flame sensor, gas valve
- Blower operation - Motor, capacitor, wheel
- Heat exchanger - Visual inspection for cracks
- Ductwork - Accessible sections checked for leaks
- Temperature rise - Measuring input vs. output air temp
- Gas pressure - Confirming adequate supply
This comprehensive diagnosis takes 45-60 minutes and identifies exactly why your home isn’t heating properly.
Prevention: Keep Your Furnace Performing
Avoid the “running but cold” problem with these habits:
- Change filters monthly during heating season
- Schedule annual maintenance before winter
- Keep vents open in all rooms
- Don’t block returns with furniture
- Listen for changes in furnace sounds
- Watch your bills - sudden increases indicate problems
The Bottom Line
A furnace that runs but doesn’t heat is frustrating, but usually fixable. Start with the simple stuff (filter, vents, thermostat), and call a professional for anything involving gas, electrical, or internal components.
Bay Area Climate Control diagnoses and repairs all furnace brands. We’ll tell you exactly what’s wrong and give you honest options-whether that’s a $150 repair or a conversation about replacement.
📞 Call us: (510) 391-5597
Related Resources
- HVAC Diagnostic Tool - Interactive troubleshooting
- Furnace Noises Guide - What those sounds mean
- Repair vs. Replace - Decision framework
- Why Is My PG&E Bill So High? - Efficiency problems