Quick Answer: If your heater isn’t working, check these first: thermostat settings (make sure it’s set to HEAT), circuit breakers, furnace door switch, air filter, and gas valve (should be parallel to the pipe). In Denver, the #1 cause of furnace failure is a dirty or clogged flame sensor ($200-$300 cleaning/replacement, 1 hour repair). Other common issues include pilot light problems, thermocouple failures, or limit switch trips. Blue Collar Heating & Air has diagnosed hundreds of “no-heat” calls across Northglenn, Thornton, and Westminster – most are fixable same-day without full replacement.
The “Complete Silence” Problem
You wake up to a freezing house. You check the thermostat – it says 58°F when it should be 70°F. You walk to the furnace and… nothing. Complete silence. No click, no hum, no warm air.
This is the call I get 5-10 times every winter in Denver. And here’s what I’ve learned after 20+ years: 80% of “dead” furnaces aren’t actually dead. They’re usually tripped by a safety mechanism or have a simple fix that doesn’t require replacement.
8 Common Reasons Your Heater Won’t Work
Dirty flame sensor – #1 cause of furnace lockouts
1. Thermostat Issues (30% of calls)
Symptoms: Furnace completely silent, no clicking, no fan.
Check these first:
- Batteries: If you have a battery-powered thermostat, replace batteries first
- Settings: Verify it’s set to HEAT (not AC or OFF)
- Display: If screen is blank, check furnace breaker
DIY Fix: Replace batteries, verify settings.
Cost if you need a pro: $150-$300 thermostat replacement
2. Tripped Circuit Breaker (20% of calls)
What to check: Open electrical panel, look for “Furnace” breaker. If tripped (middle position), flip fully OFF then back ON.
Denver-specific: Single-digit nights cause furnaces to work harder, potentially tripping weak or old breakers.
Cost: Breaker replacement $150-$300. Component causing trip: $300-$800
3. Dirty or Failed Flame Sensor ($200-$300)
Symptoms: Furnace starts, igniter clicks, burners light for 3-5 seconds, then shut off. Cycle repeats.
Why common in Denver: High altitude = less oxygen. Furnaces run richer fuel mixture, producing more soot on flame sensors.
Cost: $200-$300 for cleaning/replacement (30-60 min job)
4. Clogged Air Filter (25% of calls)
Symptoms: Furnace starts, runs briefly, shuts off. Weak airflow. May restart after 15-30 minutes.
Denver’s dirty secret: Our dry, dusty air clogs filters FAST. Change every 30-60 days during heating season, not the manufacturer’s “90 days.”
Cost: Filters $15-$30. Damaged limit switch from overheating: $150-$300
5. Pilot Light or Igniter Problems ($200-$400)
Older furnaces: Check if pilot light is lit. Won’t stay lit = failing thermocouple.
Newer furnaces: Electronic igniter glows orange but burners don’t light = cracked igniter.
Cost: Thermocouple $150-$250. Hot surface igniter $200-$400
6. Condensate Drain Clog (High-Efficiency Furnaces)
Symptoms: Furnace won’t start, or starts then shuts down. Water pooling near furnace.
What’s happening: High-efficiency furnaces produce condensation. Clogged drain = safety switch trips.
DIY Fix: Clear drain line, pour cup of vinegar monthly.
Cost for pro: $150-$250 line cleaning. Pump replacement $300-$500
7. Gas Valve Issues or Gas Supply
Check: Gas shutoff valve should be parallel to pipe (OPEN). If perpendicular, it’s closed.
Emergency: If you smell rotten eggs/sulfur (gas leak), GET OUT and call Xcel Energy 1-800-895-2999
Cost: Gas valve replacement $300-$600
8. Limit Switch or Pressure Switch Failure ($200-$400)
Symptoms: Furnace doesn’t start at all, or starts then immediately shuts down.
Denver altitude factor: Pressure switches must be altitude-calibrated. Out-of-state installers sometimes use sea-level parts.
Cost: Limit switch $200-$300. Pressure switch $200-$400
Emergency Heating Safety: What You Should NEVER Do
DON’T:
- Use oven or stove for heat (carbon monoxide risk)
- Leave space heaters unattended
- Bypass safety switches
- Run furnace that smells like gas
DO:
- Call emergency service if you smell gas
- Use space heaters with tip-over protection
- Keep one room warm while waiting for repair
What Our Customers Say
Ignitor glowing but burners not lighting? Gas valve issue
Gas valve troubleshooting – check ON/OFF position first
“Incredibly responsive. Fast reliable and honest work. Got my heat back on before the storm.”
— Justin L., Denver Metro
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