Furnace Not Heating in Denver? 9 Things to Check Before You Call

Denver winters do not forgive a broken furnace. When the heat drops in a Front Range home, the fix is usually one of nine things — and most of them are quick. Here is the exact order our Aurora-based techs run through before we ever pull a component.

1. Confirm the thermostat is calling for heat

Set the thermostat 5°F above room temperature, mode to Heat, fan to Auto. If you have a smart stat, check that a schedule or geofence has not dropped the setpoint. A dead battery in a Honeywell or ecobee is the number one cause of a 'dead furnace' call we run in Denver every fall.

2. Change the air filter

A clogged filter starves the burner of return air, trips the high-limit switch, and shuts the furnace down for safety. If your filter is grey and you cannot see light through it, that is likely your whole problem. Colorado's dry, dusty air loads filters faster than most homeowners expect — check monthly during heating season.

3. Check the furnace switch and breaker

Every furnace has a service switch that looks like a light switch, usually near the unit. Anyone (a painter, an inspector, a curious kid) can flip it off. Then check the breaker panel — a tripped 15-amp furnace breaker is common after a power blip.

4. Verify gas is on

Look at the gas shutoff valve on the pipe feeding the furnace. Handle parallel to the pipe = open. Handle perpendicular = closed. If you have a gas stove, confirm it lights — that rules out a utility-side outage.

5. Listen for the ignition sequence

A modern furnace runs a set order: inducer motor spins up, pressure switch closes, igniter glows, gas valve opens, flame lights, blower engages after 30 to 60 seconds. If you hear the inducer but no ignition, it is almost always a flame sensor or hot-surface igniter — a same-day fix for a Denver HVAC contractor.

6. Check the condensate drain (high-efficiency units)

90%+ AFUE furnaces make water, and that water has to drain. A clogged condensate line trips a float switch and the furnace refuses to start. Look for a small PVC line leaving the unit and a plastic float in-line. If you see standing water in the pan, clear the drain.

7. Reset the furnace

Turn the service switch off, wait 60 seconds, turn it back on. That clears soft lockouts from previous failed ignition attempts. If the unit fires and runs a full cycle, monitor for the next 24 hours — one lockout can be a fluke, three in a week is a component failure.

8. Watch for error codes

Almost every furnace built since 2000 blinks an LED diagnostic code through the sight glass. Two flashes typically means pressure-switch fault, three means limit trip, four means flame sensor. The legend is printed on the inside of the burner-compartment door — take a photo and share it when you book service.

9. Know when to stop and call

If you smell gas, see soot around the burners, hear booming ignition, or the furnace shuts down and re-tries repeatedly — stop. Those are safety issues that need a licensed technician with a combustion analyzer. Call Ventilation Pro Mechanical at 720-256-5118 for same-day service across Aurora, Denver, Centennial, Parker, and Littleton.