Cold Air from Furnace
Furnace lights but the supply air stays cool. Most callbacks on this complaint come from techs swapping whatever the board points at instead of proving what actually failed.
- Weak or lost flame proving (lights, can't prove flame, drops out)30-40%
- High-limit tripping from low airflow (overheats across the exchanger)25-35%
- Draft, pressure switch, or condensate problems on 90+ units
- 1
Watch one full sequence before touching anything
Where the cycle fails (no ignition vs. light-and-drop vs. heats-then-cools) sends you to a different branch.
- 2
If it lights and drops, meter flame current in series with the rod
Most boards want 2-6 µA, brand-specific. Weak signal also means check ground, polarity, burner carryover — not just rod cleaning.
- 3
Verify 24 VAC at the gas valve and manifold pressure (typically 3.5" WC NG)
Voltage with no flow points downstream; erratic pressure points upstream to regulator or supply.
- 4
If it heats then blows cold, measure temperature rise across the furnace
Rise at or above plate top means airflow — check filter, coil, wheel, blower speed, zoning, static — not a bad limit.
- 5
On a 90+ that stalls before ignition, prove draft with a manometer at the pressure-switch tap
Should run at least 0.1" WC beyond switch close setpoint. Don't replace the switch until you've measured what it sees.
- 6
Rollout open, or flame moves when blower starts? Stop and treat as combustion/HX
These are safety events, not reset-and-walk events.
Stop and treat as combustion / heat exchanger problem if rollout is open or flame moves when the blower starts. Don't reset and leave.
Don't swap pressure switches, high limits, or flame sensors without measuring first — that's the callback trap on this call.