Why the Blower Motor Shuts Down Your Burners: A Diagnostic Guide

In modern HVAC systems, the timing of the blower motor is critical. If your furnace ignites and runs smoothly for 30–60 seconds, but the flames cut out the moment the indoor blower kicks in, you are likely dealing with a failure of the Pressure Switch or a breach in the Heat Exchanger.

1. Induced Draft vs. Atmospheric Draft

To understand this failure, we have to look at how modern furnaces "breathe."

• Older Atmospheric Units: These relied on natural buoyancy (hot air rises). In these units, a crack in the heat exchanger would allow the blower to push carbon monoxide into the home.

• Modern Induced Draft Units: A small inducer motor creates a negative pressure (vacuum) inside the heat exchanger. Because the pressure inside the tubes is lower than the pressure outside them, a crack actually sucks "house air" into the burner assembly rather than blowing exhaust out.

2. The Pressure Switch: Your Safety Sentinel

The pressure switch is the "brain" that monitors that vacuum. It ensures the inducer motor is moving enough air to vent exhaust safely.

• The Failure: When the indoor blower turns on, it creates a significant pressure change in the furnace cabinet. If there is a crack or hole in the heat exchanger, the blower forces house air into the vacuum-sealed combustion path.

• The Result: This "leak" disrupts the vacuum levels. The pressure switch senses the drop in negative pressure and immediately opens its contacts, cutting power to the gas valve to prevent unsafe combustion.

3. Visual Signs of a Breach

Even though the negative pressure prevents CO from immediately entering the ductwork, the air "sucked in" by the vacuum still disrupts the flame.

• Flame Disturbance: Watch the burners closely when the blower starts. If you see the flames waver, turn yellow, or "lift" off the burner rack, air is entering the combustion chamber from a breach.

• Flame Rectification Failure: If the air turbulence is strong enough, it can push the flame away from the flame sensor. If the sensor doesn't detect the flame for even a few seconds, it shuts the system down.

4. Secondary Heat Exchanger Clogs (90%+ Units)

In high-efficiency furnaces, the indoor blower moves air across a secondary heat exchanger made of stainless steel coils.

• If these coils are clogged with dust or debris, or if the "fins" have rotted due to acidic condensate, the blower can create an abnormal pressure differential that trips the pressure switch—even if there isn't a "crack" in the traditional sense.

The Technician’s Verdict

While modern induced draft systems are safer because they tend to "fail-safe" (shutting down the gas rather than venting CO), a tripped pressure switch when the blower starts is a major red flag. It indicates that the sealed combustion environment has been compromised.

Next Step: Perform a "Static Pressure Test" and a "Water Column Test" on the pressure switch to see exactly how many inches of WC (Water Column) are being lost when the blower engages.