How Frost Starts

Frost forms when the exhaust-air temperature at the core surface drops below 0 C while the air still holds moisture. The vapour deposits as ice rather than liquid.

The Driving Conditions

• Cold outdoor intake (large DeltaT to warm exhaust)
• Moisture in the exhaust (high humidity ratio)
• Core surface below the frost point

Why It Grows

Once ice forms, it narrows channels, raises velocity and pressure drop, and insulates the surface, accelerating buildup. Left alone it blocks the core.

Prevention in Principle

Keep the exhaust above freezing by preheating intake, purging with warm air, or recirculating return air. The setpoint must follow the coldest real temperature, not the average.