In cold climates, exhaust air can be warm and humid while intake is far below freezing. When that exhaust cools below its dew point inside the core, frost forms. Left unchecked, ice blocks the airflow entirely.
Why Frost Happens
Frost appears when exhaust outlet temperature drops near 0 C and the core surface is below freezing. The risk rises with higher exhaust humidity and lower outdoor temperature.
Common Protection Methods
- Defrost bypass: Periodically send a share of untreated air around the core to melt ice. Simple, widely used on plates.
- Pre-heat the intake: A small heater raises supply-air temperature so the core stays above freezing. Reliable but uses energy.
- Frost wheel / glycol spray: Rotary units may use a frost sector or periodic warm-air purge.
- Heat-pipe self-balance: Heat pipes naturally moderate the cold-end temperature, reducing frost risk.
Design Rules
- Model the winter design day, not the average.
- Set the defrost cycle by exhaust outlet temperature, not a timer alone.
- Size pre-heaters for the coldest hour, with a safety margin.
- Add drainage so meltwater leaves the core.
Good frost control is mostly about sensing and timing. Place the exhaust-outlet temperature sensor where it sees the real core exit, not the duct wall, and trigger defrost on that signal rather than a fixed clock so the cycle matches the weather. Adaptive defrost that shortens or skips when frost risk is low saves recovered energy across the season. Where a glycol spray or pre-heat is used, confirm the additive is compatible with the core coating and that drains handle the meltwater. Heat-pipe cores that self-balance the cold end need less active control but still benefit from a winter setpoint review. Our frost protection guide covers purge and pre-heat strategies in detail.
Frequently Asked Questions
Does defrosting waste recovered heat?
Briefly, yes, but a short, well-timed defrost prevents a full blockage that would waste all recovery.
Which technology frosts least?
Heat pipes and pre-heated plate cores handle cold climates best; unprotected rotary wheels need active frost control.