In a cold climate the same core that saves a fortune in October can freeze solid in January and block the airstream. Sub-zero outdoor air chills the exhaust-side plate below freezing, and any moisture in the exhaust condenses and ices over, slashing airflow and tearing the core. This guide covers the strategies that keep recovery working through a -25 degrees C winter, building on the frost-control reference.
Why cold exhaust freezes a core
Indoor exhaust leaves at about 22 degrees C and 40% RH. As it gives up heat to incoming -15 degrees C air, the plate surface on the exhaust side falls below 0 degrees C while still carrying moisture, so water condenses and freezes. The exhaust outlet can reach -5 to -10 degrees C in a counterflow core, the coldest point and the first to ice. Once ice bridges the plates, pressure drop spikes and the fan stalls, so prevention is cheaper than a defrost shutdown.
Preheat the supply, not the exhaust
The cleanest fix is a preheat coil on the incoming outdoor air, lifting it from -15 to roughly 0 degrees C before it enters the core. The exhaust-side surface then stays above freezing and no ice forms. Preheat energy is small (warming 10,000 m3/h by 15 degrees C is about 50 kW) and is largely offset by the recovery that the unfrozen core then delivers — a frozen core recovers nothing. Size the coil for design-day and let the recovery do the rest of the work.
Exhaust recirculation
Blending a fraction of warm supply (or bypassed exhaust) back into the exhaust inlet raises the exhaust entering temperature above the frost point without a coil. Recirculating 10% to 20% typically lifts the exhaust inlet 3 to 6 degrees C, enough to suppress ice in moderate cold. The trade-off is slightly lower net effectiveness and, for separation duties, the need to keep the recirculation on the exhaust side only so no exhaust reaches supply — a plate or heat-pipe core keeps the streams separate.
Automatic bypass and frost-control logic
A motorised bypass opens only when the exhaust outlet drops toward the frost point, routing air around the core until it warms again. The control logic is simple: measure exhaust outlet temperature, open the bypass below a setpoint (often -1 to -3 degrees C), close it above. This protects the core while maximising recovery hours. The same damper serves as the summer free-cooling bypass, so one actuator earns its keep in every season.
| Strategy | Best for | Energy cost | Eff. impact |
|---|---|---|---|
| Supply preheat | Severe cold, clean air | Low (offsets recovery) | None |
| Exhaust recirculation | Moderate cold | None (fan only) | -2 to -5 pts |
| Automatic bypass | All climates | None when closed | 0 when closed |
| Heat-pipe core | High frost risk | None | 55% to 65% |
Materials, sequencing and verification
Choose materials for the stream per the materials guide; coated plates resist both corrosion and ice adhesion. Sequence the controls so the recovery core runs first and the preheat coil only tops up, and confirm the bypass modulates rather than latches. Finally, commission at the design-day low to prove the frost strategy holds, and trend exhaust-outlet temperature in the BAS so a stuck-open bypass is caught in days, not at next winter's audit. For everyday residential duty the residential HRV notes and the commercial recovery overview translate the same logic to smaller systems, and the efficiency article shows how to read the rated number at your actual velocity.
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EN159 builds plate, rotary, heat-pipe and counterflow air-to-air exchangers for commercial, industrial and healthcare duty. Send airflow (m3/h), inlet temperatures and available space and we will size it.
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