Frost formation in rotary heat recovery (also called a rotary heat exchanger) usually occurs on the fresh-air side, especially under the following conditions:
❄️ Reasons frost forms on the fresh-air side:
Under winter operating conditions:
When the outdoor temperature is very low (usually below 0°C) while the indoor exhaust-air temperature is relatively high (around 20°C), the wheel continuously rotates between the exhaust and fresh-air sides, transferring heat from the exhaust side to the fresh-air side.
During the heat-exchange process:
The wheel carries heat away from the exhaust side and releases it to the fresh air when it rotates to the fresh-air side. However, because the outdoor fresh air is too cold, the water vapor in the air condenses upon contacting the wheel and forms frost on the fresh-air side.
🌬️ Why doesn't frost form on the exhaust side?
The air on the exhaust side comes from indoors and is relatively warm and humid.
The exhaust side mainly "loads" heat onto the wheel and is unlikely to cool down to the frosting point.
In contrast, the fresh-air side is the "receiving end" of heat; the temperature difference can lower the wheel surface temperature below 0°C, causing frost to form.
✅ Engineering countermeasures
Install a bypass or preheating device: to avoid frosting.
Add defrost control logic: such as timed reverse rotation or wheel stoppage.
Use a heater to preheat the fresh air: to avoid excessively low fresh-air temperature.
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