Frost Formation Temperature Calculator

Cold-climate heat recovery freezes when the cooled exhaust drops below freezing while still holding moisture. Enter your conditions to find the exhaust outlet temperature, its dew point and the outdoor temperature at which frost begins.

How frost forms

After recovery the exhaust leaves at T_out = T_exhaust − ε(T_exhaust − T_outdoor). If that temperature falls below 0°C while the air is still humid, condensate on the cold side freezes and blocks channels. The frost threshold outdoor temperature is the supply temperature that makes the exhaust outlet exactly 0°C: T_threshold = T_exhaust(1 − 1/ε). Below it, you need frost control (preheat, bypass or a defrosting wheel).

Frequently asked questions

My exhaust outlet is above 0°C — am I safe?

From freezing, yes. But if outlet temp is below the exhaust dew point, you still get liquid condensate (normal, drains away). Frost only occurs when outlet < 0°C.

Does higher effectiveness mean more frost?

Paradoxically yes — a more effective core cools the exhaust further, lowering the frost threshold. That is why the best cold-climate units pair high effectiveness with active defrost.

What humidity ratio should I assume?

Use the actual exhaust RH. Kitchens, pools and labs run high (70–95%) and frost earliest; dry industrial exhaust may never frost.